Optical inspection system calibration

Abstract

According to a first aspect of the invention, there is provided a method of measuring the optical reflectivity R of an object using a detection system, the detection system comprising a light emitter and a light detector spaced apart from each other. The method comprises illuminating the object with the light emitter, detecting light reflected from the object using the light detector, wherein the light detector provides an electrical output signal S indicating the intensity of the detected light S , and the optical reflectivity R of the target is determined according to the following formula: #imgabs0# where R R is the spectral reflectance of the reference standard, S R is the electrical output signal of the detector with the reference standard in place, S H is the detector electrical output signal in the absence of a target in front of the light emitter and light detector, and M is the calibration factor.

Description

Technical Field

[0001] The present invention relates to the calibration of those optical detection systems consisting of a non-directional light source, a detector and a target (material to be detected), and more particularly to such systems configured to measure the optical reflectivity of a target. Background Art

[0002] Many systems are known for measuring the optical reflectivity of a target, such as a piece of material, such as plastic, metal, etc. Some systems are capable of determining reflectivity over a range of wavelengths. Determining the reflectivity of a target is desirable in many different applications, including, for example, food analysis, color sensing, infrared sensing, biomedical sensors, spectral sensing, imitation, cosmetic analysis, pharmaceutical analysis, process control, thickness measurement, high temperature temperature measurement, LED measurement, reaction analysis, and fluorescence analysis. Such a system will typically include a light emitter, a light detector (e.g., configured to scan a wavelength range when reflectivity versus wavelength characteristics are required), and a chassis to which the emitter and detector are attached or contained. In some cases, the chassis may be a module that facilitates installation of the system in some larger device, so that the system may be a standard component sold to manufacturers of different devices. In other cases, the chassis may be the device itself, such as a smartphone in which an emitter and detector are integrated. In other cases, the chassis may be a combination of a module and a device.

[0003] Current detection systems may assume that the target reflectivity R at a given wavelength λ is S The linear relationship between the (electrical) output signal of the detector across the wavelength range of interest. This can be expressed as follows:

[0004]

[0005] in:

[0006] R R is the spectral reflectance of the reference standard;

[0007] S R is the signal with the reference standard in place (e.g., a 99% reflectivity target);

[0008] S S is the signal with the calibration item in place;

[0009] S H is a signal with an open port, i.e. there is no target in front of the transmitter.

[0010] However, in many cases, due to the multiple paths of light that strike the target and the chassis that houses the optical system before reaching the detector, the amount of light intensity collected by the detector will not be linear with the reflectivity of the target. This will be more pronounced for highly reflective targets. In other words, the intensity collected by the detector includes not only the light that strikes the target and then strikes the detector (the direct path), but also the light that is reflected by the target and the chassis that houses the optical system (the nonlinear portion). This is particularly true in Figure 1 As schematically shown in Figure 1 A target 1 and an optical reflection detection system 2 are shown, which includes an emitter 3, such as an LED (usually emitting white light) and a detector 4, such as a photodiode. The emitter and detector are housed in a chassis 5, which has an opening just above the emitter and detector to allow light to enter and exit the target. The spacing between the upper surface of the chassis and the target is represented by h, while the spacing between the emitter and detector is represented by d. Figure 1 Shown are a first ray 6, which leaves the emitter at an angle so that it strikes the detector after a single reflection from the target, and a second ray 7, which strikes the target after two reflections from the target and a single reflection from the top of the chassis.

[0011] Of course, from Figure 1 It can be clearly seen in Figure 1 that not all light that encounters multiple reflections between the chassis and the target will hit the detector. Therefore, methods that rely on the linear relationship of equation (1) lead to an overestimation of the measured intensity (emitter-target-detector) and, as a result, an underestimate of the calculated reflectivity. In addition, the calculated reflectivity as well as the collected intensity strongly depends on the containing chassis (e.g., surface reflectivity, geometry).

[0012] In order to provide a fully characterized optical module (printed circuit board and optical components), it must be paired with a specific housing. In addition, the processed reflection spectra are not only affected by nonlinear modulations of the amplitude, but they also differ between devices with different or even the same chassis. Another problem with current systems is that the device may only be calibrated for a specific distance (h) from the target. This is acceptable in cases where h does not vary, for example, where the chassis is a smartphone and the target is always pressed against the device screen, but otherwise it will lead to reduced flexibility. Summary of the invention

[0013] According to a first aspect of the invention, there is provided a method of measuring the optical reflectivity R of an object using a detection system, the detection system comprising a light emitter and a light detector spaced apart from each other. The method comprises illuminating the object with the light emitter, detecting light reflected from the object using the light detector, wherein the light detector provides an electrical output signal S indicating the intensity of the detected light S, and the optical reflectivity R of the target is determined according to the following formula:

[0014] in

[0015] R R is the spectral reflectance of the reference standard, S R is the electrical output signal of the detector with the reference standard in place, S H is the detector electrical output signal in the absence of a target in front of the light emitter and light detector, and M is the calibration factor.

[0016] The method may include determining a height z of the target above the detection system, using the height z to determine a height scaling factor η(z), and using the scaling factor to scale the determined reflectivity according to:

[0017]

[0018] The reflectivity R can be measured for one or more wavelengths λ according to the following formula:

[0019]

[0020] The electrical output signal may be integrated over a range of wavelengths, and the reflectivity represents the reflectivity averaged over the range of wavelengths.

[0021] The step of determining the optical reflectivity R of the target may include obtaining R from a memory R , S R , S H and the value of each of M, and evaluate the equation:

[0022]

[0023] The step of determining the optical reflectivity R of the target may comprise using the output signal S S As a lookup to a lookup table populated with the equation Evaluated reflectance values ​​and R R , S R , S H and the value of each of M.

[0024] The calibration factor M may depend on the target material type. The memory may contain a table mapping the target type to the corresponding value of M.

[0025] According to a second aspect of the present invention, there is provided a method for obtaining calibration data for a detection system, the detection system being configured to measure optical reflectivity using the method for measuring the optical reflectivity R of a target according to the present disclosure. The method comprises: operating a calibration detection system comprising a light emitter and a light detector spaced apart from each other for a plurality of targets of known optical reflectivity R, and obtaining a corresponding electrical output signal S of the detector R , and solve the following equation to determine the calibration factor M;

[0026]

[0027] The calibration detection system may be the detection system for which the calibration data is being obtained, or it may be a different system to the detection system for which the calibration data is being obtained, in which case the method includes providing the calibration factor M to the detection system for which the calibration data is being obtained.

[0028] The method may include performing the steps of operating and solving for one or more wavelengths and determining a value of M(λ) for each wavelength.

[0029] The method may include repeating the operating and solving steps for a plurality of different heights z above the calibration detection system to determine values ​​of M and η(z) for the corresponding heights.

[0030] According to a third aspect of the present invention, there is provided an optical reflectance measurement system, comprising a chassis and a light emitter and a light detector mounted to the chassis. The system further comprises one or more processors configured to cause the light emitter to illuminate a target, cause the light detector to detect light reflected from the target, wherein the light detector provides an electrical output signal S indicating the intensity of the detected light. S , and the optical reflectivity R of the target is determined according to the following formula:

[0031] in

[0032] R R is the spectral reflectance of the reference standard, S R is the electrical output signal of the detector with the reference standard in place, S H is the detector electrical output signal in the absence of a target in front of the light emitter and light detector, and M is the calibration factor.

[0033] The system may include a proximity sensor for determining a height z of the target above the detection system, the one or more processors being configured to determine a height scaling factor η(z) using the height z, and to scale the determined reflectivity using the scaling factor according to the following equation:

[0034]

[0035] The one or more processors may be configured to determine the reflectivity R for one or more wavelengths λ according to the following formula:

[0036]

[0037] The system may include a memory, wherein the one or more processors are configured to obtain R from the memory R , S R , S H and the value of each of M, and evaluate the equation Or the one or more processors are configured to use the output signal S S As a lookup to a lookup table stored in the memory, the lookup table is populated with the equation The estimated reflectance values ​​and R R , S R , S H and the value of each of M.

[0038] The invention can be applied to proximity sensors, color sensors and miniaturized spectrometers, for example. A particular use of the invention involves using the acquired spectral signal and calculating the target reflectivity when the target is very close to the light source and detector (small h / d as described below). BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 schematically illustrates an optical reflectivity detection system for measuring the reflectivity of a target;

[0040] Figure 2a shows simulation results for an optical reflectivity detection system, plotting the intensity of the detected signal S versus the number N of light rays striking the detector at right angles;

[0041] Figure 2b Shows Figure 2a Part of the result where for lower S values ​​the signal S scale is amplified;

[0042] Figure 3 The signal strength versus reflectivity characteristics of compensated and uncompensated detection systems are shown;

[0043] Figure 4 A reflectivity detection system is schematically shown;

[0044] Figure 5 is a flow chart showing a reflectivity detection method; and

[0045] Figure 6 is a flow chart illustrating a method of obtaining compensation data for a reflectivity detection system;

[0046] Figure 7 A number of accepted modes are schematically shown;

[0047] FIG8 shows a simplified maximum light path scheme; and

[0048] Fig. 9 The FoI and FoV of a simplified reflectivity detection system are shown. DETAILED DESCRIPTION

[0049] A method of measuring the optical spectral reflectance of an object that takes into account the light intensity losses caused by multiple reflection paths between the system's emitter and detector will now be described. This novel method exploits the assumption that each ray originating from the emitter is scattered into M rays of equal intensity at each reflection, with the reflection reducing the total intensity by a factor equal to the reflectivity R(λ) of the reflecting surface.

[0050] For the purpose of illustration, assume that the chassis is metallic and has a reflectivity R2(λ)=1, while the target has a reflectivity R1(λ)=R(λ). Furthermore, at each reflection from the target and the chassis, the light intensity I reaching the detector is D will be reduced by a factor of M. Thus, if the total number of reflections between the emitter and the detector is j, the total effect of the scattering will be to reduce the intensity at the detector by M to the jth power, i.e., M j If we assume that N is at right angles θ n The number of rays striking the detector, and I0 is the light intensity of the emitter, then:

[0051]

[0052] in:

[0053]

[0054] Therefore, the analog (electrical) signal produced by the detector will be:

[0055]

[0056] where k is a multiplication factor (taking into account the responsivity of the photodetector, potential filter responses and / or intervening media), and γ takes into account the amount of stray light reaching the detector, dark counts, and crosstalk.

[0057] Figure 2a Simulated data for an optical detection system is shown, showing the signal intensity (x-axis) versus the number N of light rays striking the detector at right angles for a hypothetical range of values ​​of M of 1 to 10. Figure 2bThe same data are shown, but the x-axis is expanded and only small values ​​of the signal intensity are shown (the scale goes up to 1.6). It is obvious from these figures that the signal intensity tends to saturate as N increases. Since equation (4) is a geometric series and converges (because R / M<1), the series can be replaced by its sum; for N=6 and M=2, the sum of the series has been reached. In practice, the amount of light allowed must be very high (theoretically infinite) and certainly greater than 6, which means:

[0058]

[0059] The advantage of equation (5) is that nonlinear effects can be directly compensated by removing the offset γ and multiplying the remainder by a factor of (1-R / M). This is given by Figure 3 Shown, Figure 3 The normalized integrated signal intensity is shown versus the (actual) target reflectivity, assuming that M has a value of 1.686, with the upper trace (dashed line) showing the uncompensated data and the lower trace (solid line) showing the compensated data.

[0060] Now considering the wavelength response of the signal (necessary in the case of a spectrometer) and relating it to the target reflectivity, the usual linear formula is:

[0061] s(λ)=k·I0(λ)·R(λ) (6)

[0062] In this configuration, the assumption of a linear relationship with the reflectivity of the material being measured (Equation (1)) creates a potential flaw in the calibration algorithm. Using the formula of Equation (4) instead, Equation (6) becomes:

[0063]

[0064] Substituting the series with the sum that satisfies equation (5) yields:

[0065]

[0066] This can be reformulated to provide an expression for determining the target reflectivity:

[0067]

[0068] Note that in equations (6) to (8), s(λ) is considered to be the signal generated only by the reflectivity at the target and includes dark counts and crosstalk. By providing calibration, dark counts and crosstalk are removed. In equation (9), S H (λ) corresponds to the offset factor γ of equation (5).

[0069] The parameter M provides a way to quantify Figure 1means of the nonlinearity of the optical system shown. The smaller the value of M, the stronger the nonlinearity of the optical system. If M tends to infinity, the system is linear in the reflectivity and equation (9) is equal to equation (1). The use of equation (9) linearizes the response of each optical system. Therefore, by prior calibration in terms of M (and γ), each optical system can be used no matter how "nonlinear" the situation it represents.

[0070] The process of compensating the integrated signal from the detector (integrated over wavelength if from a spectrometer) can be described as follows:

[0071] 1. Collect signals from various targets with different reflectivity.

[0072] a. If the output detector signal is a spectrum, then integration over wavelength (to provide an average value of M over a range of wavelengths) can be used.

[0073] b. Alternatively, M is derived for each wavelength.

[0074] 2. Fit the obtained data pair [reflectivity, detector signal] using equation (5).

[0075] 3. Extract parameters γ and M from the fit (step 2)

[0076] It should be understood that changing the target height relative to the detection system (i.e. Figure 1 h in the figure, the light field will be different as a function of the light intensity reaching the target and the amount of light from multipath interactions. Therefore, if the calibration is performed for a given target height, a system calibrated with equation (1) will only provide satisfactory results for that height (subject to the nonlinearity caused by multipath reflections). However, if the calibration is performed using equation (5), the system will still exhibit linear behavior at target heights that are different from the calibration target height, just at different M.

[0077] This is provided by equation (9) which depends on the target height (variable z);

[0078]

[0079] where η(z) is S(z REF ) / S(z) (the integral of S over the wavelength s(λ)), and z REF is the reference altitude.

[0080] The procedure for additionally calibrating the system for altitude is as follows:

[0081] 4. Repeat steps 1, 2, and 3 for different heights to obtain M(z) at each height.

[0082] 5. Obtain the ratio η(z) at each height by measuring S(z).

[0083] It should be understood that M(z) increases linearly with height until a saturation value is reached. The higher the height of the target above the system, the lower the contribution of multipath reflections to the detected signal, and thus the higher M will be. On the other hand, η(z) has a nonlinear behavior and it increases with increasing z (approximately according to a quadratic polynomial of the second order).

[0084] In normal use, the height of the target above the system may be determined using, for example, an onboard range sensor, and equation (10) may be used to determine the reflectivity of the target, using the values ​​of M and η for that height, and γ which is independent of height.

[0085] The above described methods of calibrating the detection system and calculating the reflectance essentially allow each system response to be considered linear with the reflectance. Furthermore, it is easier to apply altitude compensation. The method allows equalizing the output signals provided by multiple sensors located at different positions but still illuminated by the same source. This is an interesting situation, particularly useful when the system includes multiple detectors used as miniaturized spectrometers, which have overlapping wavelength ranges. Since these methods effectively eliminate the influence of the chassis housing the detectors and emitters, it is possible to sell optical detection modules without any restrictions on the end use case.

[0086] Other advantages achieved by the described method may include:

[0087] • In the case of miniaturized spectrometers, the measured and processed spectra can have much less dispersion between devices sharing the same chassis or using different chassis.

[0088] They can improve measurement accuracy.

[0089] Altitude compensation can be applied.

[0090] Spectra / signals at different altitudes can be compensated to provide consistent results.

[0091] The described method can be implemented into those systems which measure and thus provide the calculated reflectivity of the target by embedding appropriate algorithms in the software / firmware of the system. After the parameters M(λ) have been determined by a priori calibration using a reference reflectivity target, they can be compared with the array R R (λ), S R (λ), and S H (λ) is stored in a register (memory location) together with the photodetector. When a measurement is performed on a target, the raw signal S generated by the photodetector is processed according to equation (9): S(λ) to provide the target reflectivity R(λ). Where height compensation is provided and the detection system is capable of measuring the target height (e.g., using a proximity sensor), the parameter η(z) is determined during calibration and stored in the system. The raw signal is then processed according to equation (10). Alternatively, the parameter η(z) is determined during calibration and equation (11) is applied to the raw signal. However, it will be appreciated that the processing operations may be distributed, with certain tasks being performed external to the device in which the detection module is provided.

[0092] Figure 4 An optical reflectivity detection system is shown that includes a light emitter 3, a light detector 4 and a chassis 5. In addition, the system includes a processor 8 and a memory 9, wherein the processor 8 is configured to operate as described above. The memory 9 can store various parameters required to evaluate equation (9) or equation (10), in which case the system also includes a proximity sensor 10. Alternatively, the memory 9 can store a pre-calculated lookup table that maps sensor output values ​​to reflectivity.

[0093] Figure 5 The various steps (S1 to S3) associated with determining the reflectivity value of a target are described, and Figure 6 Steps ( S100 to S200 ) associated with performing calibration using the system itself or calibrating the system using a calibration system having the same characteristics are described.

[0094] It will be appreciated that the M(λ) and η(z) parameter calculations may be performed using an onboard computer of a device including a reflectivity detector (e.g., on a smartphone). Alternatively, raw or pre-processed data may be sent via a network connection to a server or the like that performs the parameter calculations, where the calculated parameters are returned to the device where they are stored for later use.

[0095] Systems that can integrate reflectance detectors include, for example, miniaturized MEMS-based spectrometers, tunable light sources (UV, visible, or infrared).

[0096] Those skilled in the art will appreciate that various modifications may be made to the above-described embodiments without departing from the scope of the present invention.

[0097] Additional technical information

[0098] To better understand the above proposal, please refer to the following detailed technical discussion.

[0099] Figure 7 Some accepted patterns are shown.

[0100] ·I vs.

[0101] Three main mechanisms are considered to study the behavior of intensity with respect to height (or better ζ=z / d). These mechanisms limit the amount of light reaching the detector (PD) and are:

[0102] Maximum number of reflections (N M )

[0103] Maximum optical path

[0104] Minimum acceptance angle

[0105] Maximum number of reflections

[0106] The allowed modes are not infinite. In fact, modes with high N values ​​will experience a lot of reflections, and if their exit intensity is low, they will not contribute to the signal read by the PD. In other words, for a certain target reflectivity R, the maximum number of accepted modes N has an upper limit imposed by the detector sensitivity N M .

[0107] Assuming that the PD is only able to read intensities 'I' above a certain threshold 'I0', i.e. the universal mode, in the absence of attenuation through the medium, the following inequality must hold:

[0108]

[0109] This relationship is important because it relates the PD sensitivity I0, the emission intensity from the illumination source I S , target reflectivity R and parameter M. This function Represents the floor function.

[0110] Maximum optical path (θ max )

[0111] Assume that due to attenuation through the medium, the ray cannot travel longer than a certain distance. Therefore, although the number of reflections it must undergo is equal to or less than N M , but if the optical path is longer than a certain limit, the mode will not reach the PD.

[0112] Assume that FIG8A depicts that the light has reached the maximum optical path (at z1, θ max If the height is increased to Z2 (Fig. 8B), and the same angle θ is maintained max (N=3), the light has to travel a longer path, so the same mode or higher modes are not allowed. If the light path is shorter than this limit, only the inferior modes (N1, N2) can reach the PD.

[0113] The longest path sets a limit angle θ at a certain altitude max , beyond this limiting angle, the light will not reach the PD.

[0114]

[0115] Substituting (13) into (12):

[0116]

[0117] In equation (15), the medium is also taken into account by the attenuation coefficient α, N ≤ N M .

[0118] ·θ min

[0119] Fig. 9 A simplified scheme of FoI and FoV is shown. Due to the characteristic field of view (FoV) of the PD and the field of illumination (FoI) of the emitter, there is a minimum angle θ min , below which the minimum angle θ min , light cannot exit the luminaire or enter the PD. Although modes below such limits will be allowed, the PD will not read power.

[0120] Referring to Figure 8,

[0121]

[0122] ·Effective mode (N max -N min )

[0123] These three limiting mechanisms are set for specific systems (I0, I S ,R,M,α,FoI,FoV,ζ) provides the number of possible patterns, N max -N min The amount of light will clearly determine the amount of light sensed by the PD. By plotting the expression:

[0124] θ n =tan -1 (2ζn) (17)

[0125] It is easy to explain how the limiting factor determines N max -N min , which is formed by the line θ = θ max and θ = θ min With curve θ n By increasing ζ, the curve becomes steeper toward the y-axis, below θ max The mode is more concentrated at 90°, so N max Smaller.

[0126] By reducing ζ, the difference N max -N min increases, the intensity also increases.T The maximum intensity is reached when N is 0. Below this value, the intensity decreases again because N M is less than θ max At the same time, N min (The minimum angle at which light reaches PD) increases, the difference N max -N min When ζ = ζ0, the intensity will reach its minimum value.

[0127] According to equation (16), we can M Substitute n and ζ T , ζ0 is substituted into ζ to derive tan(θ max ), tan(θ min ) value.

[0128] tan(θ max )=2ζ T N M (18)

[0129] tan(θ min )=2ζ0N M (19)

[0130] Obviously, the number of rays is finite, and it is no longer reasonable to use the sum from 1 to ∞, but rather to use the sum from N min To N max For simplicity, the attenuation of each ray is excluded. Only the rays exceeding θ are considered. max mode.

[0131]

[0132] The first N of the geometric series max and N min The sum of the -1 terms is:

[0133]

[0134] (Note that M does not vary with ζ).

[0135]

[0136] Therefore, what actually varies with ζ is the weight of the following factors on the rest of the function.

[0137]

[0138] The higher ζ is, the lower the modes are. At higher altitudes, when only one ray is theoretically allowed to hit the PD, the intensity versus reflectivity curve becomes linear within a certain range. Conversely, when ζ is low, the number of modes increases and the term Become very small.

[0139] As already explained, in ζ T Below this, the strength decreases (experimental verification), because it is not allowed to exceed N M The mode of the emitter or PD has a field of illumination (FoI) and a field of view (FoV), respectively. When ζ=ζ0, the intensity is close to zero. ζ T Both ζ0 and ζ0 depend on R / M as expected.

[0140] The experimental results show that the model is valid under the described conditions.

Claims

1. A method for measuring an optical reflectivity R of a target using a detection system, the detection system comprising a light emitter and a light detector spaced apart from each other, the method comprising: illuminating the target with the light emitter; The light reflected from the target is detected using the light detector, wherein the light detector provides an electrical output signal S indicative of the intensity of the detected light. S ;as well as The optical reflectivity R of the target is determined according to the following formula: in R R is the spectral reflectance of the reference standard, S R is the electrical output signal of the detector with the reference standard in place, S H is the detector electrical output signal in the absence of a target in front of the light emitter and the light detector, and M is a calibration factor, wherein the calibration factor M depends on the target material type.

2. The method according to claim 1, comprising: determining a height z of the target above the detection system; determining a height scaling factor η(z) using the height z; The determined reflectivity is scaled using the scaling factor according to the following formula:

3. The method according to claim 1, wherein the reflectivity R is measured for one or more wavelengths λ according to the following formula:

4. The method of claim 1, wherein the electrical output signal is integrated over a wavelength range and the reflectivity represents the reflectivity averaged over the wavelength range.

5. The method according to claim 1, wherein the step of determining the optical reflectivity R of the target comprises obtaining R from a memory. R , S R , S H and the value of each of M, and evaluating the equation 6. The method of claim 1, wherein the step of determining the optical reflectivity R of the target comprises using the output signal S S As a lookup to the lookup table, the lookup table is populated with the equation R = M· Evaluated reflectance values ​​and R R , S R , S H and the value of each of M.

7. A method of obtaining calibration data for a detection system configured to measure optical reflectivity using a method according to any one of the preceding claims, the method comprising: For a plurality of targets of known optical reflectivity R, a calibrated detection system comprising a light emitter and a light detector spaced apart from each other is operated, and corresponding electrical output signals S of the detectors are obtained. R ;as well as Solve the following equation to determine the calibration factor M; Wherein, I0 is the light intensity of the light emitter and k is the multiplication factor.

8. The method according to claim 7, wherein: The calibration test system is the test system for which calibration data is being obtained, or The calibration detection system is a different system to the detection system for which the calibration data is being obtained, and the method includes providing a calibration factor M to the detection system for which the calibration data is being obtained.

9. The method of claim 7, wherein the detection system for which calibration data is being obtained is a detection system configured to use the method of claim 3, the method comprising performing the steps of operating and solving for one or more wavelengths, and determining a value of M(λ) for each wavelength.

10. The method of claim 7, wherein the detection system is configured to use the method of claim 2 and includes repeating the steps of operating and solving for a plurality of different heights z above the calibration detection system to determine values ​​of M and η(z) for the respective heights.

11. An optical reflectance measurement system, comprising a chassis and a light emitter and a light detector mounted to the chassis, the system further comprising one or more processors, the one or more processors being configured to: causing the light emitter to illuminate a target; causing the light detector to detect light reflected from the target, wherein the light detector provides an electrical output signal S indicative of the intensity of the detected light S ;as well as The optical reflectivity R of the target is determined according to the following formula: in R R is the spectral reflectance of the reference standard, S R is the electrical output signal of the detector with the reference standard in place, S H is the detector electrical output signal in the absence of a target in front of the light emitter and the light detector, and M is the calibration factor.

12. The optical reflectivity measurement system of claim 11, comprising a proximity sensor for determining a height z of the target above the detection system, the one or more processors being configured to: determining a height scaling factor η(z) using the height z; The determined reflectivity is scaled using the scaling factor according to the following formula:

13. The optical reflectance measurement system of claim 11, wherein the one or more processors are configured to determine the reflectance R for one or more wavelengths λ according to the following formula:

14. The optical reflectance measurement system of claim 11, comprising a memory, wherein: The one or more processors are configured to obtain R from the memory R , S R , S H and the value of each of M, and evaluating the equation or The one or more processors are configured to use the output signal S S As a lookup to a lookup table stored in the memory, the lookup table is populated with the equation Evaluated reflectance values ​​and R R , S R , S H and the value of each of M.

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