Method for controlling exposure in color photographic printers

In conventional color photographic printers, it is emperically known that the LATD balance of RGB is substantially constant. If the exposures of three primary color components are controlled to be constant in photographic printing, standard color film negatives may be printed with excellent color balance. However, the above LATD control method is not always effective for the film negatives where a specific color is dominant, and those films are often printed with poorly balance color. The present method performs exposure control of RGB with color compensation filters controlling the color with the color compensation filter amount dependent upon factors such as the filter position, filter density, transmission factor, amount of light, etc. This method can therefore control the exposure control values at a high precision without disturbing color balance for all types of films. This method can also prevent fluctuations in density which might otherwise be entailed by the changes in color correction level so as to thereby precisely deal with changes in the sensitivity of the materials of the development process.

BACKGROUND OF THE INVENTION 
This invention relates to a method for controlling exposure in color 
photographic printers. 
In order to print color photographs of a high quality, it is necessary to 
appropriately control the accurate measuring of a color original picture, 
and to precisely control the exposure and to appropriately determine the 
amount of light impinging onto a photosensitive material, and to optimally 
control the conditions in a photographic printer. Color original pictures 
may be color negative films, color reversal films, or color papers. 
Photosensitive materials may be color papers or color positive films. 
In color negative paper system of an ordinary photographic printer 
currently used, a reference film negative (the film negative having an 
average density of the users' photographs) is usally used as a reference 
to control the printing conditions so as to secure printing at a 
predetermined density. 
There are currently two exposure control processes in use; i.e. the 
additive color process and the subtractive color process. The additive 
color process is classified into the consecutive exposure process and the 
simultaneous exposure process of the primary colors. The additive process 
is the process of high-correction control. The subtractive process is 
adapted to consecutively control the exposure of RGB by restricting white 
light with filters (cut filters) of C (cyan), M (magenta) and Y (yellow). 
The subtractive process is classified into high-correction and 
lowered-correction types. The color compensation filter process is another 
process widely used in enlargers conventionally. 
FIG. 1 shows an embodiment of the photographic printers of color 
compensation filter type wherein a film negative 1 is illuminated with 
light from a light source 3 via color compensation filters 2 of yellow 
(Y), magenta (M) and cyan (C). The light transmitted from the film 
negative 1 is guided onto a photographic paper 6 for printing via a lens 
unit 4 and a black shutter 5. The photographic paper 6 is reeled out from 
a supply reel 61. The photographic paper 6 after exposed at the printing 
section is processed at the processing section 7 for development, 
bleaching, fixing, washing and drying and then rolled on a take-up reel 
62. At a location near the film negative 1 on the side of the lens unit 4 
is arranged photosensors 8 such as photodiodes for detecting the image 
density for the three primary colors of red (R), green (G) and blue (B). 
The printing conditions are determined by the density detection signals 
for each of RGB from the photosensors 8 and the film negative 1 which has 
been conveyed to the printer section is printed under such conditions. 
The filters 2 provided for color compensation may have a structure such as 
that shown in FIGS. 2A and 2B. Three filter plates 21 (21A through 21C) 
having a sectral quadrant shape are combined for each of the three colors 
of yellow (Y), magenta (M) and cyan (C). The light transmitted through a 
central light path 22 is controlled for each color by horizontal relative 
movement of each pair of filter plates 21A through 21C. The movement of 
the filter plates 21A through 21C are controlled by a control device (not 
shown) for respective colors. The filters plates 21A through 21C are 
approximated to the spectral transmittance distribution of the film 
negative dye so that exposure control can be performed precisely. 
In such a color photographic printer, color failure or density failure is 
artificially corrected. It is emperically known that the LATD (Large Area 
Transmittance Density) balance in blue (B), green (G) and red (R) is 
substantially constant on a frame of standard color negative films. It is 
therefore a general practice in printing to measure the LATD of the three 
primary colors of BGR and control the exposure for the three primary color 
components at a constant value. In this way an excellent print of 
well-balanced colors can be obtained from a standard color film negative. 
The above mentioned LATD control method, however, is not necessarily 
effective for the color film negative on which a specific color is 
dominant, and frequently produces defective prints with ill-balanced 
color. In order to deal with those problems, a photographic printer is 
generally equipped with color correction means of lowered-correction, 
normal-correction and high-correction levels to compensate the colors in 
negative films. More particularly, the lowered-correction method is a 
control process to apply correction in the amount of light of relatively 
low level against the relative changes in the LATD for three primary color 
components of a film negative and is suitable for the color failures 
caused by uneven color distribution of an object. The full-correction 
method is a control process to apply a certain amount of light to neutral 
(gray color) which is the result of the integration of three primary 
colors. This is suitable for correcting the negative films affected from 
different light sources or the negative films where latent images fade in 
the layer sensitive to a specific color. 
The correction in the level of light suitable for the majority of negative 
films is referred to as a normal-correction which is lowered from the 
full-correction with respect to the amount of light to be exposed. The 
high-correction is at a level of light which is higher than the 
normal-correction level. Conventionally, the spectral characteristics of a 
light receiver of a printer, and the filter characteristics of the 
exposure control filter, etc. of a printer are insufficient to be used in 
full-correction, often resulting in the mixing of three primary color 
exposures and inevitably lowering the light level of correction. 
Full-correction can not be attained even if the intensity of the 
correction is increased so far as this condition prevails. Furthermore, 
high-correction is ineffective for color correction of different light 
sources under the conditions where full-correction can not be achieved. 
When color correction is changed with respect to the level of light, the 
print density is visually changed. Therefore, density as well as color 
should be corrected. This presents another difficulty. 
Color compensation filters realize precise photomeasure and exposure 
control and therefore enable preparing conditions for full-correction. 
Then, a correction which is lowered from the full-correction with respect 
to its light amount enables performing an exposure control with good color 
balance on all of the films. Correction performance will also be improved 
for gradation changes in a film negative. This compensation filter type 
process can perform printer light source change correction at the same 
time as the exposure control. In the prior art, however, above mentioned 
advantages were not fully exploited because there was no process for 
precisely determining the exposure and controlling the filters. 
SUMMARY OF THE INVENTION 
This invention is conceived in view of the aforementioned situations and 
aims at providing an exposure control method of excellent precision and 
color balance for color photographic printers. 
Another object of this invention is to provide an exposure control method 
of excellent color balance for color photographic printers using 
compensation filters and, more particularly a simple method for changing 
the amount of light in color correction. 
According to this invention, in one aspect thereof, for achieving objects 
described above, there is provided a control method for controlling the 
exposure in a color photographic printer which changes and controls the 
amount of light in three primary colors for printing, which is 
characterized in that the density of a film to be printed is 
photographically measured, and a printing exposure is obtained and a 
printing density is estimated, and an amount of correction is obtained 
from an intended value and said estimated value, and the amount of 
correction is either added or subtracted from said printing exposure. 
According to this invention, in another aspect thereof, for achieving 
objects described above there is provided a control method for controlling 
the exposure in a color photographic printing system changes and controls 
the amount of light in the three primary colors for printing, which is 
characterized in that color compensation filters are driven for correction 
changes caused by the light source during printing, and the density of a 
film negative is measured under a predetermined light source, and an 
amount of filter correction for the changes caused by the changes in the 
correction level is obtained from the amount of printing filter under the 
reference color correction conditions and an estimated printing density, 
and the displacement of said color compensation filters caused in the 
above operation is calculated and said color compensation filters are 
driven in accordance with the displacement. 
According to this invention, in still another aspect thereof, for achieving 
objects described above, there is provided a control method for 
controlling the exposure in a color photographic printing system which 
changes and controls the amount of light of the three primary colors for 
printing, which is characterized in that exposure control values after 
said change are corrected in a manner that the weighted coefficient for 
the exposure control values and said three primary colors before said 
change or the first functions corresponding to specific visual sensitivity 
coefficient are made to coincide with the weighted coefficients for said 
exposure control values and said three primary colors after the change or 
the second functions corresponding to said specific visual sensitivity 
coefficients. 
Further, according to this invention, in another aspect thereof, for 
achieving objects described above, there is provided a control method for 
controlling the exposure in a color photographic printing system which 
changes and controls the exposure of the three primary colors for 
printing, which is characterized in that the exposure for said three 
primary colors is corrected first by the full-correction and then by the 
lowered-correction, and the printing density by said full-correction is 
made to coincide with the printing density by said lowered-correction with 
respect to the specific visual sensitivities by correcting the exposure 
control values. 
Still further, according to this invention, in another aspect thereof, for 
achieving objects described above, there is provided a control method for 
controlling the exposure in a color photographic printing system which 
changes and controls the amount of light in the three primary colors for 
printing, which is characterized in that said amount of color compensation 
filter which changes the amount of color correction is determined by the 
functions of said amount of color compensation filter before the changes 
in the color correction level. 
The nature, principle and utility of the invention will become more 
apparent from the following detailed description when read in conjunction 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
The exposure correction according to this invention is performed by means 
of color compensation filters and controlled with the functions of the 
amounts of color compensation filters such as filter density, transmission 
factors, light amounts, etc. In the photographic printer shown in FIG. 1, 
when color compensation filters 2 are controlled in a manner to make the 
sum of the density of a film negative 1 and the density of color 
compensation filters 2 constant, full-correction can be achieved if the 
spectral sensitivity distribution of RGB of photosensors 8 coincides with 
that of a photographic paper 6. 
Full-correction represents a condition under which a print having a 
predetermined three color density (gray) can be obtained even from 
negative films of different color balances as the spectral sensitivity 
distribution of the photometric system of the photosensors 8 which 
determines the exposure of the printer completely coincides with the 
spectral sensitivity distribution of the photographic paper 6. In order to 
prepare such a condition, it is necessary to make the spectral sensitivity 
distributions of the photometric system including the photosensors 8 and 
of the photographic paper for printing, but it is difficult to make both 
completely coincide with each other merely by adjusting the sensitivity 
distributions with photometric filters 9. It is therefore necessary to 
make a correction table corresponding to the curve shown in FIG. 3 for 
each of the colors R, G and B in order to perform the full-correction for 
the measured value by the photosensors 8. Alternatively, the operation 
formula shown below may be referred using all through a33 as coefficients 
for the measured values B', G' and R'. 
##EQU1## 
The correction table and the operation formula (1) may be prepared by 
obtaining values in advance either by computation or experiments using the 
spectral characteristics of the photographic paper which is to be used for 
printing, and then measuring the actual amount of light of the paper with 
photosensors 8 and then comparing both values. 
Optical conditions in full-correction may be affected by the following 
factors: spectral distribution of the light source 3, the spectral 
transmission factor distribution of the color compensation filters 2, the 
spectral sensitivity distribution of the film negative 1, the spectral 
sensitivity distribution of the combination of the photosensors 8 and the 
photometric filters 9 and spectral sensitivity distribution of the 
photographic paper 6. When the spectral sensitivity distribution differs 
between the photographic paper and the photometric system, each of the 
above factors may affect the condition in various complicated manners 
depending on the types of negative films 1 and photographic paper 6. It is 
therefore desirable to store, in a memory of the printer, the values 
corrected by the above correction table or the formula (1) concerning each 
of the photographic materials for use and select the suitable one at the 
time of printing. 
When these conditions for full-correction are met, the controlled values of 
each color exposure and the color variation in printing can be caused to 
correspond to each other in a one-to-one relationship and the printing 
process can be quantified. More specifically, the relationship between the 
logarithmic values of the exposure E onto the photographic paper 6 and the 
print density D has the characteristics shown in FIG. 4 and the exposure 
E' after transmission through the film negative 1 can be obtained by the 
density value D.sub.o of the film negative measured by the photosensors 8 
and the exposure time. This is because the effective illumination onto the 
photographic paper 6 can be estimated according to the density value 
D.sub.o. When the gradation characteristics of the photographic paper 6 is 
given, the printing density D.sub.o can be estimated. The density value 
D.sub.o of negative films is equivalent to the printing density generally 
used in the color production theory (refer to "Shashin Kogaku no Kiso 
(Basics of Phototechnology)", Nihon Shashin Gakkai, 1979, p 387). The 
value D.sub.p measured by the photosensors 8 can be expressed as the 
formula below shows if the luminance of the light source 3 is 
J.sub..lambda., the transmission factor of the negative film is 
T.sub..lambda. and the spectral sensitivity of the photographic paper 6 
(=spectral sensitivity of the photosensors 8) is S.sub..lambda.. 
##EQU2## 
Since .intg.J.sub..lambda. S.sub..lambda. d.sub..lambda. is constant, the 
effective illumination .intg.J.sub..lambda. T.sub..lambda. S.sub..lambda. 
d.sub..lambda. onto the photographic paper 6 can be obtained from measured 
value D.sub.p from the photosensors 8 and the exposure E' can be estimated 
from the exposing time. As shown in FIG. 5, the values of slope control 
can be made constant (nearly equal to 1.0) against the linear section of 
the characteristic curve of the negative film 1 by the full-correction 
irrespective of gradation; it is not necessary to adjust slope-controlled 
values against the gradation changes of the film negative 1 (such as the 
fluctuation of the types of photographic materials, fluctuation in 
processing, etc.). However, in the case of negative films which have color 
failure factors, even if slightly, the negative films should be subjected 
to the lowered-correction by changing the amount of the color compensation 
filters 2 depending on the full-correction shown in FIG. 6. If we assume 
that the measured values of each color at full-correction are Bf, Gf and 
Rf and the density values of each color of the film negative 1 are Bn, Gn 
and Rn, the compensation filter volumes Bc, Gc and Rc can be obtained from 
the formula below as shown in FIG. 6. 
##EQU3## 
Measured values Bf, Gf and Rf represent herein the values which do not 
include a correction amount for light source variations and an exposure 
correction amount for obtaining the printing density. 
For lowered-correction, the exposure at full-correction is calculated as a 
function of the exposure amount of the color compensation filters and the 
amount of the color compensation filter is changed in order to perform the 
control as shown in FIG. 7. The photometric values adjusted for 
lowered-correction or the sum of the density value of the film negative 
and the density value of the color compensation filter becomes as below 
wherein the color correction coefficients are LB, LG and LR. 
##EQU4## 
wherein Df=(Bc+Gc+Rc)/3. 
If it is assumed that the above mentioned color correction coefficients 
have the relationship that LB=LG=LR=0.0, then the above formula can be 
converted to: 
##EQU5## 
which means that the same amount of the color compensation filter can be 
applied for all of RGB to completely achieve lowered-correction. On the 
other hand, if the color correction coefficients have the relationship 
that LB=LG=LR=1.0, the above formula (4) can be converted to: 
##EQU6## 
and full-correction can be performed. Accordingly, corrections can be 
switched from lowered-correction to full-correction by changing the color 
correction coefficients LB, LG and LR based on the above formula (4). 
In the case of the coefficients b11 through b33 are 
b11=(1+2.multidot.LB)/3, b12=(1-LB)/3, b13=(1-LB)/3, b21=(1-LG)/3, 
b22=(1+2.multidot.LG)/3, b23=(1-LG)/3, b31=(1-LR)/3, b32=(1-LR)/3, 
b33=(1+2.multidot.LR)/3, the above formula (4) can be converted to: 
##EQU7## 
This formula (7) is generally expressed as follows: 
##EQU8## 
Color correction is controlled in a manner to determine the printing 
exposure at Bl, Gl and Rl by means of the relationship in density between 
color compensation filters and the film negative before color correction. 
Although the exposures for each of the three colors are changed by varying 
the correction volume, it is necessary to make the visual densities before 
and after the change the same. FIG. 8A shows the printed densities of Y, M 
and C of an object of neutral color in the case where color failure occurs 
in the color of blue. In this case, it is necessary to make the densities 
on the photographic paper (as shown in FIG. 8B) coincide with each other 
visually further by lowered-correction. Even in a case when color 
correction is made manually by manipulating color correction keys which 
are generally provided on photographic printer, the density should be 
simultaneously corrected as the density variations. For such a case the 
method of this invention is applicable, too. 
If it is assumed that i=B, G and R, the light controlled values before the 
changing is expressed by EOi, and the light controlled values after the 
changing Ei, and the .gamma. value of the photographic paper 6 .gamma.i, 
then the relative spectral reflection density Di.sub..lambda. of the 
photographic paper 6 (Di.sub..lambda. is the relative spectral reflection 
density of the dyes of Y, M and C of respective photosensitive layers of 
B, G and R), and the specific visual sensitive function as shown in FIG. 9 
is expressed as V.sub..lambda., the relationship of the formula below 
theoretically holds in order to make the visual reflection factors before 
and after the correction visually coincide with each other. 
EQU .intg.V.sub..lambda. 
.multidot.10.sup.-(.SIGMA.EOi.multidot..gamma.i.multidot.Di.sbsp..lambda.. 
sup.) d.sub..lambda. =.intg.V.sub..lambda. 
10.sup.-(.SIGMA.(Ei+K).multidot..gamma.i.multidot.Di.sbsp..lambda..sup.) 
d.sub..lambda. (9) 
The letter K in the above formula denotes the amount of exposure correction 
necessary to make the reflections identical before and after the 
correction. The specific visual sensitivity function V.sub..lambda. is 
determined by CIE. (See the Shikisai Kagaku Handbook, Nippon Shikisai 
Gakkai, 1980. P. 26). However, the formula (9) is not practical and it is 
advisable to use the contribution ratio of the three dyes (YMC.fwdarw.1, 
2, 3) of the photographic paper for visual density. 
##EQU9## 
D11' denotes the contribution ratio for the visual density of the B 
component of yellow dye, D21' that for the G component of yellow dye and 
D31' that for the R component of yellow dye and they are respectively 
obtained by experiments. For a simple procedure, the relationship below 
holds wherein i.noteq.j at Dij', and it is 0 or when an auxiliary 
absorption of the three primary colors is not taken into account, 
EQU D11'.multidot.E01.multidot..gamma.1+D22'.multidot.E02.multidot..gamma.2+D33 
'.multidot.E03.multidot..gamma.3=D11'.multidot.(E1.multidot.K).multidot..ga 
mma.1+D22'.multidot.(E2+K).multidot..gamma.3+D33'.multidot.(E3+K).multidot. 
.gamma.3 (11) 
From the above formula, the value k can be calculated and the value of 
light amount control after the change can be corrected. Dij' and .gamma.i 
are characteristics of a photosensitive material and prepared in advance. 
Reflective materials have non-linear characteristics, but errors can be 
minimized if the density range is limited to be within 0.5 to 1.0. The 
printing density can be estimated without actually conducting printing if 
the precise photometric values are obtained for a film negative and the 
exposure is precisely controlled. Thus, the obtained estimate printing 
density values are corrected further to obtain more desirable printing 
density values to control the exposure. The exposure control with 
estimated printing density values is not limited to a compensation control 
process but can be applied to any process. This invention includes all the 
exposure control methods which are deduced based on the above concept even 
if the operation formulae or the control formulae do not include a term 
corresponding to the estimated printing density value. 
The left-hand members of the formulae (9) through (11) represent intended 
values or the estimated printing density values. More particularly, the 
amount in exposure correction is obtained first from an estimated values 
(the left-hand side of a formula) of printing density at the reference 
condition and from another estimated printing density at a different 
condition (the right-hand side of the formula), and then either the former 
or the latter exposure amount is corrected for control. This method can be 
applied to all cases not only for visual density control for changes in 
color correction but also for control and correction of changes of 
printing density caused by the changes in conditions. Printing conditions 
of a type of film can be obtained from the printing condition of the 
reference film. Furthermore, from the printing conditions of reference 
development performance can be obtained by printing conditions after 
development process. Moreover, over- or under-printing conditions can be 
obtained from the printing conditions of the reference exposure film 
negative. In order to achieve such estimation at a high precision, it is 
sufficient to store in advance the characteristic of copying materials or 
spectral and gradation characteristics thereof. 
FIG. 10 shows the flow of the correction procedure of the color 
compensation filters 2 (steps S1 through S8) wherein the filter amount at 
the time of the change in color correction is obtained by the above 
formula (3) (Step 5) and the correction amount of the color compensation 
filters 2 caused by the change in correction can be obtained from the 
right-hand side of the above formula (10) (Step S7). The final correction 
amount required for driving the color compensation filters 2 is obtained 
by adding the driving volume of the color-compensation filters 2 for 
printing the light source correction obtained at Step S1, and the 
displacement volume obtained at Step S5 and the correction volume obtained 
at Step S7 (Step S8). With this correction value, the color compensation 
filters 2 are driven by a drive means (Step S9). 
The foregoing description mentioned above that it was a general practice to 
change the level of correction from normal-to high- or normal-to 
lowered-correction. But color change tends to be inaccurate due to optical 
mixture of three lights at the time of exposure and photometry. FIG. 11 
shows the densities in hexagonal color coordinates on a photographic paper 
printed with changed color correction both by this invention method and 
the conventional method. Due to the aforementioned reason, no lines pass 
through the origin point or the 0 point even if correction is intensified 
as shown in FIG. 11. This means color correction is insufficient for the 
negative films affected from different light sources or old negative 
films. Conversely, the full-correction like this invention method can 
accurately correct colors of negative films which have been exposed to 
different light sources when photographed. The negative films can further 
be corrected by lowered correction at small degree of color change (hue 
change) although non-linear type lines caused by color mixing on the 
photographic paper remain. The area GA circumscribed by broken lines shows 
the scope of the printing density which can be permitted as gray area and 
the points FL1 and FL2 denote the correction levels for the negative films 
photographed under fluorescent lamps. The points M1 and M2 denote the 
correction level for the negative films of lowered green sensitivity while 
the points G1 and G2 denote the correction level for the negative films of 
increased green sensitivity. If the correction level is changed from 
normal-correction FL1, M1, G1 to either high-correction H or 
lowered-correction L, as in the case of conventional process, it is 
obvious that the correctable area does not overlap the gray area GA. On 
the other hand, if the correction moves from the full-correction (FL2, M2, 
G2) to the lowered-correction L as in the case of this invention, all the 
movements overlap the gray area GA. 
The color compensation filter amount can be calculated by the formula (4) 
as LB.multidot.(Bc-Df)+Df, LG.multidot.(Gc-Df)+Df, LR.multidot.(Rc-Df)+Df 
and by the formula (7) as below: 
##EQU10## 
Exposure control is performed by obtaining an amount of compensation 
filter by either the formula (4) or (7) and controlling filters. It can 
also be performed by obtaining photometric values B1, G1 and R1 first and 
then adjusting compensation filters to meet the values. In the latter 
case, exposure is controlled by obtaining the printing density volumes B1, 
G1 and R1 with the functions of color compensation filters and the 
negative film density before the change in color correction level. 
Although in actual exposing operation, correction for the change in the 
light sources and correction for intended printing density should be added 
to the photometric values Bf to Rf by the full-correction or the 
photometric values Bl to Rl by the lowered-correction, they are not used 
as the amount of compensation filter in mathematical operation of color 
correction. Instead of the formulae (4) and (7), exposure may be 
controlled according to the formula (1) for instance, by changing color 
correction first and converting the negative densities (Rn, Gn, Bn) to 
other three color density values. In this case, the amount of color 
compensation can be obtained by using the relation between the 
compensation filter volume and the negative film density before the change 
and by using the above mentioned converted three color densities instead 
of the density of the negative film. 
Although the foregoing description was given to color negative paper, this 
invention may be applied for various original prints and copy materials. 
The exposure control process according to this invention can control values 
of exposure control at a higher precision without disturbing color balance 
for all types of films. The negative films which have been exposed to 
different light sources or those with color failure can be printed 
desirably. Even if the correction level is changed from the 
full-correction, slope-control is not necessary, or hardly necessary, with 
a slight control added if only visual densities are made to coincide with 
each other. No density change occurs with the change in color correction. 
When intended values are clearly set, this invention process may be widely 
applied, facilitating condition-setting, controlling and manipulating of a 
color photographic printer and performing stable printing even if a 
fluctuation takes place in the film processing, etc. 
It should be understood that many modifications and adaptations of the 
invention will become apparent to those skilled in the art and it is 
intended to encompass such obvious modifications and changes in the scope 
of the claims appended hereto.