Abstract:
A trichromatic colorimeter for obtaining the CIE chromaticity coordinates for an object to be measured. By utilizing a logarithmic compression type light measuring circuit, a higher accuracy and measuring range are obtainable in comparison to prior art linear light measuring type colorimeters.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a trichromatic colorimeter by which chromaticity coordinates (x and y) of the CIE standard are obtained. 
     2. Description of the Prior Art 
     The prior art trichromatic colorimeter obtains the CIE chromaticity coordinates (x and y) using 3 or 4 sets of filters and photosensitive elements, which obtain X, Y and Z represented by equation (1) below: ##EQU1## 
     The X, Y and Z are obtained using a linear light measuring circuit, and are then applied as inputs to an operation circuit. The x and y are finally obtained by the operation of equation (2) below: ##EQU2## 
     It should be noted that P.sub.λ represents a relative spectral distribution of a light source, ρ.sub.λ, the spectral reflectance for an object to be measured, and x.sub.λ, y.sub.λ and z.sub.λ, the spectral tristimulus values of the CIE standard colorimetric observer in equations (1) and (2) above. The x.sub.λ, y.sub.λ and z.sub.λ have their own characteristics as shown in FIG. 1. It is also noted that ρ.sub.λ is equal to 1 (ρ.sub.λ =1) when a light source color is an object to be measured. 
     When a linear light measuring circuit is used for the above light measuring device, a high power voltage is required to widen a measuring range. Also, this poses a problem of low measuring accuracy. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to provide a photoelectric colorimeter which obviates the above problems using a logarithmic compression type light measuring circuit, by which a measuring range is widened for the same power voltage as supplied by a linear light measuring, and high accuracy display of chromaticity coordinates is possible. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a graph of spectral tristimulus values. 
     FIG. 2 is a schematic block diagram of an embodiment of the present invention. 
     FIG. 3 is a block diagram schematically illustrating an embodiment of the computation circuit of FIG. 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Description will now be given of the preferred embodiment of the present invention in conjunction with the accompanying drawings. 
     FIG. 2 is a schematic block diagram of the embodiment of the present invention. Filters are labeled F1 through F3, photosensitive elements, (silicon photodiodes), are labeled PD1 through PD3. A combination of F1 and PD1 produces a spectral sensitivity of spectral stimulus value x.sub.λ &#39; (FIG. 1), a combination of F2 and PD2 produces a spectral sensitivity of spectral stimulus value y.sub.λ, and a combination of F3 and PD3 produces a spectral sensitivity of spectral stimulus value z.sub.λ or x.sub.λ&#34;. Diodes D1 through D3 convert the output currents of photosensitive elements PD1 through PD3 into the respective logarithmically compressed voltages. Operation amplifiers OA1 through OA3 are connected to diodes D1 through D3. Variable voltage sources E1 through E3 are used for adjustment. Consequently, the voltage available from operation amplifier OA1 corresponds to the logarithmic compression value of a light quantity (stimulus value X&#39;) for spectral stimulus value xλ&#39;. 
     
         QV.sub.X &#39;=k.sub.X &#39;·log.sub.2 X&#39;                 (3) 
    
     The voltages available from operation amplifiers OA2 and OA3 correspond to the logarithmic compression values of light quantities (stimulus values Y,Z,X&#34;) for spectral stimulus values y.sub.λ and z.sub.λ x.sub.λ&#34;, respectively. 
     
         QVy=k.sub.y ·log.sub.2 y                          (4) 
    
     
         QV.sub.z =k.sub.z ·log.sub.2 Z=α+k.sub.x &#34;·log.sub.2 X&#34;                                   (5) 
    
     
         (k.sub.x &#39;=k.sub.x &#34;=k.sub.y =k.sub.z =1) (α is a constant) 
    
     The voltages are supplied through an analog multiplexer 2 to A-D converter 4 in a time series sequence for conversion into digital signals, which are entered in a register inside computation circuit 6. Based on the three data, words QV x  &#39;, QV y  and QV z  entered in the register, predetermined calculations are carried out by the computation circuit 6 thereafter to obtain chromaticity coordinates (x and y) which are digitally displayed by display circuit 8. It should be understood that control circuit 10 controls multiplexer 2, A-D converter 4, computation circuit 6 and display circuit 8. Description is now given of the actual calculations out by computation circuit 6. First, QV is obtained from logarithmic compression values QV x&#39;  and QV z  of light quantities (stimulus values) for spectral stimulus values x.sub.λ &#39; and z.sub.λ. It should be noted that light quantity (stimulus value) X is written as X=2 QV  x &#39;  +2 QV  x &#34;  =2 QVx . 
     First, QV z  -α=QV x  &#34; is obtained. Then, QV x  &#39;-QV x  &#34;=Δ 1  is obtained. 
     This results in ##EQU3## or 2 QV .sbsp.x =2 QV .sbsp.x &#34;  (1+2.sup.Δ.sbsp.1). 
     Therefore, the following equation is formed: 
     
         QV.sub.x =QV.sub.x &#39;+log.sub.2 (1+2.sup.-Δ.sbsp.1)(Δ.sub.1 ≧0) 
    
     or 
     
         QV.sub.x =QV.sub.x &#34;+log.sub.2 (1+2.sup.Δ.sbsp.1)(Δ.sub.1 &lt;0) 
    
     Therefore, QV x  is obtained by adding the value from log 2  (1+2 - Δ1) to QV x  &#39; when Δ1 is greater than 0 or by adding log 2  (1+2.sup.Δ1) to QV x  &#34; when Δ1 is negative. 
     To obtain log 2  (1+2 - Δ1) or log 2  (1+2.sup.Δ1), log 2  (1+2 - |Δ1|) corresponding to |Δ1|, for example, may be prestored in a ROM (read only memory), and a ROM address is designated by the data corresponding to the absolute value |Δ1| of Δ1 so that the data corresponding to log 2  (1+2 - |Δ1|) stored therein is generated as an output. Furthermore, log 2  (1+2 - |Δ1|) is obtained here to minimize the area of the ROM where the data log 2  (1+2 - |Δ1|) corresponding to |Δ1| is stored since log 2  (1+2 - |Δ1|) is converged into 0 as |Δ1| becomes greater. Consequently, log 2  (1+2 - Δ1) or log 2  (1+2.sup.Δ1) may be obtained regardless of Δ 1 being positive or negative, without discriminating the positive or negative sign of Δ1. In this case, however, a ROM area for storage becomes very large. In addition, QV x  can be obtained in a similar procedure by obtaining QV x  &#34;-QV x  &#39;=Δ 1  &#39;. Also, when the ROM area has no room for prestoring the data log 2  (1+2 - Δ.sbsp.1), for Δ1 value to an area belong to is identified by identifying the higher to lower bits of Δ1 in sequence, whereby log 2  (1+2 - |Δ1|) corresponding to the identified region may be obtained. 
     Thus, a light quantity (stimulus value x) in logarithmic form is obtained. In a similar procedure, the following is obtained. 
     
         QV.sub.T1 =QV.sub.x +log.sub.2 (1+2.sup.-Δ.sbsp.2) 
    
     or 
     
         QV.sub.T1 =QV.sub.x +log.sub.2 (1+2.sup.Δ.sbsp.2) 
    
     from 
     
         QV.sub.x -QV.sub.y =Δ2. This is subject to T1=2.sup.QV.sbsp.T1 =2.sup.QV x+2.sup.QV y. 
    
     Next, the following is obtained from QV T1  -QV z  =Δ 3 . 
     
         QV.sub.T =QV.sub.T1 +log.sub.2 (1+2.sup.-Δ3) 
    
     or 
     
         QV.sub.T =QV.sub.z +log.sub.2 (1+2.sup.Δ3) 
    
     (wherein T=2 QV .sbsp.T1 =2 QV .sbsp.T1 +2 QV .sbsp.z) 
     From QV T , QV x  and QV y  obtained as above, the following are obtained: 
     
         QV.sub.x -QV.sub.T =Δx 
    
     
         QV.sub.y -QV.sub.T =Δy 
    
     From Δx and Δy, chromaticity coordinates are obtained as follows to calculate x and y. x=2.sup.Δx y=2.sup.Δy 
     Calculations of 2.sup.Δx and 2.sup.Δy may be done using a known method as that used in electronic calculators. 
     For example, p, q, and r defined by the following formula are to be firstly obtained from Δ: 
     
         Δ=4p-q-r 
    
     wherein p is an integer; q=0 or 1 or 3; and 1&gt;r≧0. And 2 -r  is to be obtained by the following formula from r: ##EQU4## Finally 2 -r  is to by multiplied by 2 -q  and 2 4p  to obtain 2.sup.Δ. Thus, x and y are obtainable by applying the above method. 
     FIG. 3 is a schematic block diagram of an embodiment of the computation circuit of FIG. 2. Description will be given of the operation and block diagram thereof. Data on logarithmic compression values of light quantities (stimulus values X&#39;, Y and Z) form A-D converter 4 are respectively set in registers 62 through 66 through multiplexer 60 in a time sequence. For example, data corresponding to QV x , is first set in register 62, data corresponding to QVy in register 64 and data corresponding to QVz then in register 66. After setting data in registers 62 through 66, data from constant data output circuit 600 is subtracted from data QVz from register 66 in subtraction circuit 602 to calculate QVx&#34;. Data QVx&#34; and data QVx&#34; from register 62 are both supplied as inputs to subtraction circuit 604 and multiplexer 606. 
     Output terminal 604a of subtraction circuit 604 generates data corresponding to |Δ1|=1QVx&#39;-QVx&#34;1, and terminal 604b generates a &#34;high&#34; level signal when QVx&#39;≧QVx&#34; and a &#34;low&#34; level signal when QVx&#39;&lt;QVx&#34;. 
     Multiplexer 606 generates data corresponding to logarithmic compression value QVx&#39; of a light quantity (stimulus value X&#39;) for spectral stimulus value x.sub.λ, when terminal 604b is at &#34;high&#34; level, while it generates data corresponding to logarithmic compression value QVx&#34; of a light quantity (stimulus value X&#34;) for spectral stimulus value x&#34;λ when terminal 604b is at &#34;low&#34; level. Furthermore, an ROM address is designated by data |Δ1| from terminal 604a, and data corresponding to log 2  (1+2 - |Δ1|) stored therein is generated as an output. Data QVx&#39; or QVx&#34; from multiplexer 606 and log 2  (1+2 - |Δ1|) from ROM 608 are supplied as inputs to addition circuit 610, where the logarithmic compression value of a light quantity for spectral stimulus value xλ, QVx=QVx&#39;+log 2  (1+2 - Δ1) or QVx=QVx&#34;+log 2  (1+2.sup.Δ1) is calculated depending on the positive or negative sign of Δ1, data corresponding to QVx is computed and stored in register 632. 
     Data corresponding to QVx from register 632 and data from register 64 are supplied as inputs to subtraction circuit 614 and multiplexer 616, and calculations of QVx-QVy=Δ2 are performed by subtraction circuit 614, whereby data corresponding to 1Δ21 is generated from terminal 614a and a &#34;high&#34; level signal when Δ2 is greater than 0 and a &#34;low&#34; level signal when Δ2 is negative are generated from terminal 614b. In response to the above signals, multiplexer 616 generates data corresponding to QVx when the signal is at &#34;high&#34; level, and data corresponding to QVy when the signal is at &#34;low&#34; level. The data is added to data corresponding to log 2  (1+2 - |Δ2|) from ROM 618 by addition circuit 620 to calculate the following: 
     
         QV.sub.T1 =QVx+log.sub.2 (1+2.sup.-Δ2) 
    
     or 
     
         QV.sub.T1 =QVy+log.sub.2 (1+2.sup.Δ2). 
    
     This data is stored in register 634. 
     Data corresponding to QV T1  from register 634 and data corresponding to QVz from register 66 are supplied as inputs to subtraction circuit 624 and multiplexer 626, and calculations of QV T1  -QVz=Δ3 are performed by subtraction circuit 624, and data corresponding to 1Δ21 is generated from terminal 624a, and a &#34;high&#34; level signal when Δ3 is greater then 0 and a &#34;low&#34; level signal when Δ3 is negative are generated from terminal 624b. 
     In response to the signal from terminal 624b, multiplexer 626 generates QV T1  when the signal is at &#34;high&#34; level and QV z  when the signal is at &#34;low&#34; level. Data corresponding to QV T1  or QV z  from multiplexer 626 and data corresponding to log 2  (1+2 - |Δ3|) from ROM 628 are supplied as inputs to addition circuit 630 where calculation of QV T  =QV T1  +log 2  (1+2 - Δ3) or QV T  =QVz+log 2  (1+2.sup.Δ3) are performed, and data corresponding to QV T  is supplied as an input to register 636. 
     Next, data from registers 636 and 632 are supplied as inputs to subtraction circuit 638, where calculations of Δ x  =QV x  -QV T  are carried out. As a result, the exponential part of 2 in X/(X+Y+Z)=2.sup.Δx has been calculated, and 2.sup.Δx =x is calculated by exponential conversion circuit 642, and the data is digitally displayed by diaplay circuit 8. 
     In addition, data from registers 636 and 634 are supplied as inputs to subtraction circuit 640 where Δ y  =QV y  -QV T  is calculated, and 2.sup.Δy =y is further calculated by exponential conversion circuit 642, and the data is displayed by display circuit 8. 
     With the above embodiment, the subtraction circuit, ROM, register and multiplexer are described one by one to facilitate understanding. Since these are used in common, and the exponential operation circuit is feasible using known circuitry, their description is omitted. It should be understood that the above circuitry may be effected by using random logic circuitry consisting of circuits in response to individual operations by using a microcomputer actuated by a central processing unit (CPU) based on a program stored in memory ROM. 
     Furthermore, the above embodiment uses 3 photosensitive elements (silicon diodes). However, the present invention is applicable to a type consisting of a total of 4 photosensitive elements including 2 elements corresponding to spectral stimulus values x.sub.λ &#39; and x.sub.λ &#34; for color measurement. In addition, the present invention is applicable to another type using one photosensitive element and the filter in front thereof changeable in a time series sequence. 
     With the present invention, the output current from the photosensitive elements is logarithmically compressed and stored and chromaticity coordinates x and y are processed for operation based on the logarithmic compression value. This causes the maximum amplitude of the signal handled by each circuit to become small, thereby widening a measuring range and ensuring high accuracy CIE chromaticity coordinates.