Spectroscopic apparatus comprising a pulsed light source

Abstract

An apparatus (500) for spectral measurements comprises: - an illuminating unit (110) to form narrowband light pulses (B2t1, B2t2) at different wavelengths (λ1,λ2) for illuminating an object (OBJ1) with the narrowband light pulses (B2t1, B2t2) - a sensor (SEN1) to detect light (B3) received from the object (OBJ1), - a control unit (CNT1) to control operation of the sensor (SEN1), wherein the illuminating unit (110) comprises: - a broadband light source (LS1) to generate broadband light pulses (B1), - a Fabry-Perot interferometer (FPI1) to form narrowband light pulses (B2) by filtering the broadband light pulses (B1), wherein the broadband light source (LS1) is arranged to generate a first broadband light pulse (B1t1) at a first trigger time (t1), and to generate a second broadband light pulse (B1t2) at a second trigger time (t2), wherein the Fabry-Perot interferometer (FPI1) is tunable by moving a mirror (M2) of the Fabry-Perot interferometer (FPI1), wherein the illuminating unit (110) is arranged to operate such that a movement of the mirror (M2) is not stopped between the first trigger time (t1) and the second trigger time (t2), wherein a start time (tA1) of a first exposure time (ΔtEX1) of the sensor (SEN1), and a duration of the first exposure time (ΔtEX1) are controlled such that the illuminating unit (110) forms the first broadband light pulse (B1t1) and the second broadband light pulse (B1t2) during the first exposure time (ΔtEX1).

Description

[0001]SPECTROSCOPIC APPARATUS COMPRISING A PULSED LIGHT SOURCE FIELD The present invention relates to a spectroscopic apparatus, which is arranged to form light pulses at different wavelengths, and which comprises a sensor to detect light. BACKGROUND A known hyperspectral imaging system comprises a light source to illuminate an object with white light, and a spectrally selective camera to capture spectral images of the object. SUMMARY An object is to provide an apparatus, which comprises a tunable light source and a sensor to detect light. An object is to provide a method for detecting light from a sample, which is illuminated with light pulses. An object is to provide a spectrometer. An object is to provide a method for measuring a spectral property of a sample. An object is to provide a spectral imaging apparatus. An object is to provide a method for spectral imaging. According to an aspect, there is provided an apparatus according to claim 1. Further embodiments are defined in the other claims. The scope of protection sought for various embodiments of the invention is set out by the independent claims. The embodiments, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention. The apparatus comprises an illuminating unit to form illuminating light pulses at different wavelengths. The apparatus may be a spectroscopic apparatus, i.e. an apparatus, which is suitable for measuring one or more spectral properties of an object. The illuminating unit comprises a broadband light source, and a tunable Fabry-Perot interferometer. The broadband light source may generate broadband light pulses. The Fabry-Perot interferometer may form narrowband light pulses at different wavelengths by filtering the broadband light pulses. An object may be illuminated with the narrowband light pulses. The illuminating unit may illuminate the object in a spectrally selective manner. The object may e.g. transmit, reflect, scatter and / or fluoresce light in response to the illuminating light pulses. Light received from the object may be transmitted, reflected, scattered and / or fluoresced from the object. The sensor may detect the light received from the illuminated object. The sensor may be e.g. a non-imaging sensor, a line sensor, or a two-dimensional sensor. The sensor may be arranged to capture a one- dimensional or a two-dimensional image of the object, which is illuminated with the with the narrowband light pulses in the spectrally selective manner. The illuminating unit may form a plurality of narrowband light pulses at different wavelengths. The illuminating unit may form e.g. an uninterrupted stream of narrowband light pulses. The exposure time of the sensor may define a predetermined group of narrowband light pulses from the stream of pulses. The exposure time may be controlled such that the group has a predetermined effective spectral bandwidth. Narrowband light pulses of the group may illuminate the object in the spectrally selective manner. A signal formed by the sensor during the exposure time may represent light, which is received from the object, and which is formed in response to the group of illuminating light pulses. The wavelength of the spectral transmittance peak of the Fabry-Perot interferometer depends on the mirror gap. The wavelength of each narrowband light pulse may be defined by the mirror gap. The mirror gap of the Fabry-Perot interferometer may be controlled according to a modulating waveform. The modulating waveform may be e.g. sinusoidal or triangular waveform. The mirror gap may be varied e.g. as a sinusoidal function of time. The start of an exposure time of the sensor, and the duration of the exposure time of the sensor may be controlled with respect to the modulating waveform, so as to define a group of illuminating narrowband light pulses at desired wavelengths. The start of an exposure time of the sensor, and the duration of the exposure time of the sensor may be controlled define a group of illuminating narrowband light pulses, such that the group represents a desired spectral bandwidth. The apparatus may form a narrowband light pulse at a selected wavelength by controlling the timing of a broadband light pulse with respect to a reference point of the modulating waveform. The wavelength of the narrowband light pulse may be changed by changing the timing of the light pulse. Each different wavelength may correspond to a different trigger time of a broadband light pulse. Each different wavelength may correspond to a different time delay between a reference time and a trigger time of broadband light pulse. The wavelengths of the narrowband light pulses may be changed by changing the mirror gap of the Fabry-Perot interferometer. Each narrowband light pulse may be formed by triggering a broadband light pulse when the mirror gap has been changed to a value, which corresponds to the wavelength of said narrowband light pulse. The mirror gap is changed by moving a mirror of the Fabry-Perot interferometer. A first narrowband light pulse at a first wavelength may be formed at a first trigger time, and a second narrowband light pulse at a second wavelength may be formed at a second trigger time such that the movement of the mirror is not stopped between the first and the second trigger time. The position of the mirror may be estimated based on the modulating waveform of the Fabry-Perot interferometer. The mirror gap at a trigger time may be estimated based on the modulating waveform. Stopping the movement of the mirror may cause a deviation between the estimated mirror gap and the actual mirror gap. Forming a plurality of light pulses without stopping the movement of the mirror may provide improved speed and / or improved accuracy. The mirror gap values may be estimated accurately based on the modulating waveform of the mirror gap. Forming a group of narrowband light pulses without stopping the movement of the mirror may facilitate providing said group of pulses during a single exposure time period of the sensor. The mirror of the Fabry-Perot interferometer may be arranged to move continuously during forming the pulses of said group. The mirror of the Fabry-Perot interferometer may be arranged to move continuously during the whole exposure time. The exposure time and the wavelengths of the narrowband light pulses may be selected such that the group of narrowband light pulses has a predetermined effective spectral width. The effective spectral width may be changed by changing the length of the exposure time. In an embodiment, the sensor may have a first exposure time and a second exposure time. The first exposure time may define a first group of narrowband light pulses, wherein the first group may have a first effective spectral width. The second exposure time may define a second group of narrowband light pulses, wherein the second group may have a second different effective spectral width. The mirror gap of the Fabry-Perot interferometer may be modulated at a modulation frequency fMOD. In particular, the mirror gap of the Fabry-Perot interferometer may be modulated at a constant modulation frequency fMOD, so as to minimize a deviation between an estimated mirror gap and the actual mirror gap. The light pulses may be generated at an average pulse repetition rate f0. The average pulse repetition rate f0may be e.g. higher than or equal to two times the modulation frequency fMOD. The duration of the individual light pulses may be short when compared with the modulation time period TMOD. The duration of the individual light pulses may be e.g. shorter than 1% of the modulation time period TMOD. Consequently, the spectral width of an individual narrowband light pulse may be substantially equal to the spectral width of the transmittance peak of the Fabry-Perot interferometer. The mirror gap may be modulated according to a modulating waveform. The broadband light pulses may be emitted at specific instances during the modulation of the mirror gap. The wavelength of each narrowband light pulse may be determined by the timing of the broadband light pulses with respect to the modulating waveform. The method may allow accurate control of the wavelength even at a high repetition rate of the light pulses. The control unit may form e.g. a substantially sinusoidal modulating signal for the Fabry-Perot interferometer, and the control unit may form trigger signals for timing the light pulses of the broadband light source. The trigger signals may control the timing of the broadband light pulses, and also the timing of the narrowband light pulses. The broadband light source may be e.g. a pulsed supercontinuum light source. The broadband light source may comprise a laser light source to generate monochromatic primary light pulses, and an optical fiber to form broadband light pulses from the primary light pulses. The broadband light source may provide sufficient power for enabling the operation of the apparatus. The laser light source may be e.g. a master oscillator power amplifier (MOPA). The master oscillator power amplifier comprises a seed laser and an optical amplifier to boost the output power. The control unit may form trigger signals for timing the laser pulses of a seed laser of the broadband light source. The timing of the laser light pulse of the seed laser can be controlled with high speed and with high accuracy. The broadband light source may be arranged to generate light pulses at an average repetition rate f0. The average time period T0between consecutive light pulses of the broadband light source is equal to 1 / f0. BRIEF DESCRIPTION OF THE DRAWINGS In the following examples, several variations will be described in more detail with reference to the appended drawings, in whichFig. 1a shows, by way of example, in a cross-sectional side view, a Fabry-Perot interferometer,Fig. 1b shows, by way of example, a spectral transmittance function of theFabry-Perot interferometer,Fig. 2 shows, by way of example, a spectral apparatus, which comprises atunable illuminating unit,Fig. 3a shows, by way of example, a light source to generate broadband lightpulses,Fig. 3b shows, by way of example, spectral intensity distribution ofbroadband light pulses,Fig. 3c shows, by way of example, spectral transmittance function of theFabry-Perot interferometer,Fig. 3d shows, by way of example, spectral intensity distribution of anarrowband light pulse,Fig. 4a shows, by way of example, forming narrowband light pulses atdifferent wavelengths,Fig. 4b shows, by way of example, using an exposure time to define a groupof narrowband light pulses,Fig. 4c shows, by way of example, providing a selectable effective spectralwidth by using a group of narrowband light pulses of different wavelengths,Fig. 5a shows, by way of example, forming narrowband light pulses atdifferent wavelengths by filtering broadband light pulsed with the Fabry-Perot interferometer, wherein the mirror gap of the Fabry- Perot interferometer is modulated according to a triangular waveform,Fig. 5b shows, by way of example, forming narrowband light pulses atdifferent wavelengths by filtering broadband light pulsed with the Fabry-Perot interferometer, wherein the mirror gap of the Fabry- Perot interferometer is modulated according to a sinusoidal waveform,Fig. 6a shows, by way of example, using a first exposure time to define afirst group of narrowband light pulses, using a second exposure time to define a second group of narrowband light pulses, and using a third exposure time to define a third group of narrowband light pulses,Fig. 6b shows, by way of example, using a first exposure time to define afirst group of narrowband light pulses, using a second exposure time to define a second group of narrowband light pulses, and using a third exposure time to define a third group of narrowband light pulses,Fig. 7 shows, by way of example, selected bands of a spectrum of anobject,Fig. 8a shows, by way of example, measuring spectral data by using a non-imaging spectrometer,Fig. 8b shows, by way of example, capturing spectral images with a spectralimaging apparatus,Fig. 8c shows, by way of example, capturing a linear spectral image with aspectral imaging apparatus,Fig. 9 shows, by way of example, measuring the mirror gap by using acapacitive sensor, Fig.10a shows, by way of example, an arrangement for calibrating the spectral scale of the Fabry-Perot interferometer, Fig.10b shows, by way of example, associating control signal values with wavelengths by using calibration detectors, Fig.11a shows, by way of example, an illuminating unit, which comprises optical filters for selecting different orders of interference of the Fabry-Perot interferometer, Fig.11b shows, by way of example, using optical filters for selecting different orders of interference of the Fabry-Perot interferometer. Fig.12a shows, by way of example, a Fabry-Perot interferometer, which is arranged to operate in vacuum, Fig.12b shows, by way of example, a Fabry-Perot interferometer, which is arranged to operate in vacuum. DETAILED DESCRIPTION Referring to Fig.1a, the apparatus 500 comprises a Fabry-Perot interferometer FPI1 to form narrowband light pulses B2 from broadband light pulses B1. The Fabry-Perot interferometer FPI1 forms the narrowband light pulses B2 by optically filtering the broadband light pulses B1. The Fabry-Perot interferometer FPI1 comprises a first semi-transparent mirror M1 and a second semi-transparent mirror M2. The first mirror M1 is parallel with the second mirror M2. The mirror gap dGAPbetween the mirrors M1, M2 is adjustable. The mirror gap dGAPmeans the distance between the mirrors M1, M2.The wavelength ^ of the spectral transmittance peak PEAK1 of the Fabry-Perotinterferometer FPI1 depends on the mirror gap dGAP. The wavelength ^ of thespectral transmittance peak PEAK1 of the Fabry-Perot interferometer FPI1 maybe changed by changing the mirror gap dGAP. The wavelength ^ of eachnarrowband light pulse B2 is determined by the mirror gap dGAPof the Fabry- Perot interferometer FPI1. The Fabry Perot interferometer FPI1 comprises one or more actuators ACU1 for changing the mirror gap dGAP. The actuator ACU1 may be e.g. a piezoelectric actuator or an electrostatic actuator. At least one of the mirrors M1, M2 may be moved by the one or more actuators ACU1. The first mirror M1 may be implemented e.g. on a first mirror plate PLA1. The second mirror M2 may be implemented e.g. on a second mirror plate PLA2. The apparatus 500 comprises a control unit CNT1 for controlling operation of the Fabry-Perot interferometer FPI1. The control unit CNT1 may form a modulating control signal SGAPfor changing the mirror gap dGAP. The modulating signal SGAPhas a modulating waveform. The apparatus 500 may be arranged to change the mirror gap dGAPaccording to the modulating waveform of the signal SGAP. The modulating signal SGAPmay have e.g. sinusoidal or triangular waveform. The control unit CNT1 may provide e.g. a digital modulating signal SGAP. The apparatus 500 may comprise a driving unit DRV1 to form an analog driving signal HV1 for driving the one or more actuators ACU1. The driving unit DRV1 may e.g. convert a digital modulating signal SGAPe.g. into an analog voltage signal HV1 for driving the one or more actuators ACU1. The analog driving signal HV1 may be coupled from the driving unit DRV1 to an actuator ACU1 e.g. via conductors CON1, CON2. SX, SY, and SZ denote orthogonal directions. Referring to Fig.1b, the spectral transmittance function T(^) of the Fabry-Perot interferometer FPI1 has a spectral transmittance peak PEAK1. The spectralposition ^PEAK1 of the spectral transmittance peak PEAK1 depends on the mirrorgap dGAP. For example, a first wavelength ^1 may correspond to a mirror gapvalue dGAP,1. The wavelength of narrowband light pulses B2 transmitted throughthe Fabry-Perot interferometer may also be equal to said first wavelength ^1 whenthe mirror gap dGAPis equal to the value dGAP,1. The spectral position of the spectral transmittance peak PEAK1 may be changed by changing the mirror gap dGAP. The spectral position of the spectral transmittance peak PEAK1 may be changed by moving the mirror M2. The wavelength of the narrowband light pulses B2 may be changed by changing the mirror gap dGAP. The wavelength of the narrowband light pulses B2 may be changed by moving the mirror M2. The Fabry-Perot interferometer may have a spectral operating range MSR1. The spectral operating range MSR1 may also be called e.g. as the spectral measurement range. The spectral operating range MSR1 may have a lower cut-off wavelength ^LP and an upper cut-off wavelength ^SP. The spectral operatingrange MSR1 may be defined e.g. by one or more optical filters FIL1, FIL2. Forexample, the lower cut-off wavelength ^LP may correspond to a minimum valuedGAP,MIN of the mirror gap, and the upper cut-off wavelength ^SP may correspondto a maximum value dGAP,MAXof the mirror gap. The spectral apparatus 500 may optionally comprise one or more optical filters to define the spectral operating range MSR1, so that the spectral transmittance function T(^) of the Fabry-Perot interferometer FPI1 may have only one spectral transmittance peak PEAK1 at a time. Referring to Fig.2, the spectral apparatus 500 comprises an illuminating unit 110to form narrowband light pulses B2 at different wavelengths ^1, ^2, ^3. Theilluminating unit 110 comprises a broadband light source LS1 to form broadband light pulses B1, and a Fabry-Perot interferometer FPI1 to form the narrowband light pulses from the broadband light pulses B1. An object OBJ1 may be illuminated with the narrowband light pulses B2. The apparatus 500 comprises a sensor SEN1 to detect light B3 from the object OBJ1. The sensor SEN1 may be e.g. an image sensor, and the apparatus 500 may be a spectral imaging apparatus. The apparatus 500 comprises a control unit CNT1 to control operation of the sensor SEN1. The control unit CNT1 may also control operation of the illuminating unit 110. The control unit CNT1 may control operation of the broadband light source LS1. The control unit CNT1 may control operation of the Fabry-Perot interferometer FPI1. The control unit CNT1 may form a trigger signal SLS1for triggering emission of broadband light pulses B1. The control unit CNT1 may form a modulating waveform SGAPfor modulating the mirror gap dGAPof the Fabry-Perot interferometer FPI1. The control unit CNT1 may comprise one or more data processors. The control unit may comprise e.g. a field-programmable- gate array (FPGA).The control unit CNT1 may modulate the transmission wavelength ^ of the Fabry-Perot interferometer FPI1 by modulating the mirror gap dGAP. The mirror gap dGAPmay be modulated according to a periodic modulating waveform SGAP(t). The wavelength of each narrowband light pulse B2 may be determined by the timing of a broadband light pulse B1 with respect to the modulating waveform SGAP(t). The control unit CNT1 may control the timing of the broadband light pulses B1,so as to form narrowband light pulses B2 at desired wavelengths ^1, ^2, ^3. Thewavelengths ^1, ^2, ^3 may be selected from the spectral operating range MSR1of the Fabry-Perot interferometer FPI1. The broadband light source LS1 may be arranged to generate a broadband light pulse B1 according to a trigger signal SLS1formed by the control unit CNT1. The trigger signal SLS1may control the time of emission of a broadband light pulse B1. The timing of a broadband light pulse B1 may be determined according to the trigger signal SLS1. The trigger signal SLS1may also control whether the broadband light pulse B1 is emitted or not. The broadband light source LS1 may be arranged to generate a broadband light pulse B1 only when instructed by the trigger signal SLS1to do so. The apparatus 500 may comprise a clock CLK1 for measuring the lengths of time intervals and for forming timing signals. The control unit CNT1 may synchronize exposure times of the sensor with the modulating waveform SGAP(t) by using the clock CLK1. The control unit CNT1 may synchronize triggering of the light pulses B1 with the modulating waveform SGAP(t) by using the clock CLK1. The illuminating unit 110 may be arranged to illuminate the object OBJ1 with the narrowband light pulses B2. The narrowband light pulses B2 may illuminate at least a region REG1 of the object OBJ1. The narrowband light pulses B2 may illuminate a region REG1 of the surface SRF1 of the object OBJ1. The illuminating unit 110 may optionally comprise illuminating optics OPT1 e.g. to focus or distribute the narrowband light pulses B2 to a desired region of the object OBJ1. The illuminating optics OPT1 may comprise e.g. a focusing lens. The apparatus 500 may optionally comprise an imaging unit CAM1 to capture spectral images IMG11, IMG12, IMG13, of one or more illuminated regions REG1 of the object OBJ1. The imaging unit CAM1 may comprise imaging optics LNS1 and an image sensor SEN1. The sensor SEN1 may form a sensor signal SSEN1. The sensor signal SSEN1may comprise e.g. a captured image IMG11. A first spectral image IMG11may be captured when the object is illuminated with a first group GRP1 of narrowband light pulses B2. The group GRP1 of pulses may comprise e.g. at least 2, 3, 4, 5, 6, 7, 8, 9, or 10 narrowband light pulses B2 at individually selectable wavelengths. The first group GRP1 may represent a first spectral band of illuminating light. A second spectral image IMG12may be captured when the object is illuminated with a second group GRP2 of narrowband light pulses B2. The second group GRP2 may represent a second spectral band of illuminating light. A third spectral image IMG13may be captured when the object is illuminated with a third group GRP3 of narrowband light pulses B2. The third group GRP3 may represent a third spectral band of illuminating light. The apparatus 500 may comprise a memory MEM2 for storing data DATA1 obtained from the sensor SEN1. The sensor data DATA1 may comprise e.g. captured images IMG11, IMG12, IMG13. The illuminating unit 110 may be arranged to illuminate the object OBJ1 in a spectrally selective manner. The object OBJ1 may also be called e.g. as a sample or as a target. The illuminating unit 110 may be used together with the image sensor SEN1 for hyperspectral imaging. The spectral selectivity may be provided by the spectrally selective illumination. The illuminating unit 110 may be used e.g. together with a 1D or 2D image sensor SEN1 for hyperspectral imaging. The image sensor may also be panchromatic, i.e. all detector pixels of the image sensor may have similar spectral response. The detector pixels of the 1D image sensor are arranged in a one-dimensional array. The detector pixels of the 2D image sensor are arranged in a two-dimensional array. The light pulses B1, B2 may be generated e.g. as an uninterrupted stream of pulses. The light pulses B1, B2 may be generated e.g. as an uninterrupted stream of pulses at constant equal intervals T0. The wavelength of each narrowband light pulse B2 may also be selected within certain constraints by selecting the timing of the broadband light pulses B1 with respect the modulating waveform. The apparatus 500 may comprise a memory MEM1 for storing control parameters PAR1. The control parameters PAR1 may specify e.g. a start time tAof anexposure time ^tEX, of the sensor SEN1, and a duration of the exposure time ^tEX.The control unit CNT1 may control operation of the sensor SEN1 according to the control parameters PAR1. The control unit CNT1 may be optionally arranged to determine output values OUT1 from the captured spectral images. For example, the control unit CNT1 may calculate spectral reflectance values of the object OBJ1 or spectral transmittance values of the object OBJ1 from pixel values of the captured images IMG11, IMG12, IMG13. The output values OUT1 may be e.g. spectral reflectance values or spectral transmittance values. The apparatus 500 may comprise a memory MEM3 for storing the output values OUT1. The control unit CNT1 may determine the output values OUT1 by using calibration data CAL1. For example, the control unit CNT1 may be configured to determine spectral reflectance values from pixel values of a captured spectral image (IMG11) by using calibration data CAL1. The apparatus 500 may comprise a memory MEM4 for storing calibration data CAL1. The control unit CNT1 may perform the steps of the present method by executing computer program code PROG1. The apparatus 500 may comprise a memory MEM5 for storing the computer program code PROG1. The apparatus 500 may optionally comprise a communication unit RXTX1 receiving and / or transmitting data. The communication unit RXTX1 may communicate e.g. via wired and / or wireless communication. The communication unit RXTX1 may communicate e.g. via a mobile communications network. For example, the communication unit RXTX1 may communicate data DATA1 and / or output values OUT1 to a remote device. The communication unit RXTX1 may communicate e.g. with a control unit of an industrial manufacturing process. The apparatus 500 may optionally comprise a user interface UIF1 for receiving user input and / or for providing information to user. The user interface UIF1 may comprise e.g. touchscreen and / or a keypad. The illuminating unit 110 may be arranged to form a first group GRP1 ofnarrowband light pulses B2 during a first exposure time ^tEX of the sensor SEN1.The start time tAand the duration of the exposure time may be selected with such that the first group GRP1 represents a desired spectral band (e.g. RNG1 in Fig. 7). The start time tAand the duration of the exposure time may be selected with respect to the modulating waveform SGAP(t) such that the first group GRP1 represents a desired first spectral band. The sensor SEN1 may capture a firstimage IMG11 of the object OBJ1 during the exposure time ^tEX, when the objectOBJ1 is illuminated with the narrowband light pulses B2 of the first group GRP1. The first image IMG11may represent the response of the object OBJ1 to the spectral band of the first group GRP1. At least a first narrowband light pulse B2^1and a second narrowband light pulseB2^2 may be formed during the exposure time ^tEX. The illuminating unit 110 mayilluminate a region REG1 of the object OBJ1 with the first narrowband light pulseB2^1. The field-of-view FOV1 of the imaging unit CAM1 may overlap the illuminated region REG1. The region REG1 may contribute to a first image IMG11,which is captured by the sensor SEN1 during the exposure time ^tEX. The firstimage IMG11may comprise an image of the illuminated region REG1. The first image IMG11may be an image of the illuminated region REG1. The first imageIMG11 may represent at least the first wavelength ^1 and the second wavelength^2. The illuminating unit 110 may be arranged to generate a plurality of narrowbandlight pulses B2 at different wavelengths ^1, ^2, ... by changing the mirror gap dGAPaccording to a modulating waveform. A first narrowband light pulse B2^1may be formed at a first trigger time t1. A second consecutive narrowband light pulse B2^2may be formed at a second trigger time t2. The mirror gap dGAPof the Fabry-Perot interferometer FPI1 may be modulated such that the movement of the second mirror M2 is not stopped between the times t1and t2. The mirror M2 may be moved in the same direction at the first trigger time t1and at the second trigger time t2. Modulating the mirror gap dGAPwithout stopping the movement of the mirror M2 during the exposuretime ^tEX may e.g. improve speed and / or accuracy of operation of the apparatus500. The illuminating unit 110 may optionally comprise a beam splitter BS1, and a reference detector DET1, wherein a part of the light of the light pulses B1, B2 may be directed to the reference detector DET1 via the beam splitter BS1 so as to measure the energy and / or intensity of the light pulses B1, B2. The reference detector DET1 may form a signal SDET1indicative of the energy and / or intensity of the light pulses B1, B2. The apparatus 500 may optionally comprise an actuator ACU2 for causing relative movement between the apparatus 500 and the object OBJ1. The apparatus 500 may optionally comprise an actuator ACU2 for causing relative movement between the illuminating unit 110 and the object OBJ1. The apparatus 500 may optionally comprise an actuator ACU2 for causing relative movement between the imaging unit CAM1 and the object OBJ1. The actuator ACU2 may comprise e.g. conveyor belt and / or a robot. The actuator ACU2 may also change angular orientation of the apparatus 500 with respect to the object OBJ1. The apparatus 500 may be mounted on a moving arm of a robot. The actuating unit ACU2 may be e.g. a turret, which may be arranged to rotate the apparatus 500 with respect to a stationary object OBJ1. The apparatus 500 may also be mounted e.g. on a vehicle. The vehicle may be e.g. selected from the following list: a piece of equipment designed to transport people, cargo or both, a motor car, a truck, a bus, a train, a tram, a ship, an underwater vehicle, an aircraft, a helicopter, an unmanned vehicle, an unmanned aerial vehicle (drone), a spacecraft, a satellite. The sensor signal SSEN1may comprise a main signal component, which is proportional to the intensity of light impinging on the sensor SEN1. The sensor signal SSEN1may also comprise signal noise. The effect of the signal noise may be reduced or eliminated e.g. by measuring a dark signal without illuminating the object OBJ1, and by subtracting the dark signal from the sensor signal SSEN1. In an embodiment, generating of the broadband light pulses B1 may be temporarily stopped for measuring the dark signal. Alternatively, or in addition, the apparatus 500 may comprise a shutter SHUT1 to temporarily prevent propagation of the light pulses B1 or B2, for measuring the dark signal. The shutter SHUT1 may be e.g. a mechanical shutter. The broadband light source LS1 may be arranged to generate an uninterrupted stream of broadband light pulses B1 at a constant interval T0, e.g. in order to minimize variation of pulse energy of the broadband light pulses B1. Variation of the pulse energy may be minimized by using the shutter SHUT1 to prevent propagation of the broadband light pulses B1 of the uninterrupted stream. The narrowband light pulses B2 may have a uniform or a non-uniform spatial intensity distribution on the surface SRF1 of the object OBJ1. A non-uniform spatial intensity distribution of the narrowband light pulses B2 may be optionally compensated by using calibration data. The calibration data may be determined e.g. by illuminating a white test surface with the narrowband light pulses B2, and by measuring the spatial intensity distribution formed on the illuminated white test surface. The spatial intensity distribution may also depend on the wavelength of the narrowband light pulses B2. The calibration data may also be determined for two or more different spectral bands (RNG1, RNG2) separately. The apparatus 500 has an optical path from the broadband light source LS1 to the sensor SEN1. The optical path includes, among other things, the Fabry-Perot interferometer FPI1, optional illuminating optics OPT1, an object OBJ1, optional imaging optics LNS1, and the sensor SEN1. The optical filter FIL1, FIL2 may be positioned at any position in the optical path between the broadband light source LS1 and the sensor SEN1. The apparatus 500 may optionally comprise one or more actuators for moving the filter FIL1, FIL2 into the optical path and / or for moving the filter FIL1, FIL2 away from the optical path. In an embodiment, the apparatus 500 may optionally comprise two Fabry-Perot interferometers FPI1. Broadband light pulses B1 obtained from the broadband light source LS1 may be distributed to a first Fabry-Perot interferometer and to a second Fabry-Perot interferometer e.g. by using a spectrally selective beam splitter. The first Fabry-Perot interferometer may be arranged to form narrowband light pulses within a first spectral range, and the second Fabry-Perot interferometer may be arranged to form narrowband light pulses within a second different spectral range. Narrowband light pulses B2 formed by the first and the second Fabry-Perot interferometer may be arranged to illuminate the same object OBJ1. In an embodiment, the apparatus 500 may optionally comprise two broadband light sources. First broadband light pulses obtained from a first broadband light source and second broadband light pulses obtained from a second broadband light source may be coupled to pass through the same Fabry-Perot interferometer FPI1. Referring to Fig. 3a, the broadband light source LS1 may be e.g. a pulsed supercontinuum light source. The pulsed supercontinuum light source may e.g. provide sufficient optical power for the whole operating bandwidth of the apparatus 500. The pulsed supercontinuum light source may also enable collimation of the broadband light pulses B1 for the Fabry-Perot interferometer FPI1. The pulsed supercontinuum light source may enable providing sufficient optical power in a situation where the broadband light pulses B1 are collimated or substantially collimated for the Fabry-Perot interferometer FPI1. The broadband light source may comprise a laser light source LAS1 to generate monochromatic primary light pulses B0, and an optical waveguide FIB1 to form broadband light pulses B1 from the primary light pulses B0. The optical waveguide FIB1 may be e.g. an optical fiber. The pulsed operation may facilitate providing high efficiency. The laser light source LAS1 may be e.g. a master oscillator power amplifier (MOPA). The master oscillator power amplifier comprises a seed laser SEED1 and an optical amplifier OPA1 to boost the output power. The timing of the laser light pulse B00 of the seed laser SEED1 can be controlled with high speed and with high accuracy. The optical amplifier OPA1 may provide the primary light pulses B0 by amplifying the seed light pulses B00. The laser light source LAS1 may form monochromatic primary light pulses B0. The optical waveguide FIB1 may form broadband light pulses B1 by spectrally broadening the spectral bandwidth of the primary light pulses B0. The laser light source LAS1 may emit monochromatic primary laser pulses B0, which have high peak power. The spectrum of the monochromatic primary pulses B0 is broadened due to non-linear effects in an optical waveguide. The maximum power of the primary pulses B0 may be e.g. greater than 5 kW, advantageously greater than10 kW. The duration ^tDUR of the generated light pulses B0, B1, B2 may be e.g.shorter than 10 ns, advantageously shorter than 3 ns. The maximum power ofthe primary pulses B0 may be e.g. greater than 10 kW, and the duration ^tDUR ofthe generated light pulses B0, B1, B2 may be e.g. shorter than 3 ns. The average repetition rate f0of the light pulses B0, B1, B2 may be e.g. greater than or equal to 1 kHz, greater than or equal to 10 kHz, greater than or equal to 50 kHz, greater than or equal to 100 kHz, or even greater than or equal to 200 kHz. The average repetition rate f0of the light pulses B0, B1, B2 may be e.g. in the range of 1 kHz to 500 kHz. Referring to Fig.3b, the broadband light source LS1 may be arranged to operate such that the spectral bandwidth of the broadband light pulses B1 covers the intended spectral operating range MSR1 of the Fabry-Perot interferometer FPI1. The broadband light pulses B1 may have a spectral intensity distribution IB1(^).Referring to Figs. 3c and 3d, the spectral width ^^FWHM of the spectraltransmittance peak PEAK1 of the Fabry-Perot interferometer FPI1 may be e.g. smaller than 20% of the spectral operating range MSR1 of the Fabry-Perot interferometer FPI1, advantageously smaller than 10%, and preferably smallerthan 5%. The symbol ^^FWHM denotes the full spectral width at half maximum.The spectral width ^^FWHM of the formed narrowband light pulse B2 may besubstantially equal to the spectral width ^^FWHM of the spectral transmittance peakPEAK1. A narrowband light pulse B2 may have a spectral intensity distribution IB2(^). Referring to Fig.4a, the broadband light source LS1 may be arranged to generate broadband light pulses B1 at an average repetition rate f0. The average repetition rate f0may also be called e.g. as the base frequency. The average time period T0between consecutive pulses of the broadband light source is equal to 1 / f0. Pulses B1t1, B1t2, B1t3, B1t4, B1t5, B1t6, B1t7, B1t8, B1t9, ... may be formed at trigger times t1, t2, t3, t4, t5, t6, t7, t8, t9, ... The apparatus 500 may comprise a clock CLK1. The trigger times t1, t2, t3, t4, t5, t6, ... may be synchronized with the modulating waveform SGAP(t) by using the clock CLK1. The broadband light source LS1 may be arranged to generate an uninterrupted stream of broadband light pulses B1 at a constant interval T0, e.g. in order to minimize variation of pulse energy. Referring to Fig.4b, A first narrowband light pulse B2t1may be formed at a firsttrigger time t1, and may have a first wavelength ^1. A second narrowband lightpulse B2t2may be formed at a second trigger time t2, and may have a secondwavelength ^2. Pulses B2t1, B2t2, B2t3, B2t4, B2t5, B2t6, B2t7, B2t8, B2t9, ... may beformed at trigger times t1, t2, t3, t4, t5, t6, t7, t8, t9, ... The wavelength of each pulse may be determined according to the trigger time and according to the modulating waveform. The symbols B2^1, B2^2, B2^3, B2^4, B2^5, B2^6, B2^7, B2^8, ... denotenarrowband light pulses at wavelengths ^1, ^2, ^3, ^4, ^5, ^6, ^7, ^8, ...For the present purpose, each broadband light pulse B1 and the corresponding narrowband light pulse B2 may be considered to be formed at the same time, e.g. at the trigger time t1.The start time tA and the duration of the exposure time ^tEX may be controlled soas to define a desired group GRP1 of narrowband light pulses B2. The durationof the exposure time ^tEX may be controlled so as to define a group GRP1, whichconsists of a desired number of light pulses. The start time tAand the duration may be selected e.g. such that the group GRP1 consists of narrowband light pulses B2, which together represent a spectral range from a lower wavelength to a higher wavelength. The group GRP1 may e.g. consist of pulses B2^3, B2^4, B2^5, B2^6.The exposure time ^tEX of the sensor SEN1 has a start time tA and a stop time tE.The duration of the exposure time ^tEX is equal to the difference (tE-tA) betweenthe stop time tE and the start time tA. The symbol ^tEX may herein refer to theexposure time ^tEX, and also to the duration of the exposure time ^tEX. The sensorSEN1 may form a sensor signal SSEN1 during the exposure time ^tEX such thatthe sensor signal SSEN1is substantially proportional to the energy of light, whichimpinges on the sensor SEN1 during the exposure time ^tEX. The sensor SEN1may form the sensor signal SSEN1such that light impinging on the sensor SEN1 before the start time tAdoes not contribute to the sensor signal SSEN1, which is formed between the start time tAand the stop time tE. The sensor SEN1 may form the sensor signal SSEN1such that light impinging on the sensor SEN1 after the stop time tEdoes not contribute to the sensor signal SSEN1, which is formed between the start time tAand the stop time tE.Referring to Fig. 4c, the effective spectral width ^^FWHM,EFF of the group GRP1 ofnarrowband light pulses B2 may be substantially greater than the spectral width of an individual narrowband light pulse B2.The broadening of the effective spectral width ^^FWHM,EFF of a single narrowbandlight pulse B2 may be proportional to the duration ^tDUR multiplied by the spectralscanning speed (^^ / ^t) of the Fabry-Perot interferometer FPI1. The ratio(^tDUR / T0) of the duration ^tDUR of the pulse B1 (and B2) to the average timeperiod T0may be e.g. smaller than 2%, smaller than 1%, smaller than 0.5%, or smaller than 0.2%, or even smaller than 0.1%, so as to limit spectral broadening of an individual narrowband light pulse B2. Consequently, the effective spectral width of the group GRP1 of the narrowband light pulses B2 may be determinedaccurately by defining the start time and the duration of the exposure time ^tEX.The symbol IMAXdenotes a maximum value of intensity.The exposure time ^tEX may be controlled e.g. such that the effective spectralwidth ^^FWHM,EFF of the group GRP1 of narrowband light pulses B2 is substantiallyequal to a predetermined spectral width ^^PRE.The group GRP1 may comprise e.g. at least to 2 narrowband light pulses B2. The group GRP1 may comprise e.g. more than 2 narrowband light pulses B2. The group GRP1 may comprise e.g. more than 3 narrowband light pulses B2. The group GRP1 may comprise e.g. more than 5 narrowband light pulses B2. The group GRP1 may comprise e.g. more than 10 narrowband light pulses B2. Referring to Fig.5a, the uppermost curve of Fig.5a shows, by way of example, temporal evolution of the control signal SGAP(t). The second curve from the top of Fig.5a shows, by way of example, temporal evolution of the mirror gap dGAP(t). The third curve from the top of Fig. 5a shows, by way of example, temporalevolution of the peak wavelength ^(t) of the spectral transmittance function T(^)of the Fabry-Perot interferometer FPI1. The lowermost part of Fig.5a shows, by way of example, the timing, the intensity, and the wavelength of the narrowband light pulses B2. The mirror gap dGAP(t) of the Fabry-Perot interferometer FPI1 may be modulated e.g. according to a triangular waveform. The actuators ACU1 of the Fabry-Perot interferometer FPI1 may be driven with a substantially triangular waveform SGAP(t) and / or the actuators ACU1 may be driven such that the mirror gap dGAP(t) is varied as a substantially triangular function of time. The control signal SGAP(t) may be varied between a minimum value SGAP,MINand a maximum value SGAP,MAX. The mirror gap dGAP(t) may be varied between a minimum mirror gap value dGAP,MINand a maximum mirror gap value dGAP,MAX. Broadband light pulses B1 and corresponding narrowband light pulses B2 may be formed at trigger times t-1, t0, t1, t2, t3, t4, t5, t6, t7, t8, t9, t10, t11, ... The trigger times t1, t2, t3, t4, t5, t6, t7, t8may be associated with control signal values SGAP,1, SGAP,2, SGAP,3, SGAP,4, SGAP,5, SGAP,6, SGAP,7, SGAP,8. The trigger times t1, t2, t3, t4, t5, t6, t7, t8may be associated with mirror gap values dGAP,1, dGAP,2, dGAP,3, dGAP,4, dGAP,5, dGAP,6, dGAP,7, dGAP,8. The wavelength of a narrowband light pulse B2 formed at a trigger time is determined by the value of the mirror gap at said trigger time.The wavelength ^(t) of the transmittance peak of the Fabry-Perot interferometerFPI1 may vary between a minimum wavelength ^MIN and a maximum wavelength^MAXaccording to the modulating waveform. The narrowband light pulses B2 formed at the trigger times t1, t2, t3, t4, t5, t6, t7, t8may have wavelengths ^1, ^2, ^3, ^4, ^5, ^6, ^7, ^8.The start time tA and the stop time tE of an exposure time ^tEX of the sensor SEN1may be selected to define a group GRP1 of narrowband light pulses B2, such that the group GRP1 consists of narrowband light pulses B2, which have desiredwavelengths (e.g. ^3, ^4, ^5, ^6).The broadband light source LS1 may form e.g. an uninterrupted stream of broadband light pulses B1 at equal intervals T0. Forming the uninterrupted stream of broadband light pulses B1 at equal intervals T0may ensure that the broadband light pulses B1 have substantially equal energy. The Fabry-Perot interferometer FPI1 may also form an uninterrupted stream of narrowband light pulses B2 at the equal intervals T0, by filtering the broadband light pulses B1.The exposure time ^tEX may be controlled so as to define a desired group GRP1from the stream of narrowband light pulses B2. The modulating waveform dGAP(t) may have substantially linear sections. The modulating waveform dGAP(t) may be a substantially triangular waveform, which comprises or consists of linear sections. The linear section may correspond to a substantially equal spectral separation between consecutive narrowband light pulses B2. The triangular waveform may cause high acceleration at the endpoints dGAP,MINdGAP,MAX. The high acceleration may limit the scanning speed and / or may reduce accuracy. Using a sinusoidal waveform may maximize the scanning speed and / or may improve accuracy near the endpoints dGAP,MINdGAP,MAX. The apparatus 500 may be arranged to operate such that acceleration of the mirror M2 of the Fabry-Perot interferometer FPI1 is reduced, minimized or evencompletely avoided during the exposure time ^tEX. The apparatus 500 may bearranged to operate such that acceleration of the mirror M2 of the Fabry-Perot interferometer FPI1 is reduced, minimized or even completely avoided during ascanning time interval ^tSCAN, which includes the exposure time ^tEX.The illuminating unit 110 may be arranged to generate a first broadband light pulse B1 at a first trigger time (t1), and to generate a second broadband light pulse B1 a second trigger time (t2). The Fabry-Perot interferometer FPI1 may form a first narrowband light pulse B2 by filtering the first broadband light pulse B1. The Fabry-Perot interferometer FPI1 may form a second narrowband light pulse B2 by filtering the second broadband light pulse B1. The illuminating unit 110 may be arranged to operate e.g. such that a movement of the mirror M2 of the Fabry-Perot interferometer FPI1 is not stopped between a first trigger time (t1) and a second trigger time (t2). The illuminating unit 110 may be arranged to operate e.g. such that the mirror M2 moves continuously at the first trigger time (t1) and at the second trigger time (t2). The illuminating unit 110 may be arranged to operate e.g. such that themovement of the mirror M2 is not stopped during the first exposure time ^tEX1.The illuminating unit 110 may be arranged to operate e.g. such that the mirror M2moves continuously during the whole first exposure time ^tEX1.The start time tA1 of the first exposure time ^tEX1, and the duration of the firstexposure time ^tEX1 may be controlled such that the illuminating unit 110 forms afirst broadband light pulse B1t1and a second broadband light pulse B1t2duringthe first exposure time ^tEX1.The illuminating unit 110 may be arranged to operate such that the mirror M2 moves continuously at the first trigger time t1and at the second trigger time t2. The illuminating unit 110 may be arranged to operate such that the movement ofthe mirror M2 is not stopped during the first exposure time ^tEX1. The mirror (M2)may be arranged to move continuously during the whole first exposure time ^tEX1.The continuous movement of the mirror M2 may improve accuracy of the wavelength range of the group GRP1. The maximum duration of a continuous movement of the mirror M2 may depend on the modulating waveform. The maximum duration of the continuousmovement may be called e.g. as the scanning time interval ^tSCAN (see e.g. Fig.6b). In case of a sinusoidal modulating waveform dGAP(t), the scanning timeinterval ^tSCAN may be shorter than 50% of the modulation time period TMOD. Incase of a symmetric triangular modulating function, the scanning time interval ^tSCANmay be shorter than 50% of the modulation time period TMOD. In case of an asymmetric triangular modulating waveform (sawtooth waveform), thescanning time interval ^tSCAN may be shorter than 100% of the modulation timeperiod TMOD. The periodic modulating waveform dGAP(t) may be formed as a linear combination of periodic functions. A periodic modulating waveform may also be formed e.g. as an exponential function of a periodic function. A periodic modulating waveform may also be formed e.g. as a polynomial function of a periodic function.For most practical modulating waveforms, the scanning time interval ^tSCAN maybe e.g. greater than 25% of the modulation time period TMOD. The illuminating unit 110 may be arranged to operate such that: - the mirror gap dGAPof the Fabry-Perot interferometer (FPI1) is modulated according to a periodic modulating waveform dGAP(t), - the modulating waveform dGAP(t) has a modulation time period TMOD,- the mirror M2 moves continuously during a scanning time interval ^tSCAN,- the movement of the mirror M2 is not stopped during the scanning time interval ^tSCAN,- the scanning time interval ^tSCAN includes the first exposure time ^tEX1, and- the scanning time interval ^tSCAN is longer than 25% of the modulation timeperiod TMOD. The mirror M2 may have a minimum velocity vM2,MINand a maximum velocityvM2,MAX during the scanning time interval ^tSCAN. The minimum velocity vM2,MIN maybe equal to the maximum velocity vM2,MAXmultiplied by a predetermined coefficient kMIN, i.e. vM2,MIN= kMIN^vM2,MAX. As a further constraint, the scanningtime interval ^tSCAN may selected such that that the velocity of the mirror M2 isgreater than or equal to the minimum velocity value vM2,MINduring the scanningtime interval ^tSCAN. In case of a sinusoidal modulating waveform, the scanningtime interval ^tSCAN may be e.g. shorter than 40% of the modulation time periodTMOD, in order to ensure that the minimum velocity vM2,MINof the mirror M2 is greater than or equal to e.g.30% of the maximum velocity vM2,MAXof the mirrorM2. The scanning time interval ^tSCAN may be e.g. in the range of 25% to 40% ofthe modulation time period TMOD. Referring to Fig.5b, the mirror gap dGAP(t) of the Fabry-Perot interferometer FPI1 may be modulated e.g. according to a sinusoidal waveform. The uppermost curve shows, by way of example, temporal evolution of the mirror gap dGAP(t). The second curve from the top shows, by way of example, temporal evolution of thepeak wavelength ^(t) of the spectral transmittance function T(^) of the Fabry-Perot interferometer FPI1. The lowermost part of Fig. 5b shows, by way of example, the timing, the intensity, and the wavelength of the narrowband light pulses B2. The mirror gap dGAP(t) of the Fabry-Perot interferometer FPI1 may be modulated according to a sinusoidal waveform. The actuators ACU1 of the Fabry-Perot interferometer FPI1 may be driven with a substantially sinusoidal waveform SGAP(t) and / or the actuators ACU1 may be driven such that the mirror gap dGAP(t) is varied as a substantially sinusoidal function of time. The mirror gap dGAP(t) may be varied between a minimum mirror gap value dGAP,MINand a maximum mirror gap value dGAP,MAX. Broadband light pulses B1 and corresponding narrowband light pulses B2 may be formed at trigger times t-3, t-2,t-1, t0, t1, t2, t3, t4, t5, t6, t7, t8, t9, t10, t11, ... The trigger times t1, t2, t3, t4, t5, t6, t7, t8may be associated with mirror gap values dGAP,0, dGAP,1, dGAP,2, dGAP,3, dGAP,4, dGAP,5, dGAP,6, dGAP,7, dGAP,8. The wavelength of a narrowband light pulse B2 formed at a trigger time is determined by the value of the mirror gap at said trigger time. The narrowband light pulses B2 formed at the trigger times t0,t1, t2, t3, t4, t5, t6, t7,t8 may have wavelengths ^0, ^1, ^2, ^3, ^4, ^5, ^6, ^7, ^8.The start time tA and the stop time tE of an exposure time ^tEX of the sensor SEN1may be selected to define a group GRP1 of narrowband light pulses B2, such that the group GRP1 consists of narrowband light pulses B2, which have desiredwavelengths (e.g. ^3, ^4, ^5).The broadband light source LS1 may form e.g. an uninterrupted stream of broadband light pulses B1 at equal intervals T0. The Fabry-Perot interferometer FPI1 may also form an uninterrupted stream of narrowband light pulses B2 at the equal intervals T0, by filtering the broadband light pulses B1. The exposure time ^tEXmay be controlled so as to define a desired group GRP1 from the stream of narrowband light pulses B2. Referring to Fig. 6a, the apparatus 500 may be arranged to capture imagesIMG11, IMG12, IMG13 during exposure times ^tEX1, ^tEX2, ^tEX3. The uppermostpart of Fig. 6a shows, by way of example, temporal evolution of the peak wavelength of the transmittance function T(^) of the Fabry-Perot interferometer FPI1. TMODdenotes the modulation period. The central part of Fig.6a shows, by way of example, timing of the light pulses B1, B2. The lowermost part of Fig.6ashows, by way of example, the exposure times ^tEX1, ^tEX2, ^tEX3. Each exposuretime ^tEX1, ^tEX2, ^tEX3 may define a corresponding group GRP1, GRP2, GRP3of narrowband light pulses B2.A first image IMG11 may be captured during a first exposure time ^tEX1. The firstexposure time ^tEX1 may have a start time tA1 and a stop time tE1. The firstexposure time ^tEX1 may define a first group GRP1 of narrowband light pulsesB2.A second image IMG12 may be captured during a second exposure time ^tEX2.The second exposure time ^tEX2 may have a start time tA2 and a stop time tE2.The second exposure time ^tEX2 may define a second group GRP2 of narrowbandlight pulses B2.A third image IMG13 may be captured during a third exposure time ^tEX3. Thethird exposure time ^tEX3 may have a start time tA3 and a stop time tE3. The thirdexposure time ^tEX3 may define a third group GRP3 of narrowband light pulsesB2. The mirror gap may be modulated according to a periodic modulating waveform dGAP(t). The periodic modulating waveform may have reference points, e.g. the points where the mirror gap reaches the minimum value. The reference points of the periodic modulating waveform may be associated with reference times tF1, tF2, tF3, tF4. The start time tAof each exposure time may be specified e.g. by adelay ^tTRIG1 between the start time tA and the nearest preceding reference time.^A1may denote the shortest wavelength of the pulses of the first group GRP1. ^E1may denote the longest wavelength of the pulses of the first group GRP1. The first group GRP1 may consist of narrowband light pulses B2, which represent thewavelength range from ^A1 to ^E1. Capturing of the images IMG11, IMG12, IMG13may be synchronized with the modulating waveform. All groups GRP1, GRP2, GRP3 may also represent thesame wavelength range from ^A1 to ^E1.Referring to Fig.6b, the apparatus 500 may be arranged to capture two or more images IMG11, IMG12, IMG13during a single period (TMOD) of the modulating waveform dGAP(t). For example, the apparatus 500 may be arranged to capture a first image IMG11, and a second image IMG12on a positive slope of the modulating waveform dGAP(t). For example, the apparatus 500 may be arranged to capture a third image IMG13on a negative slope of the modulating waveform dGAP(t).A first image IMG11 may be captured during a first exposure time ^tEX1. The firstexposure time ^tEX1 may have a start time tA1 and a stop time tE1. The firstexposure time ^tEX1 may define a first group GRP1 of narrowband light pulsesB2.A second image IMG12 may be captured during a second exposure time ^tEX2.The second exposure time ^tEX2 may have a start time tA2 and a stop time tE2.The second exposure time ^tEX2 may define a second group GRP2 of narrowbandlight pulses B2.A third image IMG13 may be captured during a third exposure time ^tEX3. Thethird exposure time ^tEX3 may have a start time tA3 and a stop time tE3. The thirdexposure time ^tEX3 may define a third group GRP3 of narrowband light pulsesB2.The start time tA1 of the first exposure time ^tEX1 may be specified e.g. by a delay^tTRIG1between the start time tA1and a reference time tF1. The start time tA2of thesecond exposure time ^tEX2 may be specified e.g. by a delay ^tTRIG2 between thestart time tA2and the reference time tF1.The start time tA3of the third exposuretime ^tEX3 may be specified e.g. by a delay ^tTRIG3 between the start time tA3 andthe reference time tF1.All narrowband light pulses B2 formed during the first exposure time ^tEX1constitute a first group GRP1, wherein the start time tA1and the duration of thefirst exposure time ^tEX1 may be controlled such that the effective spectral width(^^FWHM,EFF) of the first group GRP1 is equal to a first predetermined spectral width (^^PRE).All narrowband light pulses B2 formed during the second exposure time ^tEX2constitute a second group GRP2, wherein the start time tA2and the duration ofthe second exposure time ^tEX2 may be controlled such that the effective spectralwidth (^^FWHM,EFF) of the second group GRP2 is equal to a second predetermined spectral width (^^PRE).The duration of the first exposure time ^tEX1 may e.g. in the range of 1% to 20%of the modulation time period TMODof the periodic modulating waveform dGAP(t).The exposure times ^tEX1, ^tEX2, ^tEX3 may have equal length, or different lengths.In an embodiment, one or more exposure times (e.g. ^tEX2) may also define onlyone narrowband light pulse B2. For example, a first group GRP1 defined by a firstexposure time ^tEX1 may comprise two or more pulses, and a second group GRP2defined by a second exposure time ^tEX2 may consist of only one pulse.Fig. 7 shows, by way of example, a spectrum XOBJ(^) of an object OBJ1. The spectrum XOBJ(^) may be e.g. a reflectance spectrum IB3(^) / IB2(^). One or more spectral ranges RNG1, RNG2, RNG3 of the spectrum XOBJ(^) may be measured by capturing one or more images IMG11, IMG12, IMG13.The first image IMG11 may be captured by using the first exposure time ^tEX1, soas to represent a first spectral range RNG1. The image IMG11may be an image of the object OBJ1, and a single pixel of the image IMG11may represent e.g. a single point of the object OBJ1. The second image IMG12may be captured by using the second exposure time ^tEX2, so as to represent a second spectral range RNG2.The third image IMG13 may be captured by using the third exposure time ^tEX3,so as to represent a third spectral range RNG3.The exposure times ^tEX1, ^tEX2, ^tEX3 may have equal length or different lengths.The exposure times ^tEX1, ^tEX2, ^tEX3 may have different lengths e.g. in order tooptimize the speed of the measurement. A longer exposure time ^tEX1 may beused e.g. to measure a spectral range RNG1 where the sensor signal is weakerand / or where spectral features could be averaged. A shorter exposure time ^tEX3,^tEX3may be used e.g. to measure a different spectral range RNG2, RNG3 where the sensor signal is stronger.The start time tA1 and the duration of the first exposure time ^tEX1 may becontrolled such that the first group GRP1 of narrowband light pulses B2 represents a first predetermined spectral range RNG1 from a first predeterminedlower wavelength limit ^A1 to a first predetermined higher wavelength limit ^E1.The start time tA1 and the duration of the first exposure time ^tEX1 may becontrolled such that the effective spectral width (^^FWHM,EFF) of the first group GRP1 is equal to a first predetermined spectral width (^^PRE).The start time tA2 and the duration of the second exposure time ^tEX2 may becontrolled such that the second group GRP2 of narrowband light pulses B2 represents a second predetermined spectral range RNG2 from a secondpredetermined lower wavelength limit ^A2 to a second predetermined higherwavelength limit ^E2.The start time tA2 and the duration of the second exposure time ^tEX2 may becontrolled such that the effective spectral width (^^FWHM,EFF) of the second group GRP2 is equal to a second predetermined spectral width (^^PRE).The start time tA3 and the duration of the third exposure time ^tEX3 may becontrolled such that the third group GRP3 of narrowband light pulses B2 represents a third predetermined spectral range RNG3 from a thirdpredetermined lower wavelength limit ^A3 to a third predetermined higherwavelength limit ^E3.The start time tA3 and the duration of the third exposure time ^tEX3 may becontrolled such that the effective spectral width (^^FWHM,EFF) of the third group GRP3 is equal to a third predetermined spectral width (^^PRE). The start times tA1, tA2, tA3may be controlled with respect to the reference time tF1of the modulating waveform dGAP(t). Figs. 8a, 8b, and 8c show arrangements for spectral measurements. Fig. 8a shows an arrangement for measuring spectral properties of a single point or area of an object OBJ1. Fig.8b shows an arrangement for capturing two-dimensional spectral images of the object. Fig.8c shows an arrangement for capturing linear spectral images of the object. The arrangement of Fig. 8a, 8b, and / or 8c may comprise e.g. the apparatus 500 described with reference to Fig.2. Referring to Fig.8a, the apparatus 500 may be a non-imaging spectrometer. The sensor signal SSEN1formed by the sensor SEN1 may represent a single point and / or a single area of an object OBJ1. The sensor signal SSEN1formed by the sensor SEN1 may represent only a single point of the object OBJ1. The sensor signal SSEN1formed by the sensor SEN1 may represent only a single area of the object OBJ1. The apparatus 500 may be arranged to measure one or more spectral properties of the single point of the object OBJ1. The apparatus 500 may be arranged to measure one or more spectral properties of the single area of the object OBJ1. The illuminating unit 110 may illuminate the object OBJ1 with narrowband light pulses B2. The sensor SEN1 may be arranged to detect light B3, which is received from the object OBJ1 in a situation where the object OBJ1 is illuminatedwith the narrowband light pulses B2 during an exposure time ^tEX.The sensor SEN1 may be arranged to form signals SSEN1during exposure times^tEX1, ^tEX2, ^tEX3. A first measured signal value SSEN1 may represent the light B3detected by the sensor SEN1 during a first exposure time ^tEX1. A secondmeasured signal value SSEN1may represent the light B3 detected by the sensorSEN1 during a second exposure time ^tEX2. A third measured signal value SSEN1may represent the light B3 detected by the sensor SEN1 during a third exposuretime ^tEX3. The sensor SEN1 may be used as a non-imaging detector. The sensor SEN1 may receive light B3 e.g. directly from the object OBJ1. The apparatus 500 may comprise e.g. an optical fiber to guide received light B3 from the object OBJ1 to the sensor SEN1. The apparatus 500 may comprise the imaging unit CAM1 also when measuring spectral properties of a single point or area of the object OBJ1. The imaging unit CAM1 may comprise the sensor SEN1, and focusing optics LNS1 to focus light B3 from a selected object point to the image sensor SEN1. The sensor SEN1 may comprise only one detector pixel e.g. to provide a fast response, to provide low noise and / or in order to reduce costs. Alternatively, the sensor SEN1 may comprise several detector pixels, wherein the sensor signal may be formed from image data obtained from only one detector pixel. Alternatively, image data from several detector pixels may be combined to form only one sensor signal, which represents the selected area of the object. Referring to Fig.8b, the apparatus 500 may be a spectral imaging device. The apparatus 500 may be an imaging spectrometer. The illuminating unit 110 may illuminate an object OBJ1 with narrowband light pulses B2. The illuminating optics OPT1 (Fig.2) may optionally focus light of the narrowband light pulses B2 to the object OBJ1. An imaging unit CAM1 may capture a spectral image IMG1 of the object OBJ1 in a situation where the object OBJ1 is illuminated with the narrowband light pulsesB2 during an exposure time ^tEX. The apparatus 500 may be arranged to captureimages IMG11, IMG12, IMG13 during exposure times ^tEX1, ^tEX2, ^tEX3.The imaging unit CAM1 may comprise focusing optics LNS1 to focus received light B3 to the image sensor SEN1. The detector pixels of the image sensor may be arranged e.g. in a two-dimensional array. The captured image IMG11may comprise a two-dimensional array of image pixels. Image pixel values from several captured images IMG11, IMG12, IMG13may be arranged optionally e.g. as a hyperspectral image data cube. One or more spectral properties may be calculated from the pixel values of the captured image IMG11. One or more spectral properties may be calculated from the pixel values of two or more captured images IMG11, IMG12, IMG13. Referring to Fig.8c, the apparatus 500 may comprise a line scan camera CAM1, which comprises a one-dimensional image sensor SEN1. The image sensor SEN1 of the line scan camera CAM1 may comprise e.g. only one active row of detector pixels. The image sensor SEN1 may capture a one-dimensional spectral image IMG1, which comprises a 1xM array of image pixels. A region REG1 of an object OBJ1 may be located in the field-of-view FOV1 of the camera CAM1. The object OBJ1 may be illuminated with the narrowband light pulses B2. The camera CAM1 may be capture an image IMG1 of the illuminated region REG1 during anexposure time ^tEX. The captured image IMG1 may be one-dimensional. Eachcaptured image IMG1 may e.g. consist of 1xM image pixels, where the number M may be e.g. in the range of 100 to 20000. The apparatus 500 may be arrangedto capture images IMG11, IMG12, IMG13 during exposure times ^tEX1, ^tEX2, ^tEX3.Referring to Fig.9, the Fabry-Perot interferometer FPI1 may optionally comprise one or more sensors CAP1 for measuring the mirror gap dGAP. The sensor CAP1 may be e.g. a capacitive sensor. The sensor may also be e.g. an optical sensor (see Fig.10a). The capacitive sensor CAP1 may comprise e.g. electrodes E1, E2, E3. Electrodes E1, E2 may form a first sensor capacitor. Electrodes E2,E3 may form a second sensor capacitor. The electrodes E1, E3 may be stationary. The electrode E2 may be attached to the moving mirror plate PLA2. One or more sensor capacitors may also be connected in series. The capacitance of the capacitive sensor CAP1 may depend on the mirror gap dGAP. The apparatus 500 may optionally comprise a distance measuring unit DMU1, which may be arranged to measure the mirror gap dGAPby measuring the capacitance of the sensor CAP1. The distance measuring unit DMU1 may measure the capacitance e.g. by coupling a voltage signal VM1 to the sensor CAP1 and by monitoring a corresponding current IM1 of the sensor CAP1 and / or the distance measuring unit DMU1 may measure the capacitance e.g. by coupling a current signal IM1 to the sensor CAP1 and by monitoring a corresponding voltage VM1 of the sensor CAP1. The distance measuring unit DMU1 may be connected to the electrodes E1, E3 via conductors CON1, CON2. The distance measuring unit DMU1 may form a signal SD, which is indicative of the measured mirror gap dGAP. The control unit CNT1 may use the signal SDas feedback. Referring to Fig.10a, the apparatus 500 may comprise calibration light sources LS11, LS21 and calibration detectors DET11, DET21 for calibrating the spectral scale of the Fabry-Perot interferometer FPI1. For example, the control unit CNT1 may be arranged to adjust the minimum and maximum values of the modulating waveform based on signals SDET11, SDET21obtained from the calibration detectors DET11, DET21 A first control signal value SCAL1may be associated with a first spectral position ^CAL1by using a first calibration detector DET11. A first calibration light source LS11 may be e.g. a laser, which emits narrowband light B11 at a first calibrationwavelength ^CAL1. The calibration detector DET11 may detect light transmittedthrough the Fabry-Perot interferometer FPI1 only when the wavelength ^ of thetransmittance peak PEAK1 matches the first calibration wavelength ^CAL1. Asecond control signal value SCAL2may be associated with a second spectralposition ^CAL2 by using a second calibration detector DET21. A second calibrationlight source LS21 may be e.g. a laser, which emits narrowband light B21 at asecond calibration wavelength ^CAL2. The calibration detector DET21 may detectlight transmitted through the Fabry-Perot interferometer FPI1 only when thewavelength ^ of the transmittance peak PEAK1 matches the second calibrationwavelength ^CAL2.The apparatus 500 may optionally comprise one or more optical filters FIL11, FIL21 to define the bandwidth of the calibration light B11, B21. The apparatus 500 may comprise a first spectrally selective combination CMB1 of a calibration light source LS11, and a calibration detector DET11. The calibration light source LS11 may be arranged to provide first calibration light B11. The calibration detector DET11 may be arranged to detect first calibration light B11 that has passed through the Fabry-Perot interferometer FPI1. The first spectrally selective combination CMB1 may be arranged to form a calibration detector signal SDET11. The first spectrally selective combination CMB1 may be arranged to change a state of the calibration detector signal SDET11when thewavelength ^ of the spectral transmittance peak PEAK1 of the Fabry-Perotinterferometer FPI1 becomes higher or lower than the first predeterminedcalibration wavelength ^CAL1. The calibration detector signal SDET11may change state e.g. from a low (lower) value to a high (higher) value, or from a high value to a low value. For example, the combination CMB1 may provide a high calibration detector signal value SDET11when the wavelength ^ of the spectral transmittance peak PEAK1 is equal to thefirst calibration wavelength ^CAL1, wherein the combination CMB1 may provide alow calibration detector signal value SDET11 when the wavelength ^ of the spectraltransmittance peak PEAK1 is higher or lower than the first calibration wavelength ^CAL1. The light source LS11 may be e.g. a laser, which emits light at the first calibrationwavelength ^CAL1. The optical filter FIL11 is optional when the light source LS11is a laser. The combination CMB1 may optionally comprise the filter FIL11. The light source LS11 may also be a broadband light source, e.g. a light emitting diode, wherein the spectral selectivity may be provided by using the optical filter FIL11. The filter FIL11 may be positioned between the light source LS11 and the Fabry-Perot interferometer FPI1, or the filter FIL11 may be positioned between the Fabry-Perot interferometer FPI1 and the detector DET11. The filter FIL11 may have a narrow passband, to provide a calibration detector signal pulse SDET11when the spectral transmittance peak PEAK1 is momentarily at the firstcalibration wavelength ^CAL1. The filter FIL11 may also be a long pass filter or ashort pass filter. The calibration detector signal SDET11may change state from a low value to a high value, or from a high value to a low value, when the wavelength of the spectral transmittance peak PEAK1 becomes higher or lowerthan the first calibration wavelength ^CAL1. The spectrally selective combinationCMB1 may be arranged to operate such that the calibration signal SDET11changesstate when the wavelength ^ of the spectral transmittance peak PEAK1 becomeshigher or lower than the first calibration wavelength ^CAL1.The apparatus 500 may comprise a second spectrally selective combination CMB2 of a calibration light source LS21, and a calibration detector DET21. The calibration light source LS21 may be arranged to provide second calibration light B21. The calibration detector DET21 may be arranged to detect second calibration light B21 that has passed through the Fabry-Perot interferometer FPI1. The second spectrally selective combination CMB2 may be arranged to form a calibration detector signal SDET21. The second spectrally selective combination CMB2 may be arranged to change a state of the calibration detectorsignal SDET21 when the wavelength ^ of the spectral transmittance peak PEAK1of the Fabry-Perot interferometer FPI1 becomes higher or lower than the secondpredetermined calibration wavelength ^CAL2.For example, the combination CMB2 may provide a high calibration detectorsignal value SDET21 when the wavelength ^ of the spectral transmittance peakPEAK1 is equal to the second calibration wavelength ^CAL2, wherein thecombination CMB2 may provide a low calibration detector signal value SDET21when the wavelength ^ of the spectral transmittance peak PEAK1 is higher orlower than the second calibration wavelength ^CAL2.In an embodiment, the first combination CMB1 and the second combination CMB2 may also share a common light source (e.g. LS11) or a common detector (e.g. DET11). In an embodiment, also the broadband light source LS1 may be used as the calibration light source LS11. For example, light of the broadband light pulses B1 may be used as the calibration light B11 together with the optical filter FIL11 and with the detector DET11. For example, light of the broadband light pulses B1 may be used as the calibration light B21 together with the optical filter FIL21 and with the detector DET21. Referring to Fig. 10b, the first calibration detector DET11 may provide a pulsewhen the wavelength ^ of the transmittance peak PEAK1 matches the firstcalibration wavelength ^CAL1 (e.g. at times t11A, t11B). The control unit CNT1 maybe arranged to adjust the minimum value SGAP,MINof the modulating signal SGAPe.g. such that the time interval ^tCD between consecutive pulses of the calibrationdetector signal SDET11 is equal to a predetermined value ^tREF.The second calibration detector DET21 may provide a pulse when thewavelength ^ of the transmittance peak PEAK1 matches the second calibrationwavelength ^CAL2 (e.g. at times t21C, t21D). The control unit CNT1 may be arrangedto adjust the maximum value SGAP,MAXof the modulating signal SGAPe.g. such thatthe time interval ^tCD between consecutive pulses is equal to a predeterminedvalue ^tREF. The first calibration detector DET11 may provide pulses e.g. at times t11C, t11D, t12C, t12D, t13C, t13D, t14C, t14D. The second calibration detector DET21 may provide pulses e.g. at times t21C, t21D, t22C, t22D, t23C, t23D, t24C, t24D. The apparatus 500 may be arranged to associate a first (auxiliary) value SCAL1ofthe control signal SGAP with the first calibration wavelength ^CAL1 by using thecalibration signal SDET11, which is obtained from the first spectrally selective combination CMB1 of a light source LS11 and a calibration detector DET11. The apparatus 500 may be arranged to associate a second (auxiliary) value SCAL2of the control signal SGAP with the second calibration wavelength ^CAL2 by usingthe calibration signal SDET21, which is obtained from the second spectrally selective combination CMB2 of a light source LS21 and a calibration detector DET21. Referring to Figs. 11a and 11b, the maximum mirror gap of the Fabry-Perot interferometer FPI1 may also be so large that the spectral transmittance function of the Fabry-Perot interferometer FPI1 comprises simultaneously two or more spectral transmittance peaks PEAK1, PEAK2. For example, a first peak PEAK1may be at the wavelength ^1, and a second peak may be at the wavelength ^1+FSR,Adjacent peaks are separated by the free spectral range ^^FSR. The spectraltransmittance peaks PEAK1, PEAK2 correspond to different orders of interference. The illuminating unit 110 may optionally comprise an actuator ACU3 and two or more optical filters FIL1A, FIL1B for selecting only one order of interference of the Fabry-Perot interferometer FPI1. The filters FIL1A, FIL1B may be moved by the actuator ACU3, so as to select only one of the spectral transmittance peaks PEAK1, PEAK2. The actuator ACU3 may be arranged to place a first optical filter FIL1A or a second optical filter FIL1B to the optical path of the light pulses B1, B2. The first filter FIL1A may be e.g. an optical low pass filter, and the second filter FIL1B may be e.g. an optical high pass filter. When using a single spectral transmittance peak PEAK1, the formed narrowband lightpulse B2 has only one single wavelength (e.g. ^1), and the captured image IMG1represents said single wavelength. The Fabry-Perot interferometer may be arranged to operate in a gas GAS1, which is at the normal atmospheric pressure (approximately 101.3 kPa), or at a reduced pressure. Referring to Figs.12a and 12b, the Fabry-Perot interferometer may be arranged to operate in a vacuum VAC1 so as to facilitate movements of the mirror M2. The presence of ambient air at the normal pressure of 101.3 kPa may disturb or slow down the movement of the mirror M2. The vacuum VAC1 means herein a low- pressure gas GAS1, where the absolute pressure is e.g. lower than 10 kPa (i.e. less than 0.1 bar). The Fabry-Perot interferometer FPI1 may be arranged to operate at the reduced pressure (VAC1), so as to reduce of avoid an effect of ambient gas on the movement of the mirror M2. Operation at the reduced pressure may e.g. allow using a high modulation frequency of the Fabry-Perot interferometer FPI1. The Fabry-Perot interferometer FPI1 may be arranged to operate in a vacuum VAC1. The absolute pressure in the vacuum VAC1 may be e.g. lower than 10 kPa, or even lower than 1 kPa. The Fabry-Perot interferometer may be positioned in a vacuum chamber CHM1. The vacuum chamber CHM1 may optionally have optical feedthroughs WIN1, WIN2 for transmitting light pulses B1, B2 to the Fabry-Perot interferometer and / or from the Fabry-Perot interferometer. The vacuum chamber CHM1 may optionally have electrical feedthroughs FEED1 for coupling a control voltage SGAPto the actuators ACU1 of the Fabry-Perot interferometer. The control signal SGAPmay be applied to the one or more actuators ACU1 e.g. via conductors CON1, CON2. The vacuum chamber CHM1 may be optionally connected to a vacuum pump PUMP1, optionally by using a duct DUCT1, so as to provide the vacuum VAC1 during operation of the Fabry-Perot interferometer FPI1. The vacuum chamber CHM1 may contain gas GAS1. The gas GAS1 may be pumped away from the vacuum chamber CHM1 by using the pump PUMP1. The internal pressure of the vacuum chamber CHM1 may be reduced by using the vacuum pump, e.g. so that the absolute pressure is lower than 10 kPa, advantageously lower than 1 kPa during operation of the apparatus 500. Referring to Fig.12b, the vacuum chamber CHM1 may also have a permanent vacuum VAC1. The Fabry-Perot interferometer (FPI1) may be positioned in a hermetically sealed vacuum chamber (CHM1), wherein the absolute pressure inside the vacuum chamber (CHM1) is smaller than 10 kPa, or even smaller than 1 kPa. In an embodiment, the Fabry-Perot interferometer FPI1 may also be arranged to operate in a gas GAS1, which has low molar mass, so as to facilitate operation at a high modulation frequency fMOD. The gas GAS1 may be e.g. helium (He) or hydrogen (H2). The GAS1 may be e.g. at the normal pressure (101.3 kPa) or at a reduced pressure, e.g. below 10 kPa. For the person skilled in the art, it will be clear that modifications and variations of the devices and methods according to the present invention are perceivable. The figures are schematic. The particular embodiments described above with reference to the accompanying drawings are illustrative only and not meant to limit the scope of the invention, which is defined by the appended claims.

Claims

CLAIMS 1. An apparatus (500) comprising: - an illuminating unit (110) to form narrowband light pulses (B2t1, B2t2) at different wavelengths (^1,^2) for illuminating an object (OBJ1) with the narrowband light pulses (B2t1, B2t2), - a sensor (SEN1) to detect light (B3) received from the object (OBJ1), - a control unit (CNT1) to control operation of the sensor (SEN1), wherein the illuminating unit (110) comprises: - a broadband light source (LS1) to generate broadband light pulses (B1), - a Fabry-Perot interferometer (FPI1) to form narrowband light pulses (B2) by filtering the broadband light pulses (B1), wherein the broadband light source (LS1) is arranged to generate a first broadband light pulse (B1t1) at a first trigger time (t1), and to generate a second broadband light pulse (B1t2) at a second trigger time (t2), wherein the Fabry-Perot interferometer (FPI1) is tunable by moving a mirror (M2) of the Fabry-Perot interferometer (FPI1), wherein the illuminating unit (110) is arranged to operate such that a movement of the mirror (M2) is not stopped between the first trigger time (t1) and the second trigger time (t2), wherein a start time (tA1) of a first exposure time (^tEX1) of the sensor (SEN1), and a duration of the first exposure time (^tEX1) are controlled such that the illuminating unit (110) forms the first broadband light pulse (B1t1) and the second broadband light pulse (B1t2) during the first exposure time (^tEX1).

2. The apparatus (500) of claim 1, wherein the illuminating unit (110) is arranged to operate such that the mirror (M2) moves continuously at the first trigger time (t1) and at the second trigger time (t2).

3. The apparatus (500) of claim 1 or 2, wherein the illuminating unit (110) is arranged to operate such that the movement of the mirror (M2) is not stopped during the first exposure time (^tEX1).

4. The apparatus (500) according to any of the claims 1 to 3, wherein the illuminating unit (110) is arranged to operate such that the mirror (M2) moves continuously during the whole first exposure time (^tEX1).

5. The apparatus (500) according to any of the claims 1 to 4, wherein the illuminating unit (110) is arranged to operate such that: - the mirror gap (dGAP) of the Fabry-Perot interferometer (FPI1) is modulated according to a periodic modulating waveform (dGAP(t)), - the modulating waveform (dGAP(t)) has a modulation time period (TMOD), - the mirror (M2) moves continuously during a scanning time interval (^tSCAN), - the movement of the mirror (M2) is not stopped during the scanning time interval (^tSCAN), - the scanning time interval (^tSCAN) includes the first exposure time (^tEX1), and - the scanning time interval (^tSCAN) is longer than 25% of the modulation time period (TMOD).

6. The apparatus (500) according to any of the claims 1 to 5, wherein all narrowband light pulses (B2) formed during the first exposure time (^tEX1) constitute a first group (GRP1), wherein the start time (tA1) and the duration of the first exposure time (^tEX1) are controlled such that the first group (GRP1) represents a predetermined spectral range (RNG1) from a predetermined lower wavelength limit (^A1) to a predetermined higher wavelength limit (^E1).

7. The apparatus (500) according to any of the claims 1 to 6, wherein all narrowband light pulses (B2) formed during the first exposure time (^tEX1) constitute a first group (GRP1), wherein the start time (tA1) and the duration of the first exposure time (^tEX1) are controlled such that an effective spectral width (^^FWHM,EFF) of the first group (GRP1) is equal to a first predetermined spectral width (^^PRE).

8. The apparatus (500) according to any of the claims 1 to 7, being arranged to form narrowband light pulses (B2) during a second exposure time (^tEX2) of the sensor (SEN1), wherein all narrowband light pulses (B2) formed during the second exposure time (^tEX2) constitute a second group (GRP2), wherein a start time (tA2) and a duration of the second exposure time (^tEX2) are controlled such that the effective spectral width (^^FWHM,EFF) of the second group (GRP2) is equal to a second predetermined spectral width (^^PRE).

9. The apparatus (500) according to any of the claims 1 to 8, wherein the mirror gap (dGAP) of the Fabry-Perot interferometer (FPI1) is modulated according to a periodic modulating waveform (dGAP(t)), and the average repetition rate (f0) of the broadband light pulses (B1) is greater than or equal to two times the modulation frequency (fMOD) of the modulating waveform (dGAP(t)).

10. The apparatus (500) according to any of the claims 1 to 9, wherein the broadband light source (LS1) comprises a seed laser (SEED1), an optical amplifier (OPA1), and an optical waveguide (FIB1).

11. The apparatus (500) according to any of the claims 1 to 10, comprising a memory (MEM1) for storing control parameters (PAR1), which specify the start time (tA1) of the first exposure time (^tEX1), and the duration of the first exposure time (^tEX1).

12. The apparatus (500) according to any of the claims 1 to 11, wherein the sensor (SEN1) is an image sensor for capturing images (IMG1) of the illuminated object (OBJ1).

13. The apparatus (500) according to any of the claims 1 to 12, comprising a first spectrally selective combination (CMB1) of a calibration light source (LS11) and a calibration detector (DET11), wherein the calibration light source (LS11) is arranged to provide first calibration light (B11), the calibration detector (DET11) is arranged to detect first calibration light (B11) that has passed through the Fabry-Perot interferometer (FPI1), wherein the first spectrally selective combination (CMB1) is arranged to form a calibration detector signal (SDET11), wherein the first spectrally selective combination (CMB1) is arranged to change a state of the calibration detector signal (SDET11) when a wavelength (^) of a spectral transmittance peak (PEAK1) of the Fabry-Perot interferometer (FPI1) becomes higher or lower than a first predetermined calibration wavelength (^CAL1).

14. A method comprising: - generating a first broadband light pulse (B1t1) at a first trigger time (t1), - forming a first narrowband light pulse (B2t1) by filtering the first broadband light pulse (B1t1) with a Fabry-Perot interferometer (FPI1),- changing a mirror gap (dGAP) of the Fabry-Perot interferometer (FPI1) by moving a mirror (M2) of the Fabry-Perot interferometer (FPI1), - generating a second broadband light pulse (B1t2) at a second trigger time (t2), - forming a second narrowband light pulse (B2t2) by filtering the second broadband light pulse (B1t2) with the Fabry-Perot interferometer (FPI1), - illuminating an object (OBJ1) with light of the narrowband light pulses (B2t1, B2t2), - detecting light (B3) received from the object (OBJ1) by using a sensor (SEN1), wherein a start time (tA1) of a first exposure time (^tEX1) of the sensor (SEN1), and a duration of the first exposure time (^tEX1) are controlled such that the first broadband light pulse (B1) and the second broadband light pulse (B1) are formed during the first exposure time (^tEX1), wherein the movement of the mirror (M2) is not stopped between the first trigger time (t1) and the second trigger time (t2).

15. The method of claim 14, comprising capturing an image (IMG1) of the object (OBJ1) by using the sensor (SEN1).

Images