Patent Application
20050270526
This application is based on the application No. 2004-166618 filed in Japan, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a spectral sensitivity composing system in a colorimeter or the like.
2. Description of the Related Art
In a colorimeter for measuring the color values of incident light, the light need to be measured with the spectral sensitivity approximating a predetermined spectral sensitivity, specifically, the standard observer's color matching function recommended by CIE (International Commission on Illumination). With recent increase in the use of monochromatic light sources and light sources composed of a combination of monochromatic lights, such as liquid crystal displays, plasma displays, and LEDs, demand is increasing for the precision in the approximation.
In order to realize a predetermined spectral sensitivity, the following two methods have been used conveniently. In a first method, a plurality of optical filters are used in combination (a filter method, hereafter). In a second method, light to be measured is separated into a plurality of wavelength components by wavelength separating means, and then received respectively by plural elements of light receiving means (pixel sensors, hereafter). The output of each pixel sensor is multiplied each by the specific weight factor, and then added together (spectrophotometric method, hereafter). The difference between the filter and the spectrophotometric method is summarized in the following Table 1.
As shown in Table 1, in the filter method where a predetermined spectral sensitivity is approximated by a combination of optical filters, the light to be measured is received by a single light receiving element via the combined filters. Accordingly, the wavelength range of received light is wide and the light receiving element of large receiving area can be used. As the result, this method has an advantage of high signal current resulting good S/N ratio. Additionally filter method has advantages of the small circuit scale and the short processing time as all wavelength components in the spectral sensitivity are received simultaneously, and an advantage of the wide dynamic range. However, restrictions in the available filters and variation in characteristics of each filter cause a difficulty in achieving a desired precision in the approximation. Thus, in recent years, the abovementioned spectrophotometeric method becomes dominant in practice.
In the above-mentioned spectrophotometeric method, the light to be measured in the visible wavelength range of 400-700 nm is separated into 31 wavelength components with 10 nm interval for example by wavelength separator as shown in
An object of this invention is to provide a spectral sensitivity composing system which precisely approximates a desired spectral sensitivity but is small in circuit scale and high in S/N ratio.
In order to achieve the above-mentioned object, a spectral sensitivity composing system for composing a predetermined spectral sensitivity comprises: a wavelength separator for separating light to be measured into a plurality of wavelength components; a plurality of sensors for receiving the wavelength components separated by said wavelength separator; current dividers each provided to each of said pixel sensors for dividing an output current from the corresponding sensor at an arbitrary ratio; a summing amplifier for receiving the current components outputted from said current dividers and thereby outputting a signal corresponding to the sum of the received current components; and a setting circuit for setting a dividing ratio corresponding to a predetermined spectral sensitivity in each of said current dividers.
This configuration provides a spectral sensitivity composing system which precisely approximates a desired spectral sensitivity but is small in circuit scale and high in S/N ratio.
In the following description, like parts are designated by like reference numbers throughout the several drawings.
The sensor array SA consists of 31 silicon photodiodes or the like arranged at regular intervals (pixel sensors, hereafter). These pixel sensors together with other components of the polychromater 4 have respective spectral sensitivities R0(λ)-R30(λ) shown in
A shutter 8 driven by a drive unit 7 is provided behind the light receiving lens 3. The drive unit 7 is controlled by the arithmetic control unit 5 so that the incidence of the light 2 to be measured is controlled. The shutter 8 is closed when being out of measurement or during calibration, while is opened during measurement. The arithmetic control unit 5 acquires offset data from the signal processing circuit 6 when the shutter 8 is closed to block the incident light and on the basis of this data, corrects the output data for the light 2 to be measured.
For each of the current splitters SP0-SP30, dividing (splitting) ratio setting voltages (Vx0-Vx30, Vy0-Vy30, Vz0-Vz30, Vd0-Vd30) corresponding to the spectral sensitivity of the color matching functions x(λ), y(λ), z(λ) are generated by converting dividing (splitting) ratio data (Dx0-Dx30, Dy0-Dy30, Dz0-Dz30, Dd0-Dd30) outputted from the arithmetic control unit 5 to analog voltage by a digital to analog converter DA. Then, these voltages are successively set into the current splitters SP0-SP30 via a multiplexer MX1.
The FETs Qxk, Qyk, Qzk, Qdk are connected in parallel each other. The channel resistance of each FET can be controlled by the gate voltage, while the resistance element is shut down when a shut down voltage is applied to the gate. Accordingly, channel resistances of the FETs Qxk, Qyk, Qzk, Qdk can be adjusted by adjusting gate voltages such as to realize the dividing (splitting) ratios for composing the color matching functions x(λ), y(λ), z(λ). As such, the photocurrent Ik from the corresponding pixel sensor Pk is divided (split) into current components Ixk, Iyk, Izk corresponding to the color matching functions x(λ), y(λ), z(λ) and excess current Idk to be drained to the GND by the current splitter SPk.
Thus, the excess current Idk remaining after Ixk, Iyk, Izk is extracted to the summing amplifiers Sx, Sy, Sz by the FETs Qxk, Qyk, Qzk depending on the spectral sensitivity to be composed, is removed by the FET Qdk. This enables to precisely approximate predetermined spectral sensitivities by means of current splitting.
Further, the use of the FETs Qxk, Qyk, Qzk and the FET Qdk as the first and the second resistance elements simplifies a configuration for setting splitting ratios and also enables to change or modify splitting ratios easily if necessary for composing different spectral sensitivities for example. Furthermore, as described later, each individual system can be recalibrated after manufacturing without a complicated work by measuring a reference light source and adjusting the gate voltages based on the result of the measurement.
Thus, by setting the discharging time constants of the capacitors Cxk, Cyk, Czk, Cdk determined by the capacitance of the capacitor and the leakage resistance, sufficiently longer than the period of voltage setting by multiplexer MX2, the gate voltages are maintained at values realizing predetermined spectral sensitivities. This configuration allows setting all gate voltages for predetermined spectral sensitivities by a single set of the arithmetic control unit 5, the digital to analog converter DA, and the multiplexer MX1.
Output current I from the photodiode is expressed by the following equation.
Here, IS has a large temperature dependence. However, if VD is low enough, the value within the brackets [] in the equation above is close to zero, and accordingly the influence of IS is negligible. If VD=0.1 mV for example, the value within the brackets [] in the equation is 0.001 or the like as e/kT≈40 and hence the influence of IS is negligible.
Splitting ratio setting voltages Vxk, Vyk, Vzk, Vdk are set in the capacitors Cxk, Cyk, Czk, Cdk as described above. That is, the corresponding data Dxk, Dyk, Dzk, Ddk are successively outputted from the arithmetic control unit 5, converted into analog voltages by the digital to analog converter DA, and then set by the multiplexer MX1. Supposing that the time necessary for charging one of capacitor Cxk, Cyk, Czk, Cdk is 10 μsec, four capacitors of 31 pixel channels can be set with a period of approximately 1 msec. Thus, by giving time constants each determined by the capacitance and the leakage resistance of the capacitor sufficiently longer than the abovementioned 1 msec period to the capacitors Cxk, Cyk, Czk, Cdk, the gate voltages are precisely maintained at values set by the multiplexing for realizing predetermined spectral sensitivities, while the configuration for that is simply constituted with the arithmetic control unit 5, the digital to analog converter DA, and the multiplexer MX1.
The corresponding current components Ixk, Iyk, Izk from the current splitters SPk are respectively added together in the summing amplifiers Sx, Sy, Sz, and therein sum of current components are amplified and converted to voltage outputs Vx, Vy, Vz. Then, the voltage outputs Vx, Vy, Vz are converted to digital signal to be outputted to the arithmetic control unit 5 by the analog to digital converters ADx, ADy, ADz. Thus, the voltage outputs Vx, Vy, Vz from the summing amplifiers Sx, Sy, Sz have the spectral sensitivities corresponding to the spectral sensitivities of the color matching functions x(λ), y(λ), z(λ) set by the arithmetic control unit 5. As such, the arithmetic control unit 5 can realize the spectral sensitivities of the voltage outputs Vx, Vy, Vz from the summing amplifiers Sx, Sy, Sz approximating the color matching functions x(λ), y(λ), z(λ) of CIE 2 degree observer by outputting the splitting ratio data Dxk, Dyk, Dzk and setting the splitting ratios in accordance with the weights assigned to the pixel s via the splitting ratio setting voltages Vxk, Vyk, Vzk.
First, in the calibration process for the relative spectral sensitivity Rk(λ) at step S1, one summing amplifiers selected from Sx, Sy, Sz is used as a monitor summing amplifier. Then, monochromatic light having a half power bandwidth of 2 nm or the like and a constant energy is provided successively in 2 nm pitch from a reference monochromatic light source and received through the light receiving lens 3. Then, the output of the corresponding pixel for each monochromatic light is outputted through the monitor summing amplifier, and thereby the relative spectral sensitivity Rk(λ) of the pixel is measured. At that time, in order to avoid current components flowing into the monitoring summing amplifier from other pixels connected in parallel, the FET for the corresponding pixel is solely turned ON, while all FETs for the other pixels are turned OFF. The scan range is 350-750 nm for example which is the measurement range of 400-700 nm plus excessive bands at both ends. In case that the half-value width (2 nm) of the monochromatic light can affect the result, the half-value width is corrected by a known method (deconvolution). In case that a resolution below 2 nm is required, interpolation is performed.
Next, in the calculation of the weight factors at step S2, each pixel Pk is conveniently assumed as a virtual pixel having a relative spectral sensitivity Rk(λ). Then, the relative spectral sensitivity Rk(λ) of every virtual pixel k is multiplied by weights Wxk, Wyk, Wzk, and then added together as shown in the following equations so that composed spectral sensitivities x(λ)′, y(λ)′, z(λ)′ are obtained.
Then, the weights Wxk, Wyk, Wzk are determined such that the composed spectral sensitivities x(λ)′, y(λ)′, z(λ)′ approximate the theoretical color matching functions x(λ), y(λ), z(λ) as target. For the purpose, as shown in the following equations, the squared sums Ex, Ey, Ez of the differences between the composed and theoretical spectral sensitivities, x(λ)′, y(λ)′, z(λ)′ and x(λ), y(λ), z(λ) at every wavelength are used as evaluation functions. Then, the weights Wxk, Wyk, Wzk are determined such that the evaluation functions become within the threshold.
Next, in the calibration of the splitting ratios at step S3, the splitting ratio in the current splitter SPk of each pixel is adjusted such that pixel-to-pixel relative ratios of the currents flowing from each pixel into the summing amplifiers Sx, Sy, Sz are in accordance with the weights Wxk, Wyk, Wzk by means of measuring a reference light source A having a known spectral intensity A(λ). Adjustment targets for the splitting ratios are calculated as follows.
First, theoretical outputs ax0k, ay0k, az0k of each pixel k to be obtained when measuring the reference light source A are calculated according to the following equations.
Then, as shown in the following equations, the theoretical outputs ax0k, ay0k, az0k are multiplied by constants Cx, Cy, Cz, respectively so that bx0k, by0k, bz0k are obtained as the adjustment targets.
Calibration between outputs from summing amplifiers Sx, Sy, Sz corresponding to the color matching functions x(λ), y(λ), z(λ) is performed in color value calibration at step S5 described later. Thus, here, relative weighting between the pixels is performed for each of the color matching functions x(λ), y(λ), z(λ). The constants Cx, Cy, Cz are determined conveniently such as to maximize the efficiency of the photocurrent, that is, to minimize the drain currents flowing into the ground (GND).
In short, first, the ratios between the current components flowing from one pixel channel into the three summing amplifiers Sx, Sy, Sz are adjusted in the splitting ratio adjustment within each pixel channel, and second, the ratios between the current components flowing from the pixels into one of the three summing amplifiers Sx, Sy, Sz is adjusted in the splitting ratio adjustment between pixel channels. These procedures are described below in detail.
In the splitting ratio adjustment within each pixel channel, all FETs of the pixel channels other than the concerned pixel channel k are turned OFF, and so is the FET Qdk for setting the drain current component Idk of the concerned pixel channel k. In this state, the dividing data (splitting data) Dxk, Dyk, Dzk are adjusted such that the ratio pxk:pyk:pzk of the outputs from the three summing amplifiers Sx, Sy, Sz in response to the current components Ixk, Iyk, Izk from the FETs Qxk, Qyk, Qzk of the concerned pixel channel k becomes equal to the target ratio bx0k:by0k:bz0k. When the photocurrent is divided into all the three summing amplifiers Sx, Sy, Sz, the ratio of each two is adjusted successively. For example, the splitting data Dxk, Dyk is first adjusted such that the ratio pxk:pyk becomes equal to the target ratio bx0k:by0k. Then, the splitting data Dzk is adjusted such that the ratio pyk:pzk becomes equal to the target ratio by0k:bz0k. This procedure is performed successively for each pixel channel.
In the splitting ratio adjustment between the pixel channels, the splitting ratio within the pixel channel k is set by the splitting data Dxk, Dyk, Dzk obtained in the above-mentioned splitting ratio adjustment within the pixel channel and all FETs of the pixel channels other than the pixel channel k are turned OFF. In this state, the current components Ixk, Iyk, Izk of the concerned pixel channel k can solely be monitored as the outputs pxk, pyk, pzk of the summing amplifiers Sx, Sy, Sz.
First, a suitable pixel channel r is selected as a reference pixel channel. Then, reference outputs pxr, pyr, pzk in response to its photocurrents Ixr, Iyr, Izr are measured. After that, the outputs pxk, pyk, pzk of a concerned pixel channel k are monitored. In this state, the splitting ratio data Ddk for setting the drain current component Idk of the concerned pixel channel k is adjusted such that the ratios between the outputs pxk, pyk, pzk and the reference outputs pxr, pyr, pzr become equal to the target ratios. As the splitting ratio within the pixel channel is already adjusted, it is enough if one of the ratios, pyk:pyr for example, is adjusted to the target ratio by0k:by0r. This procedure is performed successively for each pixel channel.
Here, for avoiding the error due to the low monitoring output, the monitoring summing amplifier and the reference pixel channel are selected appropriately depending on the wavelength range. For example, on the basis of
As such, the splitting ratio for Ixk, Iyk, Izk is first adjusted within each pixel channel. Then, the amount of the drain current component Idk is adjusted such that the relative ratio between the pixels (wavelengths) is equal to the predetermined value.
Next, in the temperature correction at step S4, changes of the splitting ratios due to the temperature dependence of the channel resistance of FET are corrected. As the channel resistance of FET does not depend on the current level, the splitting ratios can be re-adjusted with the help of constant current provided to each current splitter by means of current injection or LED illumination.
Specifically, in the configuration for the first technique in which a constant current is injected as shown in
Under the same temperature condition as the calibration of the splitting ratios at step S3, the splitting data Dxk, Dyk, Dzk, Ddk acquired are set up and the shutter 8 is closed to eliminate the photocurrent Ik. Then, the multiplexer MX2 is controlled so as to inject the constant current IC to a concerned pixel channel k. Then, the outputs mx0k, my0k, mz0k of the summing amplifiers Sx, Sy, Sz are measured and stored as reference values. This procedure is performed successively for each pixel channel.
In actual measurements often at the different temperature condition from that at calibration, a constant current IC is injected and the outputs mxk, myk, mzk of the summing amplifiers Sx, Sy, Sz are measured by the same procedure as that when obtaining reference values mx0k, my0k, mz0k. Then, the splitting ratio data Dxk, Dyk, Dzk, Ddk for setting the gate voltages of the FETs are re-adjusted such that the measured outputs mxk, myk, mzk become equal to the reference values mx0k, my0k, mz0k.
On the other hand, in the second technique, a constant current is injected into the current splitter by means of illumination by an LED 15 provided in the polychromater 4 as shown in
As the emission intensity of the LED 15 however depends on the temperature, the emission intensity of the LED 15 is simultaneously measured by an LED monitor which comprises a monitor sensor and a signal processing circuit. The measured intensity is stored as the reference value M0. It should be noted that no device having the large temperature dependence is used in the LED monitor.
In actual measurements, the LED 15 emits light before or after the measurement and the outputs mxk, myk, mzk of the summing amplifiers Sx, Sy, Sz and the emission intensity M of the LED 15 are measured for each pixel channel in the same procedure as the calibration. Then, the data Dxk, Dyk, Dzk, Ddk for the gate voltages of the FETs is re-adjusted such that the outputs mxk, myk, mzk of the summing amplifiers Sx, Sy, Sz become respectively equal to the reference values M/M0 mx0k, M/M0·my0k, M/M0·mz0k corrected by the emission intensity ratio of the LED 15 M/M0.
Alternatively, a temperature sensor may be arranged near the current splitter SPk so that the arithmetic control unit may perform the abovementioned temperature correction when a temperature change exceeding a predetermined value is detected by the temperature sensor.
In the adjustment of the color values at step S5, tristimulus values Xa, Ya, Za of the reference light source A having a known spectral intensity A(λ) are measured with the colorimeter 1 to be adjusted having the spectral sensitivities set by the splitting ratio data Dxk, Dyk, Dzk, Ddk and corrected for temperature as described above. Then, coefficients Ex, Ey, Ez are determined according to the following equations such that the tristimulus values Xa, Ya, Za are in agreement with the theoretical tristimulus values Xa0, Ya0, Za0 calculated from the known spectral intensity A(λ) of the reference light source A.
Then, in actual measurements, the arithmetic control unit 5 outputs corrected tristimulus values X′, Y′, Z′ calculated from measured tristimulus values X, Y, Z using following equations.
As described above, in the colorimeter 1 according to this embodiment, predetermined spectral sensitivities such as color matching functions of CIE 2 degree observer is realized by means of combining the wavelength components for high precision in the approximation and not by means of combination of filters (aforementioned filter method) which has the high S/N ratio and the small circuit scale but poor precision in the approximation. That is, in the colorimeter 1 according to this embodiment, the predetermined spectral sensitivities are realized by a method in which the light 2 to be measured is separated into a plurality of wavelength components by a dispersing device G and the wavelength components are received respectively by pixel sensors of a sensor array SA, and then a signal processing is performed to the photocurrent outputted from the pixel sensors. A current splitter is provided to each pixel channel k for the processing. The current splitter SPk enables a photocurrent Ik from the corresponding pixel Pk to be divided at an arbitrary ratio, while an arithmetic control unit 5 sets the splitting ratio corresponding to the desired spectral sensitivities. That is, signal processing where the photocurrent Ik from each pixel sensor Pk is divided and extracted at a given ratio performs the weighting by the specific weight factors. After that, in the colorimeter 1, the summing amplifiers Sx, Sy, Sz add together the current components Ix, Iy, Iz from the current splitters respectively so that the voltage outputs Vx, Vy, Vz have composed spectral sensitivities approximating precisely the desired spectral sensitivities.
Thus, in comparison with a prior art (aforementioned spectrophotometric method) in which the photocurrent from each pixel is individually converted to digital data, numerically multiplied by the specific weight factor corresponding to the desired spectral sensitivities, and then numerically added together for composing the spectral sensitivities, the method according to this embodiment has as high precision in the approximation as a spectrophotometric method and improved S/N ratio due to the increase of input currents to the summing amplifiers Sx, Sy, Sz performing current to voltage conversion. Furthermore, the increase of the input currents to the summing amplifiers Sx, Sy, Sz reduces the influence of noise and leakage caused by the feedback resistors Rfx, Rfy, Rfz. Further, a reduced number of the current to voltage converting circuits reduces the scale and the cost of the circuit compared to the spectrophotometric method employing a parallel processing for the advantages in the signal to noise ratio and the dynamic range.
In the present embodiment, the current splitter SPka divides the inputted photocurrent Ik into two current components, Itk to be inputted to a single summing amplifier St and Idk to be drained to the ground (GND). The splitting ratio for each current splitter SPka is set by the arithmetic control unit 5a by outputting successively the splitting ratio data sets (Dxk, Ddk), (Dyk, Ddk), (Dzk, Ddk) of the corresponding color matching functions X(λ), y(λ), z(λ). The current component Itk outputted from each current splitter SPka is added together in the summing amplifier St. Then, the sum of current components is amplified and converted to voltage output by the feedback resistor Rft. The obtained voltage output Vt is converted to digital data by the analog to digital converter ADt, which is then inputted to the arithmetic control unit 5a. As such, spectral sensitivities precisely approximating the color matching functions x(λ), y(λ), z(λ) are composed sequentially. This configuration simplifies the system.
Although color matching functions of CIE 2 degree observer have been used as desired spectral sensitivities in the description given above, other spectral sensitivities such as color matching functions of CIE 10 degree (10°) observer may be used.
Alternatively, the arithmetic control unit can store splitting ratio data for both color matching functions of CIE 2 degree and 10 degree observers and either color matching functions of the two observers can be selectively set by outputting the corresponding splitting ratio data.
In the present embodiment, wavelength components are acquired by a polychromater provided with a dispersing device. However, the invention is not limited to this and the wavelength components may be acquired by a combination of a wedge type interference filter and a sensor array.
In the temperature correction at measurements, a constant current IC may be injected only into a part of the current splitters. Alternatively, the arithmetic control unit 5 or 5a may retain data for temperature correction obtained at manufacturing, and at measurements, modifies splitting ratio data based on the retained data and a temperature measured by a temperature sensor provided.
Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various change and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being including therein.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-166618 | Jun 2004 | JP | national |
