CN113533217B - Multi-channel synthesized wide spectrum light source for spectral confocal measurement and fusion method - Google Patents
Multi-channel synthesized wide spectrum light source for spectral confocal measurement and fusion method Download PDFInfo
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Abstract
The invention discloses a wide spectrum light source for multichannel synthesis of spectral confocal measurement and a fusion method, wherein the wide spectrum light source comprises: the LED power supply comprises a direct current switch power supply, a constant current driving circuit, an LED array, an optical fiber coupling flange and an optical fiber branching device; the single-color LED, the radiator and the optical fiber coupling flange jointly form a channel of the multi-channel LED light source module, and all the channels are mutually independent and independently controlled by power supply; the optical fiber coupling flange couples the emergent light of the monochromatic LEDs to the branching optical fiber, and the spectrum superposition principle is utilized to fuse the emergent light of the monochromatic LEDs with different wavelengths of a plurality of channels, so that the adjustable wide-spectrum light source is obtained. The invention can eliminate the modulation effect of the traditional light source on the peak value of the measurement signal, can eliminate the influence of the dispersion objective lens on the inconsistent transmittance of different wavelengths, improves the signal to noise ratio of the measurement signal so as to improve the measurement accuracy, and can adjust the output spectral characteristics according to different measurement requirements to obtain good measurement signals.
Description
Technical Field
The invention relates to a wide spectrum light source for multichannel synthesis for spectral confocal measurement and a fusion method thereof, which are particularly suitable for the manufacture of precision instruments and the measurement of samples such as films, glass, circuit boards and the like.
Background
With the development of precision manufacturing industry, the requirement for workpiece detection is higher and higher. The spectral confocal measurement technology developed from the confocal measurement technology has wide application in industrial detection due to high detection efficiency, and particularly in measurement of transparent materials such as films, glass substrates and the like.
The spectral confocal sensor uses longitudinal chromatic aberration to correspond wavelength information to spatial position codes. The reflected wavelength is detected by a spectrometer to determine the spatial position. A typical spectral confocal sensor mainly comprises: the device comprises a light source, a dispersion objective lens and a spectrometer. In order to achieve simultaneous generation of multiple foci, the illumination source chosen should be a broad spectrum source.
The light sources commonly used at present mainly comprise: halogen tungsten lamp, xenon lamp, "white light" LED, supercontinuum light source. The halogen tungsten lamp and the xenon lamp have high luminous intensity and wide generated spectrum range, but the halogen lamp and the xenon lamp have large bulb size and low illumination efficiency. The "white light" LED is one of the most commonly used light sources due to its inexpensive cost and long life, but it provides a much narrower spectral range than halogen and xenon lamps, limiting to some extent the scanning range of the spectral confocal sensor. The supercontinuum light source can provide a wider spectrum with stable intensity, is an ideal light source, but has high cost and is not beneficial to the production. In addition, the common light sources have the same characteristics, and the emitted spectrums of the common light sources show a relatively fixed and non-uniform distribution rule. The light source emission spectrum thus forms a non-uniform envelope, producing a non-uniform modulation of the signal score detected by the spectrometer. Therefore, in the whole spectrum range, the uniformity of the signal to noise ratio of the measured peak signal is poor due to the non-uniformity of the spectrum distribution of the light source, and the signal to noise ratio of partial wavelength is low, so that the accuracy of a measurement result is seriously influenced.
In addition, the dispersive objective lens is usually made of at least three glass materials with large Abbe number differences to ensure that linear dispersion is generated. This means that the dispersive objective will cause different degrees of absorption at different wavelengths in the illumination source, thereby changing the initial spectral distribution of the source. In film measurement, because the long wave signal needs to pass through the film, the energy is severely lost, the reflected signal intensity of the lower surface of the film is sharply reduced, and the signal to noise ratio is low. These influencing factors all have an influence on the signal detected by the spectrometer.
Disclosure of Invention
In order to overcome the defects of the existing illumination light source, the invention provides a wide spectrum light source for multi-channel synthesis of spectral confocal measurement and a fusion method thereof, so as to reduce the influence of the light source and a dispersion objective on a measurement signal, thereby improving the signal-to-noise ratio and the stability of the measurement signal.
The invention adopts the technical scheme for solving the technical problems that:
the invention relates to a multi-channel synthesized broad spectrum light source for spectral confocal measurement, which is characterized by comprising the following components: the LED power supply comprises a direct current switch power supply, a constant current driving circuit, an LED array, N optical fiber coupling flanges and an optical fiber branching device;
the LED array consists of N monochromatic LEDs with different dominant wavelengths and a radiator, and an ith monochromatic LED is fixed with an ith radiator through radiating silica gel; the ith optical fiber coupling flange is fixed with the ith radiator through a screw, and an ith optical channel is formed by the ith monochromatic LED, the ith radiator and the ith optical fiber coupling flange; i=1, 2, …, N;
the constant current driving circuit is provided with N current output channels and is respectively connected with N single-color LEDs in the LED array, so that the ith current output channel provides adjustable constant driving current for the ith single-color LED;
an ith ellipsoidal mirror is arranged in the ith optical fiber coupling flange, the long axis of the ith ellipsoidal mirror is coincident with the central axis of the ith optical fiber coupling flange, and the upper focus and the lower focus of the ith ellipsoidal mirror are a pair of conjugate points and all fall on the central axis of the ith optical fiber coupling flange; the ith monochromatic LED coincides with the lower focus of the ith ellipsoidal mirror;
the ith branch optical fiber of the optical fiber splitter is connected with the ith optical fiber coupling flange through the ith optical fiber coupling interface, and an incident port of a fiber core of the ith branch optical fiber is overlapped with an upper focus of the ith ellipsoidal mirror;
the outgoing light of the ith monochromatic LED in the ith optical channel is reflected by the ith ellipsoidal mirror, converged at the upper focus and coupled into the inner part of the fiber core of the ith branch optical fiber, and fused with the outgoing light in other optical channels at the fusion joint of the fiber cores of the N branches, and finally the fused broad spectrum light source is output from the total path optical fiber.
The fusion method of the multi-channel synthesized wide spectrum light source for spectral confocal measurement is characterized by comprising the following steps:
step 1.1, measuring spectral distribution data of N dominant wavelength monochromatic LEDs by utilizing a spectrometer and carrying out normalization processing;
step 1.2, calculating the peak wavelength and the spectral bandwidth of N dominant wavelength monochromatic LEDs according to the normalized spectral data;
step 1.3, establishing a maximum light intensity proportion coefficient table of the monochromatic LEDs with N dominant wavelengths by taking the monochromatic LED light intensity with the maximum spectral bandwidth as a reference value;
step 1.4, fitting the spectral distribution characteristics { S } of N dominant wavelengths according to G fitting models g,i (λ)|λ∈[λ min ,λ max ][ lambda ] represents the wavelength, [ lambda ] min ,λ max ]Representing the spectral range of the light source output S g,i (lambda) represents the spectrum distribution characteristic of the monochromatic LED with the ith dominant wavelength fitted by the g fitting model; g=1, 2, …, G; i=1, 2, …, N;
step 1.5, determining the coefficients
As an evaluation index of the fitting effect of the g-th fitting model on the spectrum characteristics of the monochromatic LED with the ith dominant wavelength;
step 1.6, evaluation index according to ith dominant wavelength
Selecting a fitting model corresponding to the maximum evaluation index as a best fitting model corresponding to the i-th monochromatic LED with dominant wavelength;
step 2.1, obtaining wavelength transmittance characteristics of the dispersion objective lens:
if the internal structure of the dispersion objective is known, the spectral transmittance characteristic T of the dispersion objective in the spectral range of the light source output is constructed according to the glass materials used by the dispersion objective and the film plating conditions of the optical surfaces Lens (λ)；
If the internal structure of the dispersion objective is unknown, a light source with known spectral distribution characteristics is used as an illumination light source, a reflecting mirror with known reflectivity is used as a measuring object, X wave peak spectrums are sampled and measured in a measuring range, the central wavelength and the light intensity of wave peaks are recorded, polynomial fitting is utilized to obtain the light source distribution characteristics,and then removing the spectral reflectance influence of the reflector to obtain a spectral curve of the illumination light source after passing through the dispersive objective, and comparing the spectral curve with a spectral curve without passing through the dispersive objective to obtain the transmittance characteristic T of the dispersive objective to the spectrum Lens (λ)；
Step 2.2, obtaining spectral reflection characteristics of the surface to be measured;
if the spectral reflectance of the surface to be measured is known, the spectral reflectance R of the surface to be measured is characterized by using a functional expression surf (λ)；
If the spectral reflectance characteristic of the surface to be measured is unknown, a light source with known spectral reflectance characteristic is used as an illumination light source to measure the spectral reflectance characteristic after being reflected by the surface to be measured, and the spectral reflectance characteristic is compared with the spectral reflectance characteristic after not being reflected by the surface to be measured, thereby obtaining the spectral reflectance characteristic R of the surface to be measured surf (λ)；
Step 2.3, calculating the spectral distribution characteristic S of the fused broad spectrum light source by using the formula (1) T (λ)：
In the formula (3), S' T (lambda) is the spectral energy distribution characteristic of the light source required for measurement in the spectral range of the light source output;
S Τ ＝S×A (2)
S T ＝[S T (λ 1 )S T (λ 2 )…S T (λ k )…S T (λ K )] T (3)
S＝[S 1 (λ)S 2 (λ)…S i (λ)…S N (λ)] (4)
S i (λ)＝[S i (λ 1 )S i (λ 2 )…S i (λ k )…S i (λ K )] T (5)
A＝[A 1 A 2 …A i …A N ] T (6)
in the formulas (4) to (7), S T Representing the spectral distribution characteristics of the fused broad spectrum light source, S T (λ k ) Representing the fused broad spectrum light source at the kth wavelength lambda k S represents the spectrum distribution characteristic collection of all the single-color LED lamp outputs, S i (lambda) represents the spectral distribution characteristic of the i-th dominant wavelength of the single-color LED, S i (λ k ) A single color LED representing the ith dominant wavelength at the kth wavelength lambda k A is a parameter set related to a constant driving current outputted from the current output channel, a i Representing a parameter related to a constant driving current of the i-th dominant wavelength monochromatic LED;
in min (S) Τ -sxa) is used as an optimization target and is solved by means of a genetic algorithm to obtain a set of optimal constant drive current related parameters a * ；
If the sample to be measured is a transparent film, after the step 2.2, the step 3 is executed after the spectral characteristics are adjusted according to the following process;
step a, measuring the surface spectral reflectance characteristic R of the film to be measured according to the process of step 2.2 Film (λ)；
Step (a)b. The light source with known spectral distribution characteristics is taken as an illumination light source, the spectral distribution characteristics of the light source after passing through the film are measured by adopting a transmission method and compared with the spectral distribution characteristics without passing through the film, thereby obtaining the spectral transmittance characteristic T of the film Film (λ)；
Step c, calculating the adjusted spectral distribution characteristic S of the light source by using the formula (1) Film (λ)。
If the sample to be measured is a transparent film, after the primary measurement in the step 5 is finished, the spectral characteristics of the secondary light source are adjusted according to the following process:
step 6.1, primarily measuring the peak wavelength corresponding to each surface of the sample to be measured, so as to enable lambda' p1 A peak wavelength representing the peak corresponding to the upper surface of the film, lambda 'is given by' p2 A peak wavelength representing a corresponding peak of the lower surface of the thin film;
step 6.2, adjusting the peak wavelength lambda 'respectively' p1 And lambda' p2 Centered, the light intensity corresponding to the wavelength within the Δλ bandwidth
And->
Thereby obtaining the spectral distribution characteristic S of the novel broad spectrum light source TFilm2 (λ)；
And 6.3, repeating the step 3, the step 4 and the step 5 to finish the measurement of the film sample.
Compared with the prior art, the invention has the beneficial effects that:
1. the invention adopts a plurality of monochromatic LEDs to combine and construct a wide spectrum light source with adjustable spectrum, can change the spectrum distribution emitted by the light source according to different test requirements, overcomes the defect that the spectrum energy distribution of the traditional light source is not adjustable, and expands the application range of the spectrum confocal sensor.
2. The invention adopts the spectrum adjustable light source as the illumination light source of the spectrum confocal sensor, overcomes the defect that the emergent spectrum distribution of the traditional light source is not adjustable, eliminates the modulation effect of the non-uniform envelope formed by the emergent spectrum of the traditional light source on the peak value of the measured signal, and reduces the influence of the non-uniformity of the spectrum distribution of the light source on the signal-to-noise ratio of the measured signal.
3. The invention adopts the spectrum adjustable light source as the illumination light source of the spectrum confocal sensor, can effectively eliminate the influence of the dispersion objective lens on the inconsistent light wave transmittance of different wavelengths in the emergent spectrum of the light source, reduces the change of the spectrum energy distribution of the light source caused by the inconsistent light wave transmittance of the dispersion objective lens, and further improves the signal to noise ratio of the measurement signal.
4. The invention adopts the spectrum adjustable light source as the illumination light source of the spectrum confocal sensor, can timely adjust the spectrum distribution output by the light source according to the test condition in the film measurement application, compensates the absorption loss of the long-wave wavelength energy in the film layer, improves the intensity of the long-wave measurement signal, and further improves the signal-to-noise ratio of the measurement signal.
Drawings
FIG. 1 is a schematic diagram of a spectrum-tunable broad spectrum light source structure for multi-channel synthesis according to the present invention;
FIG. 2 is a schematic diagram of a multi-channel synthetic spectrally tunable broad spectrum light source according to the present invention;
FIG. 3 is a schematic diagram of the structure of an ellipsoidal mirror inside the optical fiber coupling flange according to the present invention;
FIG. 4 is a schematic diagram of an optical fiber splitter according to the present invention;
FIG. 5 is a spectrum distribution curve of NCSC119BT-V1 in Nissan chemical industry;
FIG. 6 is a graph comparing the effect of fitting of NCSC119BT-V1 spectral characteristics;
FIG. 7 is a schematic diagram of the principle of measuring the spectral reflectance characteristics of the surface to be measured in step 2.2 according to the present invention;
FIG. 8 is a schematic diagram of the invention when applied to film measurement;
FIG. 9 is a schematic diagram of the measuring principle of the spectral reflectance and internal spectral transmittance of the surface of the film to be measured in step a and step b according to the present invention;
reference numerals in the drawings: 1 a single color LED;2 a radiator; 3, an optical fiber coupling flange; 4, an optical fiber coupling interface; 5 fiber cores; 6 ellipsoidal mirrors; 7, focusing on an ellipsoidal mirror; 8 ellipsoidal mirror lower focus; a 9-branch optical fiber; 10 total optical fiber.
Detailed Description
As shown in fig. 1, in this embodiment, a multi-channel synthesized broad spectrum light source for spectral confocal measurement is composed of a dc switch power supply, a constant current driving circuit, an LED array, N optical fiber coupling flanges and an optical fiber splitter;
as shown in fig. 2, the LED array is composed of N single-color LEDs with different dominant wavelengths and a radiator 2, the ith single-color LED1 is fixed with the ith radiator 2 through heat dissipation silica gel, the single-color LED1 has high luminous intensity and high power, the radiator 2 is arranged to effectively and rapidly disperse heat generated by the single-color LED1, the service life of the single-color LED1 can be ensured, the output light efficiency of the single-color LED1 can be ensured, the heat dissipation silica gel is smeared at the bottom of the substrate of the single-color LED1 to increase heat conduction and heat dissipation capacity, and meanwhile, the single-color LED1 can be fixed on the radiator 2, the use of fixing screws is reduced, and the size of the optical fiber coupling flange 3 can be reduced; the ith optical fiber coupling flange 3 is fixed with the ith radiator 2 through a screw, the inside of the optical fiber coupling flange 3 is tightly contacted with the radiator, the leakage loss of the optical energy of the monochromatic LED1 is reduced, and the ith optical channel is formed by the ith monochromatic LED1, the ith radiator 2 and the ith optical fiber coupling flange 3; i=1, 2, … and N, wherein the channels are mutually independent, the number of light source channels can be changed according to the requirement, the separated structure is convenient for increasing or reducing the number of light source channels according to the test requirement, the limitation of the installation space is avoided, the structure and the layout of the light source can be flexibly adjusted according to the test environment, and the environment adaptability is good;
the constant current driving circuit is provided with N current output channels and is respectively connected with N single-color LEDs in the LED array, so that the ith current output channel provides adjustable constant driving current for the ith single-color LED 1; the constant current driving circuit preferably adopts a driving chip with wide dimming ratio range and large driving current, the wide current adjusting capability can adjust the output power of the light source in a large range, different lighting requirements are met, and the full-scale driving current can ensure that the light source has enough lighting intensity; preferably, an analog dimming mode is adopted, continuous output of current can be realized by analog dimming, the speed of spectral confocal measurement is not influenced, pulse current is output by PWM dimming, the current output frequency of the PWM dimming does not cause stroboscopic phenomenon of an LED, but the requirements of high-speed measurement cannot be met; the constant current driving circuit supplies power and outputs each channel independently, and the constant current driving circuit is not interfered with each other, so that the output control of the light source is facilitated;
as shown in fig. 3, an ith ellipsoidal mirror 6 is arranged in the ith optical fiber coupling flange 3, the long axis of the ith ellipsoidal mirror 6 coincides with the central axis of the ith optical fiber coupling flange 3, and an upper focus 7 and a lower focus 8 of the ith ellipsoidal mirror 6 are a pair of conjugate points and all fall on the central axis of the ith optical fiber coupling flange 3; the ith monochromatic LED1 is overlapped with the lower focus 8 of the ith ellipsoidal mirror 6;
as shown in fig. 4, the optical fiber splitter comprises a plurality of branch optical fibers 9 and a total optical fiber 10, the ith branch optical fiber 9 of the optical fiber splitter is connected with the ith optical fiber coupling flange 3 through the ith optical fiber coupling interface 4, and the incident port of the optical core 5 of the ith branch optical fiber 9 coincides with the upper focal point 7 of the ith ellipsoidal mirror 6; because the size of the single-color LED1 and the port size of the branch optical fiber 9 are known and fixed, when the optical fiber coupling flange 3 is designed, the equivalent luminous point of the single-color LED1 and the position of the incident port of the fiber core 5 are brought into the design of the ellipsoidal mirror 6, so that the two points are respectively positioned on the lower and upper focuses of the long axis of the ellipsoidal mirror 6, and for some single-color LEDs 1 with different sizes, parts such as gaskets can be added to adjust the single-color LED1 to be positioned at the focus position;
compared with the traditional scheme of lens coupling optical fibers, the integrated structure of the optical fiber coupling flange 3 with the built-in ellipsoidal mirror 6 is beneficial to reducing the complexity of a system, and meanwhile, the optical fiber coupling flange 3 and the monochromatic LED1 are convenient to assemble; the ellipsoidal mirror 6 is a polished surface or an aluminized film, so that the light reflection capability is improved, as much emergent light of the monochromatic LEDs 1 as possible is converged and coupled into the fiber cores 5 of the branch optical fibers 9, and is fused with emergent light in other optical channels at the fusion positions of the N branch fiber cores 5, finally, the fused broad spectrum light source is output from the total optical fiber 10, the branch optical fibers 9 can transmit light in multiple modes by adopting multimode optical fibers, the transmission efficiency is higher, the fiber cores 5 of the branch optical fibers 9 have the same diameter, the split ratio among the branch optical fibers 8 is the same, and the fusion of the broad spectrum can be ensured not to be influenced by the coupling sequence of the branch optical fibers 8.
In this embodiment, a fusion method of a broad spectrum light source for multi-channel synthesis of spectral confocal measurement is performed according to the following steps:
step 1.1, under the same driving current and integration time, utilizing a spectrometer to measure the spectrum distribution data of N dominant wavelength monochromatic LEDs and carrying out normalization processing, wherein the external interference and operation errors in the measuring process can be eliminated by using the output result of multiple averages in the measuring process, and the truest spectrum data of the monochromatic LEDs are reserved;
step 1.2, calculating the peak wavelength and the spectral bandwidth of N dominant wavelength monochromatic LEDs according to the normalized spectral data;
step 1.3, the maximum light intensity proportion coefficient table of the monochromatic LEDs with N dominant wavelengths is established by taking the monochromatic LED light intensity with the largest spectral bandwidth as a reference value, the monochromatic LEDs 1 have the characteristic of narrow bandwidth, but the bandwidths of the LEDs with different dominant wavelengths are different, and in the process of spectrum synthesis, the monochromatic LEDs with wider bandwidths generally play an important role, so the light intensity of the monochromatic LEDs with the largest bandwidth is taken as the reference value, and the maximum light intensity proportion coefficient table of each wavelength is established, so that the role of each LED in synthesizing a wide spectrum can be solved more effectively;
step 1.4, the spectrum distribution bandwidth of the single-color LED is narrower, usually 10-20nm, and the distribution characteristics are similar to Gaussian distribution, as shown in FIG. 5, which is a NCSC119BT-V1 spectrum distribution curve of Riya chemical industry, and the characteristics determine that the single-color LED is suitable for spectrum synthesis; the spectral distribution of a typical monochromatic LED exhibits a gaussian-like distribution, so that the gaussian function can be used to extract discrete data of the measured spectral distribution or a product vendorCarrying out nonlinear fitting on the supplied measurement data to obtain a monochromatic LED spectrum distribution function S (lambda); because the Gaussian distribution function is bilaterally symmetrical, errors are generated when the Gaussian distribution function is used for fitting an asymmetric LED spectrum, and therefore the Gaussian-Lorentz piecewise function, the asymmetric Gaussian function, the asymmetric Lorentz function and the two-dimensional state density function based on the luminous characteristics of the LED material are sequentially used for fitting the monochromatic LED spectrum distribution; the fitting method applicable to different monochromatic LEDs may be different, in this embodiment, the fitting degree of the fitting method is evaluated for each monochromatic LED, table 1 is a mathematical expression and a name abbreviation of 8 fitting models used in the present invention, and the 8 fitting models are used to fit the spectral distribution characteristics { S } of the N dominant wavelengths g,i (λ)|λ∈[λ min ,λ max ][ lambda ] represents the wavelength, [ lambda ] min ,λ max ]Representing the spectral range of the light source output, visible light is typically used as the light source, i.e., lambda, in spectral confocal measurement applications min ＝400nm，λ max ＝700nm；S g,i (lambda) represents the spectrum distribution characteristic of the monochromatic LED with the ith dominant wavelength fitted by the g fitting model; g=1, 2, …,8,i =1, 2, …, N;
TABLE 1 fitting model and mathematical expression thereof
Step 1.5, determining the coefficients
As an evaluation index of the fitting effect of the g-th fitting model on the spectral characteristics of the monochromatic LED of the ith dominant wavelength, the coefficient +.>
As described by formula (1):
in the formula (1), k=1, 2, …, K represents the number of wavelengths set in the output spectrum range of the light source,
K∈N * ，λ k ＝λ min ++ (k-1) ×dλ, dλ represents the interval of the values of wavelengths, +.>
A single color LED representing the ith dominant wavelength at the kth wavelength lambda k Normalized spectral intensity on S g,i (λ k ) Represents that the g-th fitting model is applied to the single-color LED with the ith dominant wavelength at the kth wavelength lambda k Fitting light intensity on>
A mathematical expectation representing the normalized spectral intensity of an ith dominant wavelength single-color LED over K wavelengths; the closer the evaluation index is to 1, the closer the fitting effect is to the actual condition, and the better the fitting effect is;
step 1.6, evaluation index according to ith dominant wavelength
Selecting a fitting model corresponding to the maximum evaluation index as a best fitting model corresponding to the i-th monochromatic LED with dominant wavelength; as shown in FIG. 6, which is a graph showing a comparison of the fitting effect of NCSC119BT-V1 spectral characteristics, it can be seen that the two-dimensional state density function of the LED is the best fitting model, and LEDs of other models also select the best fitting model according to the method;
step 2.1, obtaining wavelength transmittance characteristics of a dispersion objective lens, wherein the dispersion objective lens is a core of a spectral confocal sensor, and in order to uniformly disperse a wide-spectrum light source in the axial direction, the dispersion objective lens is usually made of at least three glass materials with larger Abbe number differences; for a certain glass, the transmittance of the glass for different wavelengths is different, and the glass is slightly different due to the thickness variation; different kinds of glass have great difference in light transmittance of the same medium wavelength; therefore, the spectrum transmission characteristic of the whole dispersion objective lens is changed according to the wavelength, and the inconsistency of the spectrum transmission characteristic can cause the change of the intensity of a measurement signal and reduce the signal to noise ratio of a part of wavelength measurement signals, so that the spectrum transmission characteristic of the dispersion objective lens is brought in when the distribution characteristic of a target light source is set so as to eliminate the adverse effect brought in by the dispersion objective lens; the method can be specifically divided into two cases of known internal structures of the dispersion objective lens and unknown internal structures of the dispersion objective lens, wherein the internal structures of the dispersion objective lens mainly comprise glass materials used by each lens, lens thickness and film plating conditions of optical surfaces:
if the internal structure of the dispersion objective is known, the spectral transmittance of the optical glass of different materials can be obtained by inquiring a glass manual according to the glass materials used by the dispersion objective and the coating conditions of all optical surfaces, but only have the transmittance corresponding to a plurality of discrete wavelengths, and the distribution rule of the discrete transmittance data is similar to an exponential function, so that the exponential function is utilized to fit to obtain the spectral transmittance characteristic tau of each glass material in the output spectral range of the light source L_m (lambda), the transmittance of the optical surface coating can be obtained from the manufacturer S_q (lambda) and constructing the spectral transmittance characteristic T of the dispersive objective lens in the spectral range of the light source output by using the formula (2) Lens (λ)；
In the formula (2), τ L_m (lambda) represents spectral transmittance characteristics of an mth lens in a dispersive objective lens in a spectral range of light source output, τ S_q (lambda) represents the spectral transmittance characteristic of the qth optical surface of the dispersive objective in the spectral range of the light source output;
if the internal structure of the dispersion objective is unknown, a light source with known spectral distribution characteristics is used as an illumination light source, a reflector with known reflectivity is used as a measuring object, X wave peak spectrums are sampled and measured in a measuring range, the central wavelength and the light intensity of wave peaks are recorded, and then polynomial simulation is utilizedThe light source distribution characteristics are obtained, the spectral reflectance influence of the reflector is removed, so that the spectral curve of the illumination light source after passing through the dispersion objective is obtained, and the spectral curve is compared with the spectral curve without passing through the dispersion objective, so that the transmittance characteristic T of the dispersion objective to the spectrum is obtained Lens (λ)；
Step 2.2, obtaining spectral reflection characteristics of the surface to be measured;
if the spectral reflectance of the surface to be measured is known, the spectral reflectance R of the surface to be measured is characterized by using a functional expression surf (λ)；
If the spectral reflectance characteristics of the surface to be measured are unknown, as shown in fig. 7, a light source with known spectral reflectance characteristics is used as an illumination light source, and is coupled into an optical fiber and then is emitted in parallel through an optical fiber collimator, light passes through a half-reflecting half-lens and then is vertically incident to the surface to be measured, the light is reflected by the surface to be measured and then enters a light path where a spectrometer is located through the half-reflecting mirror, the spectral reflectance characteristics after being reflected by the surface to be measured are measured by the spectrometer, the influence of the half-reflecting half-lens and the optical fiber collimator is removed, and the light is compared with the spectral reflectance characteristics after being not reflected by the surface to be measured, so that K wavelengths of light { lambda of the surface to be measured, which is set in the light source output spectral range, are obtained 1 ,λ 2 ,…,λ k ,…λ K The reflectance set r (λ) = [ r (λ) 1 )r(λ 2 )…r(λ k )…r(λ K )]，r(λ k ) Represents the k-th wavelength lambda of the surface to be measured in the spectrum range of the light source k Then polynomial fitting is carried out on the reflectivity set R (lambda) to obtain the spectral reflection characteristic R of the surface to be measured surf (λ)；
Step 2.3, calculating the spectrum distribution characteristic S of the fused broad spectrum light source by using the step (3) T (λ)：
In the formula (3), S' T (lambda) is the spectral energy distribution characteristic of the light source required for measurement in the spectral range of the light source output; s'. T (λ) Can be adjusted according to different test environments and requirements, and S 'in displacement measurement' T (lambda) is preferably set to "iso-white";
if the sample to be measured is a transparent film, as shown in fig. 8, when the spectral confocal sensor is applied to film measurement, the light intensity of the second peak is reduced due to internal absorption of the film, the peak center wavelength corresponding to the upper surface of the film is smaller, because the distance between the surface and the dispersive objective lens is minimum, the peak center wavelength corresponding to the lower surface is larger, the peak wavelength distance corresponding to the two surfaces reflects the thickness of the film, and if the thickness of the film is larger, the center wavelength corresponding to the second peak is larger, and vice versa; because the light with the wavelength corresponding to the second peak passes through the film and is converged on the lower surface of the film, part of the light energy is reflected by the upper surface of the film, part of the light energy is absorbed by the inside of the film, and part of the light energy is transmitted by the lower surface of the film, the energy reflected back to the spectrometer is seriously lost, and the signal to noise ratio of the signal of the second peak is seriously influenced; the spectral characteristics output by the light source can be adjusted according to the internal spectral transmission characteristics and the surface reflection characteristics of the film, the intensity of the second peak signal is improved, and the spectral characteristics can be adjusted according to the following process;
step a, as shown in FIG. 9, the surface spectral reflectance characteristic R of the film to be measured is measured according to the procedure of step 2.2 Film (λ)；
Step b, as shown in fig. 9, using a light source with known spectral distribution characteristics as an illumination light source, measuring the spectral distribution characteristics of the light source after passing through the film by adopting a transmission method, combining the surface spectral reflection characteristics measured in the step a, and comparing the surface spectral reflection characteristics with the spectral distribution characteristics without passing through the film to obtain the spectral transmittance characteristics T of the film Film (λ)；
Step c, calculating the adjusted light source spectrum distribution characteristic S by using the formula (3) Film (λ)；
S Τ ＝S×A (4)
S T ＝[S T (λ 1 )S T (λ 2 )…S T (λ k )…S T (λ K )] T (5)
S＝[S 1 (λ)S 2 (λ)…S i (λ)…S N (λ)] (6)
S i (λ)＝[S i (λ 1 )S i (λ 2 )…S i (λ k )…S i (λ K )] T (7)
A＝[A 1 A 2 …A i …A N ] T (8)
in the formulas (4) to (7), S T Representing the spectral distribution characteristics of the fused broad spectrum light source, S T (λ k ) Representing the fused broad spectrum light source at the kth wavelength lambda k S represents the spectrum distribution characteristic collection of all the single-color LED lamp outputs, S i (lambda) represents the spectral distribution characteristic of the i-th dominant wavelength of the single-color LED, S i (λ k ) A single color LED representing the ith dominant wavelength at the kth wavelength lambda k A is a parameter set related to a constant driving current outputted from the current output channel, a i Representing a parameter related to a constant driving current of the i-th dominant wavelength monochromatic LED;
in min (S) Τ -sxa) is used as an optimization target and is solved by means of a genetic algorithm to obtain a set of optimal constant drive current related parameters a * ；
if the sample to be measured is a transparent film, after the primary measurement in the step 5 is finished, the spectral characteristics of the secondary light source can be adjusted according to the following process:
step 6.1, primarily measuring the peak wavelength corresponding to each surface of the sample to be measured, so as to enable lambda' p1 A peak wavelength representing the peak corresponding to the upper surface of the film, lambda 'is given by' p2 The peak wavelength of the corresponding peak on the lower surface of the film is represented, and during the primary measurement, the spectral reflection characteristic of the film surface is only brought in the setting process of the target light source spectrum, and the signal intensity of the second peak is not compensated because the wavelength position corresponding to the second peak cannot be determined before measurement, and the light intensity compensation can be carried out only by secondary adjustment;
step 6.2, adjusting the peak wavelength lambda 'by using the formulas (9) and (10)' p1 And lambda' p2 Centered, the light intensity corresponding to the wavelength within the Δλ bandwidth
And
thereby obtaining the spectral distribution characteristic S of the novel broad spectrum light source TFilm2 (λ)；
In the formula (9), h=1, 2, …, H represents the number of wavelengths set in the Δλ bandwidth range,
H∈N * ，/>
δλ represents the interval of the values of the wavelengths; Δλ is the peak wavelength λ' p1 And lambda' p2 The bandwidth of the filter is properly expanded on the basis of the peak wavelength, so that the error in the process of extracting the peak wavelength can be avoided; s is S test (λ′ p1_h ) And S is test (λ′ p2_h ) Respectively the wavelength lambda' p1_h And lambda' p2_h Corresponding light intensity, R Film (λ′ p1_h ) And R is Film (λ′ p2_h ) Respectively represent the film versus wavelength lambda' p1_h And lambda' p2_h T, T Film (λ′ p2_h ) Indicating the film vs. wavelength lambda' p2_h Transmittance of S TFilm (λ′ p1_h ) And S is TFilm (λ′ p2_h ) Respectively the wavelength lambda' p1_h And lambda' p2_h The adjusted corresponding light intensity; because the light with the wavelength corresponding to the first wave crest is reflected back to the spectrometer on the first measured surface, the transmission loss of the first wave crest on the first measured surface only needs to be compensated for; for the second peak, a part of energy is lost due to surface reflection when passing through the first measuring surface, a part of energy is lost due to material absorption in the film, and finally a part of light energy is lost due to transmission in the second surface, so that the spectral distribution characteristic S of the novel broad spectrum light source is obtained TFilm2 (λ)；
And 6.3, repeating the step 3, the step 4 and the step 5 to finish the measurement of the film sample.
Claims (4)
1. A multi-channel synthetic broad spectrum light source for spectral confocal measurement, comprising: the LED power supply comprises a direct current switch power supply, a constant current driving circuit, an LED array, N optical fiber coupling flanges and an optical fiber branching device;
the LED array consists of N monochromatic LEDs with different dominant wavelengths and a radiator (2), and an ith monochromatic LED (1) is fixed with the ith radiator (2) through radiating silica gel; the ith optical fiber coupling flange (3) is fixed with the ith radiator (2) through a screw, and an ith optical channel is formed by the ith monochromatic LED (1), the ith radiator (2) and the ith optical fiber coupling flange (3); i=1, 2, …, N;
the constant current driving circuit is provided with N current output channels and is respectively connected with N single-color LEDs in the LED array, so that the ith current output channel provides adjustable constant driving current for the ith single-color LED (1);
an ith ellipsoidal mirror (6) is arranged in the ith optical fiber coupling flange (3), the long axis of the ith ellipsoidal mirror (6) coincides with the central axis of the ith optical fiber coupling flange (3), and an upper focus (7) and a lower focus (8) of the ith ellipsoidal mirror (6) are a pair of conjugate points and all fall on the central axis of the ith optical fiber coupling flange (3); the ith monochromatic LED (1) is overlapped with the lower focus (8) of the ith ellipsoidal mirror (6);
an ith branch optical fiber (9) of the optical fiber splitter is connected with an ith optical fiber coupling flange (3) through an ith optical fiber coupling interface (4), and an incident port of a fiber core (5) of the ith branch optical fiber (9) is overlapped with an upper focus (7) of an ith ellipsoidal mirror (6);
the outgoing light of the ith monochromatic LED (1) in the ith optical channel is reflected by the ith ellipsoidal mirror, is converged at the upper focus (7) and is coupled into the inner part of the fiber core (5) of the ith branch optical fiber (8), and is fused with the outgoing light in other optical channels at the fusion joint of the N branch fiber cores (5), and finally the fused broad spectrum light source is output from the total optical fiber (10).
2. A fusion method based on the multi-channel synthetic broad spectrum light source for spectral confocal measurement according to claim 1, characterized by the following steps:
step 1, establishing a mathematical model of spectral distribution characteristics of N dominant wavelength monochromatic LEDs and establishing a maximum light intensity proportion coefficient table:
step 1.1, measuring spectral distribution data of N dominant wavelength monochromatic LEDs by utilizing a spectrometer and carrying out normalization processing;
step 1.2, calculating the peak wavelength and the spectral bandwidth of N dominant wavelength monochromatic LEDs according to the normalized spectral data;
step 1.3, establishing a maximum light intensity proportion coefficient table of the monochromatic LEDs with N dominant wavelengths by taking the monochromatic LED light intensity with the maximum spectral bandwidth as a reference value;
step 1.4, fitting the spectral distribution characteristics { S } of N dominant wavelengths according to G fitting models g,i (λ)|λ∈[λ min ,λ max ][ lambda ] represents the wavelength, [ lambda ] min ,λ max ]Representing the spectral range of the light source output S g,i (lambda) represents the spectrum distribution characteristic of the monochromatic LED with the ith dominant wavelength fitted by the g fitting model; g=1, 2, …, G; i=1, 2, …, N;
step 1.5, determining the coefficients
As an evaluation index of the fitting effect of the g-th fitting model on the spectrum characteristics of the monochromatic LED with the ith dominant wavelength;
step 1.6, evaluation index according to ith dominant wavelength
Selecting a fitting model corresponding to the maximum evaluation index as a best fitting model corresponding to the i-th monochromatic LED with dominant wavelength;
step 2, setting the spectral distribution characteristics of the target light source:
step 2.1, obtaining wavelength transmittance characteristics of the dispersion objective lens:
if the internal structure of the dispersion objective is known, the spectral transmittance characteristic T of the dispersion objective in the spectral range of the light source output is constructed according to the glass materials used by the dispersion objective and the film plating conditions of the optical surfaces Lens (λ)；
If the internal structure of the dispersion objective is unknown, a light source with known spectral distribution characteristics is used as an illumination light source, a reflector with known reflectivity is used as a measuring object, X wave peak spectrums are sampled and measured in a measuring range, the central wavelength and the light intensity of the wave peaks are recorded, the polynomial fitting is utilized to obtain the light source distribution characteristics, the spectral reflectivity influence of the reflector is removed, and the spectral curve of the illumination light source after passing through the dispersion objective is obtained and compared with the spectral curve without passing through the dispersion objectiveThereby obtaining the transmittance characteristic T of the dispersive objective lens to the spectrum Lens (λ)；
Step 2.2, obtaining spectral reflection characteristics of the surface to be measured;
if the spectral reflectance of the surface to be measured is known, the spectral reflectance R of the surface to be measured is characterized by using a functional expression surf (λ)；
If the spectral reflectance characteristic of the surface to be measured is unknown, a light source with known spectral reflectance characteristic is used as an illumination light source to measure the spectral reflectance characteristic after being reflected by the surface to be measured, and the spectral reflectance characteristic is compared with the spectral reflectance characteristic after not being reflected by the surface to be measured, thereby obtaining the spectral reflectance characteristic R of the surface to be measured surf (λ)；
Step 2.3, calculating the spectral distribution characteristic S of the fused broad spectrum light source by using the formula (1) T (λ)：
In the formula (3), S' T (lambda) is the spectral energy distribution characteristic of the light source required for measurement in the spectral range of the light source output;
step 3, establishing a target light source spectrum distribution characteristic solving equation by using the formulas (2) to (6), and solving driving current related parameters of each channel:
S Τ ＝S×A (2)
S T ＝[S T (λ 1 )S T (λ 2 )…S T (λ k )…S T (λ K )] T (3)
S＝[S 1 (λ)S 2 (λ)…S i (λ)…S N (λ)] (4)
S i (λ)＝[S i (λ 1 )S i (λ 2 )…S i (λ k )…S i (λ K )] T (5)
A＝[A 1 A 2 …A i …A N ] T (6)
in the formulas (4) to (7), S T Representing the spectral distribution characteristics of the fused broad spectrum light source, S T (λ k ) Representing the fused broad spectrum light source at the kth wavelength lambda k S represents the spectrum distribution characteristic collection of all the single-color LED lamp outputs, S i (lambda) represents the spectral distribution characteristic of the i-th dominant wavelength of the single-color LED, S i (λ k ) A single color LED representing the ith dominant wavelength at the kth wavelength lambda k A is a parameter set related to a constant driving current outputted from the current output channel, a i Representing a parameter related to a constant driving current of the i-th dominant wavelength monochromatic LED;
in min (S) Τ -sxa) is used as an optimization target and is solved by means of a genetic algorithm to obtain a set of optimal constant drive current related parameters a * ；
Step 4, according to the optimal constant driving current related parameter A * Solving the driving current { I ] corresponding to the N dominant wavelength monochromatic LEDs according to the maximum light intensity proportion coefficient table of the monochromatic LEDs 1 ,I 2 ,…,I N Calculating the sum of the numbers of the N dominant wavelength monochromatic LEDs according to the maximum driving current which can be born by the N dominant wavelength monochromatic LEDs and using the sum as the channel number of the synthesized wide-spectrum light source;
step 5, according to the driving current { I } 1 ,I 2 ,…,I N The single-color LEDs driving the N light channels generate stable illumination, and a fused broad-spectrum light source is output from the total path optical fiber (9) of the optical fiber branching device (3) so as to be transmitted into a dispersion objective lens to complete related measurement work.
3. The fusion method according to claim 2, wherein if the sample to be measured is a transparent film, step 3 is performed after adjusting the spectral characteristics according to the following procedure after step 2.2;
step a, measuring the surface spectral reflectance characteristic R of the film to be measured according to the process of step 2.2 Film (λ)；
Step b, taking a light source with known spectral distribution characteristics as an illumination light source, and measuring the light source passing through by adopting a transmission methodThe spectral distribution characteristics after the film are compared with those of the film which does not pass through, thereby obtaining the spectral transmittance characteristics T of the film Film (λ)；
Step c, calculating the adjusted spectral distribution characteristic S of the light source by using the formula (1) Film (λ)。
4. The fusion method according to claim 3, wherein if the sample to be measured is a transparent film, after the primary measurement in step 5 is completed, the spectral characteristics of the secondary light source are adjusted as follows:
step 6.1, primarily measuring the peak wavelength corresponding to each surface of the sample to be measured, so as to enable lambda' p1 A peak wavelength representing the peak corresponding to the upper surface of the film, lambda 'is given by' p2 A peak wavelength representing a corresponding peak of the lower surface of the thin film;
step 6.2, adjusting the peak wavelength lambda 'respectively' p1 And lambda' p2 Centered, the light intensity corresponding to the wavelength within the Δλ bandwidth
And->
Thereby obtaining the spectral distribution characteristic S of the novel broad spectrum light source TFilm2 (λ)；
And 6.3, repeating the step 3, the step 4 and the step 5 to finish the measurement of the film sample.
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