Method and apparatus for offset compensation

Abstract

An apparatus and method for rescaling a signal to remove an absorption offset from an optical measurement. During a first mode, a scaling factor is obtained from a measurement on a reference object, which is then applied during a second mode when a measurement object is measured.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to an apparatus and a method for removing or reducing an absorption bias in an optical measurement system, for example a system for measuring the concentration and / or composition of a gas in a cavity, for example a cavity of a body. In particular, the apparatus and method allow to remove the bias by rescaling the signal using a reference detector.

[0002] BACKGROUND In systems using lasers with different wavelengths, for example one for oxygen molecules and another for water vapor molecules, the amount of absorption induced by each gas gives a measure of the integrated gas concentration over the optical path length of the light propagating in the gas.

[0003] For human body measurements, assuming that the water vapor concentration in the body cavity is known, based on the fact that the relative humidity is close to 100% and that the temperature can be measured or assumed to be 37°C, the average optical path length in the gas is calculated from the integrated gas concentration of water vapor. Conversely, assuming that the average optical path length in the gas is equal for the two wavelengths, the oxygen concentration can be calculated.

[0004] In optical measurement apparatuses or setups, it is often necessary to collimate and combine the light beams and / or to transport the light beams to the measurement object. It is most convenient to house such light beam preparation in an enclosure of ambient air and pressure. Therefore, the two laser beams are subject to gas absorption by oxygen molecules and water vapor in this environment, which is considered an absorption bias to the measurement.

[0005] To eliminate this bias, it is common to flush the enclosure housing the light beam preparation with dry nitrogen at a constant low rate. This ensures that over time only a minimum amount of oxygen and water vapor is present in the enclosure.

[0006] This is not always a practical solution, depending on the training and background of the personnel operating the apparatus and the location where the apparatus is located. For example, the use of dry nitrogen and pressurized gases can be limited in locations such as pressurized bottles.

[0007] There is also the problem that the amount of bias varies constantly, so constant recalibration is necessary.

[0008] Therefore, a new apparatus and method for reducing or eliminating the bias can be advantageous. In particular, an apparatus and method that can be easily operated and do not require or require little training. This is also advantageous in any type of location where dry nitrogen is not available or cannot be used, and this would eliminate the need for reference measurements at the location of the measurement object. It would be particularly advantageous to have an apparatus and method that allow a simple design and assembly of the light beam preparation. SUMMARY

[0009] Embodiments of the present disclosure preferably mitigate, alleviate, or eliminate one or more of the deficiencies or problems in the art, singly or in any combination, by providing apparatus and / or methods in accordance with the following.

[0010] According to an aspect of the present disclosure, a method for rescaling signals to remove absorption bias from optical measurements is described. The method can include a first mode. The first mode can include:

[0011] measuring, using an external detector, a first light signal from each of the at least one light source located in the enclosure, the first light signal having transmitted through or backscattered from a reference object located outside the enclosure.

[0012] measuring, using a reference detector, a second light signal from each of the at least one light source located in the enclosure.

[0013] calculating a scaling factor for each of the at least one light source using the detected first light signal and the detected second light signal.

[0014] The method can further include a second mode, for example a continuous second mode. The second mode can include:

[0015] measuring, using an external detector, a third light signal from each of the at least one light source located in the enclosure, the third light signal having transmitted through or backscattered from a measurement object located outside the enclosure.

[0016] measuring, using a reference detector, a fourth light signal from each of the at least one light source located in the enclosure.

[0017] applying the scaling factor to each detected fourth light signal and then subtracting from each detected third light signal transmitted through or backscattered from the measurement object.

[0018] In some examples of the present disclosure, the enclosure can include a mixed gas, for example ambient air.

[0019] In some examples of the present disclosure, the method can include transmitting the first light signal from the enclosure to the reference object and / or transmitting the third light signal from the enclosure to the measurement object using a fiber optic probe.

[0020] In some examples of the present disclosure, the measurement object can be a cavity in a body, for example a lung. The body can be a human body. The present disclosure is not limited to a body, the object can be any type of object including a cavity or hole.

[0021] In some examples of the present disclosure, the third light signal can be used to measure gas absorption.

[0022] In some examples of the present disclosure, the measured gas absorption can be used to obtain a gas concentration of the free gas in the cavity.

[0023] In some examples of the present disclosure, the reference object can be free of gas.

[0024] In some examples of the present disclosure, the reference detector can be arranged within the housing.

[0025] In some examples of the present disclosure, a path length from the at least one light source to the reference detector can differ from a path length from the at least one light source to the external detector.

[0026] In some examples of the present disclosure, the scaling factor can be calculated separately for each of the at least one light source.

[0027] In some examples of the present disclosure, the measurement can be performed using a tunable diode laser technique such as used for GASM AS.

[0028] In some examples of the present disclosure, the first light signal and the second light signal can be measured simultaneously during the first mode. Further, and / or alternatively, in some examples of the present disclosure, the third light signal and the fourth light signal can be measured simultaneously during the second mode.

[0029] In another aspect of the present disclosure, a device, e.g. a medical device, for performing an optical measurement is described. The device can comprise a housing, which can contain a mixed gas, at least one light source can be arranged in the housing and configured to emit a light signal corresponding to each of the at least one light source, an external detector and a reference detector, an optical arrangement, which can be configured to guide the light signal from the housing to the external detector and to guide the light signal to the reference detector.

[0030] The device can further comprise a control unit, which can be configured to operate in a first mode. The first mode can comprise recording, using the external detector, the light signal as a first light signal, which is transmitted through or backscattered from a reference object, and recording, using the reference detector, the light signal as a second light signal. A scaling factor is calculated using the detected first light signal and the detected second light signal.

[0031] The control unit can further be configured to operate in a second mode. The second mode can comprise recording, using the external detector, the light signal as a third light signal, which is transmitted through or backscattered from a measurement object, and recording, using the reference detector, the light signal as a fourth light signal, and applying the scaling factor to the detected fourth light signal and subtracting from the detected third light signal to obtain a corrected light signal with a reduced offset.

[0032] The examples presented in this disclosure are primarily directed to medical devices, but the device can be any type of gas measuring device for non-medical applications. For these types of devices, the same components and principles are applicable.

[0033] In some examples of the disclosure, the reference detector can be arranged in the housing.

[0034] In some examples of the disclosure, the optical arrangement can be a beam sampler, such as a beam splitter, or a pick-up mirror. Other types of beam samples are described in the specification.

[0035] In some examples of the disclosure, the first light signal can be a first part of the light signal emitted during the first mode and the second light signal can be a second part of the light signal emitted during the first mode. For example, the light signal is directed to be split into two parts, the first part and the second part. Further, and / or optionally, in some examples, the third light signal can be a third part of the light signal emitted during the second mode and the fourth light signal can be a fourth part of the light signal emitted during the second mode. For example, the light signal is directed to be split into two parts, the third part and the fourth part.

[0036] In some examples of the disclosure, the control unit can be further configured to obtain the gas concentration or the gas distribution based on the corrected light signal.

[0037] It should be emphasized that the term "comprises / comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components, or groups thereof. BRIEF DESCRIPTION OF DRAWINGS

[0038] These and other aspects, features and advantages of the examples of the disclosure will become apparent and elucidated from the following description of the examples of the disclosure, taken in conjunction with the accompanying drawings, wherein:

[0039] Figure 1A and Figure 1B Exemplary schematic diagrams illustrating components of the disclosed device are shown, wherein Figure 1A is an example of the device in an offset adjustment mode, Figure 1B is an example of the device in a measurement mode; and

[0040] Figure 2 Exemplary flow diagrams illustrating the described methods are shown. DETAILED DESCRIPTION

[0041] Specific examples of the present disclosure will now be described with reference to the drawing. However, the present disclosure can be embodied in many different forms and should not be construed as limited to the examples set forth herein; rather, these examples are provided so that the present disclosure will be thorough and complete, and fully convey the scope of the present disclosure to those skilled in the art.

[0042] The inventors have found a method to eliminate or reduce the absorption spectral shift due to ambient air during a spectral measurement, which does not involve flushing dry nitrogen in the laser beam space, and further eliminates the need for a reference measurement at the measurement object location. The shift occurs because the light beam experiences gas absorption when passing through the environment.

[0043] Compared to previously known methods for reducing or eliminating the effect on absorption measurements, the present invention will simplify the operation and handling of optical measurement equipment. The method and equipment will also allow for a simpler design and assembly of the light beam preparation compared to other available methods or systems.

[0044] Figure 1A and 1B An illustrative system for rescaling a signal to remove an absorption shift from an optical measurement when detecting a gas distribution and / or a gas concentration is described. The rescaling described herein is particularly suitable for tunable diode laser absorption spectroscopy (TDLAS), such as gas absorption spectroscopy in scattering media (GASMAS).

[0045] When detecting a gas in a scattering material, such as tissue, using tunable diode laser absorption spectroscopy (TDLAS), in which free gas that absorbs light is dispersed in a cavity surrounded by the medium, the wavelength of at least one light source 1, 2, 3 is swept over an absorption peak or absorption band of the gas to be measured. Each step in the scan can be scaled down to a scale below the nanometer.

[0046] The principle of GASMAS is that a spectrally sharp gas absorption can be distinguished from the broadband absorption of liquids and solids. This makes it possible to extract a very small gas absorption signal (down to the order of one in ten thousand) from the light that has passed through the scattering and absorbing material, although only a small fraction of the injected light is transmitted, when using the GASMAS technique. For the GASMAS technique, the wavelength is tuned over a specific gas absorption line, for example by ramping the driving current.

[0047] In some examples, the detection is frequency sensitive and / or phase sensitive. Light from at least one light source 1, 2, 3, e.g. a laser, can be wavelength modulated at a selected frequency and a synchronous intensity change can be detected when modulating near a gas absorption wavelength. When the modulation is close to a gas absorption wavelength, the detected intensity of the light can change rapidly with small changes in wavelength as described in S. Svanberg, Gas Absorption Spectroscopy in Scattering Media - From Basic Research to Biomedical Applications, Laser & Photonics Reviews 7, 779 (2013), which is incorporated herein by reference.

[0048] The measured light intensity incident on the detector 7 can then be converted to a measure of gas absorption in the sample. By estimating a known gas concentration of a gas in the sample, e.g. water, the absorption of the gas can be translated to a measure of the average distance of light through the gas. The distance travelled or path length can be used to estimate the concentration of an unknown second gas.

[0049] In the system, the at least one light source 1, 2, 3 is arranged in an enclosure 16 which can comprise a mixed gas. In most cases, the air mixture is ambient air, but other compositions are possible. The enclosure 16 can also comprise beam preparation components for collimating the light beams from the light sources, combining light beams from multiple light sources and / or coupling the light beams into the probe 5 for transmission of light to the measurement location at the measurement object 6A. The components for collimating and / or combining the light beams can typically be lenses and mirrors known to the person skilled in the art.

[0050] In the schematic, the first light source 1 can be arranged to emit light along the optical axis of the combined light beam, or as shown, a mirror 8 can be used to fold the light beam emitted from the light source 1 for transmission along the optical axis.

[0051] In the shown example, a beam combiner 9, 10 can be used to combine the light beams of at least the second light source 2, 3 with the light beam of the first light source 1. The beam combiner 9, 10 can be a dichroic mirror. Collimating lenses 13, 14, 15 can be arranged in front of each light source 1, 2, 3. Alternatively, mirrors 8, 9 and 10 can be used to collimate the light beams. Alternatively, a lens system can be arranged for collimating the light beams after beam combination.

[0052] In the enclosure 16, the light is transmitted at least partly in an open beam path, whereby the light passes through the mixed gas within the enclosure 16. Thus, part of the light can be absorbed by the mixed gas in the enclosure 16 and the absorption can cause a shift in the measured absorption signal.

[0053] Using the optical elements, the individual light beams from each of the at least one light source 1, 2, 3 can be collimated and then combined into a single light beam. Thus, the single light beam can comprise light from each of the light sources 1, 2, 3 arranged in the housing 16. After the light beams from each of the at least one light source 1, 3 have been collimated and combined into a single light beam, the combined light beam can be coupled to the proximal end of the measurement probe 5 for transmitting the light to a measurement location at the measurement object 6B. The measurement object 6B can comprise a cavity with a bore having at least one kind of free gas. The combined light beam can be coupled to the proximal end of the measurement probe 5 by a lens or a lens system 12. The measurement probe 5 can be made of a waveguide, e.g. an optical fiber. The measurement probe 5 can be placed in close proximity to the measurement location or can be arranged inside the measurement object, e.g. inside a human body, for measuring a gas inside a body cavity. The waveguide or optical fiber can be arranged in a catheter or endoscope for arranging the probe 5 inside the measurement object 6B.

[0054] In examples, two or more light sources 1, 2, 3 are used for measuring at least one kind of free gas. Depending on the gas to be measured, the wavelengths of the at least two light sources 1, 2, 3 can be adjusted to match the absorption peaks of the at least two gases. For example, the wavelength can be adjusted to about 760 nm for handling oxygen and to about 820 nm or 935 nm for handling water vapor. Other wavelengths can be used depending on the gas to be detected.

[0055] The concentration of water vapor can be estimated based on the relative humidity and the measured temperature and thus can be used as a reference gas for obtaining an estimated path length for use when measuring a gas with an unknown concentration, e.g. oxygen or carbon dioxide. In some examples, other gases than water vapor can be used as a reference gas. All that is required is that the gas concentration can be calculated without using the path length of the detected light through a cavity or bore containing the gas with the unknown concentration.

[0056] Alternatively, in other examples, only one light source 1, 2, 3 can be used and the path length through the cavity and bore can be estimated by other means.

[0057] The at least one light source 1, 2, 3 can be a semiconductor laser, e.g. a distributed feedback laser (DFBL), a vertical cavity surface emitting laser (VCSEL) or other types of available lasers. The power of the emitted light is preferably in the range of 0.1 mW to 3000 mW.

[0058] The lasers can be driven by a current and temperature regulation unit comprised in a driving unit. The driving unit can be controlled by a control unit, e.g. a computer. The control unit can be used for signal processing and evaluation of the measurement data. All determinations or calculations described herein can be performed by the control unit or a data processing device (not shown).

[0059] The control unit or data processing device can be implemented by means of dedicated software (or firmware) running on one or more general purpose or special purpose computing devices. In this context, each "element" or "means" of such computing device is to be understood as a conceptual equivalent of a method step; there is not always a one-to-one correspondence between elements / means and specific hardware or software routines. One piece of hardware can contain different means / elements. For example, a processing unit acts as one element / means when executing one instruction and as another element / means when executing another instruction. Furthermore, one element / means can in some cases be implemented by one instruction, but in other cases be implemented by a plurality of instructions. Such software-controlled computing devices can include one or more processing units, such as CPUs ("Central Processing Units"), DSPs ("Digital Signal Processors"), ASICs ("Application-Specific Integrated Circuits"), discrete analog and / or digital components, or some other programmable logic device, such as an FPGA ("Field Programmable Gate Array"). The data processing device can further include system memory and a system bus that couples various system components including the system memory to the processing unit. The system bus can be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory can include computer storage media in the form of volatile and / or non-volatile memory such as read only memory (ROM), random access memory (RAM) and flash memory. The special-purpose software can be stored in the system memory or on other computer readable media that is included in or accessible to the computing device, such as magnetic media, optical media, flash memory cards, digital tape, solid state RAM, solid state ROM, etc. The data processing device can include one or more communication interfaces, such as a serial interface, a parallel interface, a USB interface, a wireless interface, a network adapter, etc., as well as one or more data acquisition devices, such as an A / D converter.

[0060] The special-purpose software can be provided to the control unit or data processing device on any suitable computer readable medium, including a recording medium and a read-only memory.

[0061] The system further comprises a detector 7 for detecting light transmitted through or backscattered from the measurement object 6B to be measured. The detector 7 can be arranged directly on the measurement object 6B to be measured or a waveguide, such as an optical fiber, can be used to collect light at the measurement object 6B to be measured and to transmit it to the detector 7.

[0062] The detector unit 7 can comprise a photodiode, a photomultiplier tube, an avalanche photodiode, a charge-coupled device (CCD) or a CMOS photosensitive device.

[0063] When measurements are made on tissue, for example on a gas in a cavity of a body, such as the lungs, the detector unit 7 can be adapted to be positioned on the skin of the subject.

[0064] Figure 1A A first mode is shown, which can be a system offset adjustment mode. The system comprises a second detector 4 as a reference detector. The reference detector 4 can be arranged within the housing 16. Optionally, the reference detector 4 can be directly connected to the housing 16 or connected to the housing 16 via a waveguide, for example an optical fiber.

[0065] In the offset adjustment mode, a first light signal from each of the at least one light source 1, 2 or 3 located in the housing 16 is transmitted through or backscattered from a reference object 6A located outside the housing 16. The light is detected by an external detector 7. The external detector 7 can be the same detector 7 as used when making measurements on a measurement object 6B, for example a human body, comprising a cavity with a gas to be measured.

[0066] Preferably, the reference object 6A is a medium that does not contain a gas. Alternatively, the reference object 6A can have a cavity or hole with a known gas concentration / absorption. Preferably, the known gas concentration / absorption consists of the same gas as the gas to be detected in the measurement object 6B.

[0067] A measurement probe 5 for making measurements on a measurement object 6B, for example a human body, is used to transmit light from the housing 16 to the reference object 6A.

[0068] A second light signal from each of the at least one light source 1, 2, 3 located in the housing 16 is measured using the reference detector 4.

[0069] The first and second light signals can be measured simultaneously or sequentially. A beam sampler 11 can be used to measure the first and second light signals.

[0070] When the first and second light signals are measured simultaneously, the first light signal can be a first part of a combined light beam and the second light signal can be a second part of the combined light beam. For example, the combined light beam is split into two parts.

[0071] The control unit can then be used to calculate a scaling factor related to each of the at least one light source 1, 2, 3 arranged within the housing 16, respectively, using the detected first light signal and the detected second light signal.

[0072] The first and second light signals can be measured simultaneously or sequentially.

[0073] Figure 1BA second mode is shown, which can be a measurement mode. The second mode can be a continuous mode and can be performed after the first mode has been completed. Alternatively, the measurement mode can be performed first and then the offset adjustment mode.

[0074] During the second mode, the measurement object 6B is measured. A third light signal from each of the at least one light source 1, 2, 3 located in the enclosure 16 is measured using the external detector 7. The detected third light signal has transmitted through or backscattered from the measurement object 6B. The light signal is transmitted from the enclosure 16 to the measurement object 6B by the measurement probe 5. The measurement probe 5 is the same probe 5 used for the measurement during the first mode, i.e. the offset adjustment mode.

[0075] A fourth light signal from each of the at least one light source 1, 2, 3 located in the enclosure 16 is measured using the reference detector 4.

[0076] The third and fourth light signals can be measured simultaneously or sequentially. For example, the fourth light signal can be measured between each measured third light signal, or after two measured third light signals, or after every third measured third light signal, or after every fourth measured third light signal, etc.

[0077] A beam sampler 11 can be used to measure the first and second light signals, and / or the third and fourth light signals.

[0078] When the first and second light signals and / or the third and fourth light signals are measured simultaneously, the first light signal and / or the third light signal can be a first part of the combined light beam and the second light signal and / or the fourth light signal can be a second part of the combined light beam.

[0079] Using the control unit, a calculated scaling factor relating to each of the at least one light source can be applied to each of the detected fourth light signals, respectively. A resulting value relating to each of the at least one light source 1, 2, 3 can be subtracted from the detected third light signal of each of the at least one light source 1, 2, 3, which has transmitted through or backscattered from the measurement object 6B. The third light signal of the at least one light source can then be rescaled and the offset caused by the gas in the enclosure 16 can be removed to obtain a corrected light signal of each of the at least one light source 1, 2, 3 based on the third light signal obtained by measuring the measurement object 6B. The corrected light signal can then be used to estimate and / or calculate a gas absorption of a free gas in a cavity or a hole in the measurement object 6B. The gas absorption can be used to obtain, estimate and / or calculate a gas distribution and / or a gas concentration of the gas free in the cavity or hole of the measurement object 6B.

[0080] Alternatively, in some examples, the measurement mode can be performed first and then the offset adjustment mode. Figure 1Bthe second mode in the information and can be performed continuously Figure 1A the first mode in the information. The information is then recorded on e.g. the control unit, after which scaling and adjustment of the offset is made.

[0081] When performing the scaling described herein, the path length from the at least one light source 1, 2, 3 to the reference detector 4 can differ from the path length 7 from the at least one light source 1, 2, 3 to the external detector.

[0082] In some examples, when calculating the scaling factor, the scaling factor can be calculated separately for each of the at least one light source.

[0083] In order to measure the first and second light signals simultaneously using the external detector 7 and the reference detector 4, various configurations of the components inside the housing can be used.

[0084] In one configuration, the beam sampler device 11 can be a diffuser arranged in the beam path, and the reference detector 4 can be arranged to directly detect a portion of the diffused light before the light is coupled to the probe 5.

[0085] In another configuration of the beam sampler device 11, the light beam from the at least one light source 1, 2, 3 can be collimated using optical components such as a lens system. The collimated light beam can have a diameter of about 1 to 4 mm or more. In some examples, the diameter of the collimated light beam can be less than 1 mm depending on the detector used. The reference detector 4 can be arranged to directly sample a portion of this collimated light beam without the signal detected by the reference detector 4 being coupled to the probe 5.

[0086] Another alternative of the beam sampler device 11 can be to arrange a beam splitter in the beam path. The beam splitter can direct a portion of the light beam emitted from the at least one light source 1, 2, 3 to the reference detector 4 and transmit another portion to be coupled to the probe 5.

[0087] Alternatively, in another beam sampler device 11, a mirror, such as a dichroic mirror or a pick-up mirror, can be arranged in the beam path. The mirror can direct a portion of the light beam to the reference detector 4 while allowing another portion to be transmitted and coupled to the probe 5.

[0088] Another alternative can be to use multiple reference detectors 4. For example, a reference detector can be used for each of the light sources 1, 2, 3. In such an arrangement, a portion of the light from each light source 1, 2, 3 can be detected by each reference detector 4 before the light from each light source 1, 2, 3 is joined into a single light beam and coupled to the probe 5.

[0089] For sequentially occurring measurements, a moving mirror such as a flip mirror can be used to deflect the light beams between the reference detectors 4 and couple to the probe 5.

[0090] The above described approach of performing a rescaling has the advantage that one adjustment measurement can be performed, which can be used even if conditions change, such as humidity, pressure, temperature, etc. Thus, only one calibration measurement is needed instead of continuously updating the calibration measurement, e.g. at certain time intervals or when environmental conditions change. The reason is that the reference signal is constantly measured simultaneously with the measurement signal.

[0091] Figure 2 An exemplary flowchart of a method for rescaling a signal during an optical measurement to remove an absorption offset in the optical signal is shown. The offset can be caused by the light beams being transmitted as open beams through a mixed gas, such as ambient air. The method can include a first mode in which a reference object is inserted into a distal end of an optical probe.

[0092] measuring, using an external detector, a first optical signal from each of the at least one light source located in the enclosure, the first optical signal having been transmitted through or backscattered from a reference object located outside the enclosure. The first optical signal from each of the at least one light source can be transmitted as a combined light beam that is the optical signal from each of the at least one light source combined into a single light beam.

[0093] measuring, using an external detector, a first optical signal from each of the at least one light source located in the enclosure, the first optical signal having been transmitted through or backscattered from a reference object located outside the enclosure. The first optical signal from each of the at least one light source can be transmitted as a combined light beam that is the optical signal from each of the at least one light source combined into a single light beam.

[0094] calculating, using the detected first optical signal and the detected second optical signal, an offset scaling factor for each of the at least one light source.

[0095] a second mode, such as a continuously performed mode, in which the reference object is removed and a measurement object is placed on the measurement object.

[0096] measuring, using an external detector, a third optical signal from each of the at least one light source located in the enclosure, the third optical signal having been transmitted through or backscattered from a measurement object located outside the enclosure. The third optical signal from each of the at least one light source can be transmitted as a combined light beam that is the optical signal from each of the at least one light source combined into a single light beam.

[0097] A fourth light signal from each of the at least one light source located in the housing is measured using the reference detector. The fourth light signal from each of the at least one light source can be transmitted as a combined light beam that is the light signal from each of the at least one light source combined into a single light beam.

[0098] A scaling factor associated with each of the at least one light source is applied to each of the detected fourth light signals, respectively. The resulting values can then be subtracted from the detected third light signal of each of the at least one light source transmitted through or backscattered from the measurement object to obtain a corrected light signal for each of the at least one light source.

[0099] The corrected light signal can then be used to obtain the gas absorption, distribution, and / or concentration in the cavity or hole of the measurement object.

[0100] The application has been described above with reference to specific examples. However, other examples than the above described are equally possible within the scope of the disclosure. Method steps described above can be provided in different order than described. Different features and steps can be combined in other combinations than the one described. The scope of the disclosure is only

[0101] The indefinite articles “a” and “an” used in the specification and in the claims, unless otherwise indicated, are to be construed as meaning “at least one” or “one or more”. The phrase “and / or” used in the specification and in the claims is to be construed as meaning “one or the other, or both”.

Claims

1. A method of rescaling a signal to remove an absorption bias from an optical measurement, the method comprising: a first mode, the first mode comprising: measuring, using an external detector, a first light signal from each of at least one light source located in an enclosure, the first light signal having transmitted through or backscattered from a reference object located outside of the enclosure; measuring, using a reference detector, a second light signal from each of the at least one light source located in the enclosure; calculating a scaling factor for each of the at least one light source using the detected first light signal and the detected second light signal; and a second mode, the second mode comprising: measuring, using the external detector, a third light signal from each of the at least one light source located in the enclosure, the third light signal having transmitted through or backscattered from a measurement object located outside of the enclosure; measuring, using a reference detector, a fourth light signal from each of the at least one light source located in the enclosure; applying the scaling factor to each of the detected fourth light signal and then subtracting from each of the detected third light signal that has transmitted through or backscattered from the measurement object.

2. The method of claim 1, wherein, the second mode comprises a continuous second mode.

3. The method of claim 1, wherein, the enclosure comprises a mixed gas.

4. The method of claim 3, wherein, the mixed gas comprises ambient air.

5. The method of claim 1, the method comprising using a fiber optic probe to transmit the first light signal from the enclosure to the reference object and / or to transmit the third light signal from the enclosure to the measurement object.

6. The method of claim 1, wherein, the measurement object is a cavity in a body.

7. The method of claim 1, wherein, the measurement object is a lung in a body.

8. The method of claim 6, wherein, the third light signal is used to measure gas absorption.

9. The method of claim 1, wherein, the third light signal is used to measure gas absorption.

10. The method of claim 8, wherein, the gas absorption is used to obtain a gas concentration of free gas in the cavity.

11. The method of any one of claims 1 to 10, wherein, the reference object is a gas-free medium.

12. The method of any one of claims 1 to 10, wherein, the reference detector is arranged within the enclosure.

13. The method of any one of claims 1 to 10, wherein, a path length from the at least one light source to the reference detector is different than a path length from the at least one light source to the external detector.

14. The method of any one of claims 1 to 10, wherein, the scaling factor is calculated separately for each of the at least one light source.

15. The method of any one of claims 1 to 10, wherein, the measurement is made using tunable diode laser technology for gas absorption spectroscopy in a scattering medium.

16. The method of any one of claims 1 to 10, wherein, the first light signal and the second light signal are measured simultaneously during the first mode, and / or wherein the third light signal and the fourth light signal are measured simultaneously during the second mode.

17. A medical device for making an optical measurement, the medical device comprising: an enclosure, the enclosure comprising a mixed gas; at least one light source, the at least one light source arranged in the enclosure and configured to emit a light signal corresponding to each of the at least one light source, an external detector and a reference detector; optical means configured to direct the light signal from the enclosure to the external detector and to direct the light signal to the reference detector; a control unit configured for operating in: a first mode, using the outer detector to record the light signal as a first light signal transmitted through or backscattered from a reference object, and using the reference detector to record the light signal as a second light signal, calculating a scaling factor using the detected first light signal and the detected second light signal; and a second mode, using the outer detector to record the light signal as a third light signal transmitted through or backscattered from a measurement object, and using the reference detector to record the light signal as a fourth light signal, applying the scaling factor to the detected fourth light signal and subtracting from the detected third light signal to obtain a corrected light signal with reduced offset.

18. The medical device of claim 17, wherein, the reference detector is arranged in the housing.

19. The medical device of claim 17, wherein, the optical device is a beam sampler.

20. The medical device of claim 19, wherein, the beam sampler comprises a beamsplitter or a pick-up mirror.

21. The medical device of any of claims 17 to 20, wherein, the first light signal is a first part of the light signal emitted during the first mode and the second light signal is a second part of the light signal emitted during the first mode, and / or wherein the third light signal is a third part of the light signal emitted during the second mode and the fourth light signal is a fourth part of the light signal emitted during the second mode.

22. The medical device of any one of claims 17 to 20, wherein, the control unit is further configured for obtaining a gas concentration or distribution based on the corrected light signal.

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