Method of and apparatus for measuring the optical density of a photographic negative

A method of and apparatus for measuring the optical density of an original, especially a three-color negative, from which photographic prints are made, to control how much printing light of each color penetrates the original when the image is projected onto a color print medium that is sensitive to these colors. The spectral sensitivity of the measuring apparatus is adjusted to that of the print medium. Measuring light is projected through the original and is resolved into at least one spectrum. The intensities of the light at the various ranges of wavelength are weighted and totaled in accordance with the spectral sensitivity of the particular print medium. Light valves with translucencies that can be adjusted to the sensitivity of the print medium to that range of wavelengths are distributed along the spectrum. The accordingly weighted intensities of the light of each color are separately sensed and measured.

BACKGROUND OF THE INVENTION 
The present invention concerns a method of measuring the optical density of 
an original photographic image, and apparatus for carrying out the method, 
to determine how much printing light of each color penetrates the original 
when making photographic prints therefrom. 
A method and apparatus of this type are described in the German Patent 
Publication No. OS 3,737,775. Measuring light travels through an original 
(either a positive or negative) and is resolved into at least one color 
spectrum by means of a spectroscope. The intensities of the light in the 
various ranges of wavelength are measured separately. Each result is 
multiplied by a factor .THETA..sub.x.sup.BGR that characterizes the 
spectral sensitivity of the color print medium at that range to one of the 
colors red, green, and blue. The totals of the weighted results for each 
color are employed to determine how much printing light the apparatus will 
use. Weighing the results for the separate ranges of the spectrum with 
wavelength-dependent factors theoretically makes it possible to simulate 
any filter transmission curve. The curves can accordingly be adapted very 
precisely to the sensitivity of the paper. 
The disclosed method, however, does have certain limitations. First, since 
the light traveling through any area of the original being sensed must be 
distributed among a large number of light sensitive elements, specifically 
the pixels of a CCD, each pixel receives only a small amount of light. 
Secondly, the method demands a relatively long series of calculations, 
specifically separate multiplication and addition for each individually 
measured range of the spectrum, to attain corrected brightness levels for 
each of the three colors at each area of the original. 
SUMMARY OF THE INVENTION 
The principal object of the present invention is accordingly to provide an 
improved method and apparatus of the aforesaid type that can detect higher 
intensities for each pixel and necessitates fewer calculations. 
This object, as well as other objects which will become apparent from the 
discussion that follows, are achieved, in accordance with the present 
invention, by a method of measuring optical density of an original, such 
as a photographic negative, and apparatus for carrying out the method, 
which (1) attenuates light with a plurality of "light valves" that are 
disposed along the spectrum and controlled in accordance with the 
sensitivity of the print medium to the respective spectral wavelengths, 
and (2) measures the resulting weighted intensities of the light at each 
color that is passed through the light valves. 
The translucency of the light valves is adjusted to the spectral 
sensitivity of the particular color print medium at a specific wavelength 
range just once, and this setting is retained for all areas of the 
negative. The various intensities at the spectral plane are thereby 
weighted in accordance with the print medium's spectral sensitivity, so 
that only one result must be obtained for the weighted and subsequently 
collected intensity values of each color. They will accordingly be 
available at approximately the same outgoing current per photocell as with 
conventional systems. Since only three measured values have to be 
processed for each point tested, the expenditures for computing and 
control are comparable to those of standard, currently available 
illumination controls. Furthermore, it is no longer necessary to adjust 
high-precision filters to the spectral sensitivity of each type of paper. 
Apparatus for carrying out the aforesaid method is described in detail 
below.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1a illustrates an optical density measuring apparatus having an 
illuminating device that comprises a source 1 of measuring light, a 
reflector 2, a cylinder 3 with its inner surface silvered, and a ground 
glass 4. The illuminating device is positioned directly over a film 5 
containing several original negatives. Below the film and perpendicular to 
the direction of film transport indicated by arrow 32 is a slit 6. The 
slit 6 is demarcated by thin strips of metal limited to the format of the 
negatives being printed from. Above the slit 6 and between the film 5 and 
the ground glass 4 is a rotating disk 11 with several radial slots 11a 
precisely as wide as the areas of the negative being sensed. Rotating the 
disk in the direction indicated by the arrow 33 will accordingly allow 
light from the source 1 to travel along the width of film directly above 
the slit 6 in a succession of points. 
The slit 6 is at the focus of a collimator 8, which the light leaving the 
slit travels through. The collimated beam enters a direct-view prism 9 
which is conventionally composed of several subsidiary prisms 9a, 9b and 
9c, of different types of glass. Aside from a slight spectral deflection 
of the center of the beam, the passage of light through the prism 9 is 
essentially straight through. The light leaving the prism 9 is focussed by 
a lens 10, producing a sharp image of the slit 6 on photosensors 
(photocells) 15, 16, and 17. The prism 9 resolves the light, derived from 
the width of the negative 5 over the slit 6, by wavelength into a spectrum 
extending along photocells 15, 16, and 17. The blue component of light, 
for example, can impact photocell 15 at the left and the red can impact 
photocell 17 on the right. The length of photocells 15, 16, and 17 along 
the slit 6--i.e., perpendicular to the plane of illustration--corresponds 
to the width of the negative image. 
Above the photocells 15, 16, and 17 is a series 12 of "light valves" 12a 
which control the amount of light passing through. These light valves can 
be liquid crystals for example, although other types of devices can also 
be employed. Each light valve 12a is individually controlled through a 
separate line 13 by a driver 14. The series 12 of components 12a extends 
over the same distance (length) in the plane of the illustration as the 
photocells 15, 16, and 17. 
Alternatively, the light valves 12a and photocells 15, 16, and 17 can be 
divided into a number of partial elements which corresponds to the number 
of sensing points along the slit 6. In this event the rotating disk 11 
becomes unnecessary. 
FIG. 2a illustrates the spectral sensitivities of the various layers of a 
color print medium being processed in the apparatus illustrated in FIG. 1 
as a function of wavelength. As illustrated by curve B, the blue layer is 
sensitive to light waves between 400 and 500 Nm long. The green layer, 
represented by curve G, is sensitive to wavelengths of approximately 460 
to 590 Nm. The sensitivity of the red layer, represented by the curve R, 
extends from 580 to approximately 740 Nm. 
The translucencies of the light valves 12a, liquid crystals for example in 
the series 12, as controlled by the driver 14, are programmed such that 
the curves of translucency precisely match the curves of sensitivity in 
FIG. 2a with respect to height and position. The intersections between the 
photocells 15 and 16 and 16 and 17 are positioned precisely at the points 
of intersection or contact between the particular curves of sensitivity. 
This means that the light supplied to the blue photocell can only be 
evaluated up to a wavelength of approximately 475 Nm and the light for the 
green photocell from 480 Nm on. 
The light valves 12a are narrow enough in width to ensure that the weights 
they assign to their ranges of the spectrum at mean translucency will 
eliminate error, even though the cost of providing control for all the 
individual light valve components is kept within reason. 
The operation of the apparatus illustrated in FIG. 1 will now be described. 
A film 5 of negatives that are to be used to produce prints is conveyed, 
one negative at a time, across the slit 6 at such a rate that one of the 
slots 11a in the disk 11 will sweep all the way along the slit 6 while the 
film is advanced one slit width. This procedure ensures the complete 
interpretation of every point in the advance of the light along the slit. 
The lengths of the photocells 15, 16, and 17 and the liquid-crystal light 
valves 12a (i.e., in a direction perpendicular to the plane of 
illustration in FIG. 1) are great enough to ensure that the photocells can 
effectively sense even the points at the very ends of the slit 6. It is 
alternatively possible to use a pivoting or polygonal mirror to advance 
the light, point by point, from the presentation region demarcated by the 
slit 6 to the spectroscope arrangement 8, 9 and 10. The result in either 
case is a wavelength total, weighted in accordance with the translucency 
of each liquid-crystal component 12a in the series 12, for each point on 
the negative, in each photocell 15, 16 and 17. The levels for the red, 
green, and blue densities are forwarded through appropriate lines to a 
processor 18 that operates generally in the manner disclosed in the German 
Patent No. 2,840,287 (and corresponding U.S. Pat. No. 4,279,502). As 
described therein, the film travels through rollers 7 at a constant speed 
while every area thereof, demarcated by a measuring stripe that travels 
over the film at an angle, is sensed in succession by the photocells 15, 
16, and 17. The emergence of one slot 11a at one edge of the film is 
followed by the coincidence of another slot 11a with slit 6, and the 
negative, which is being advanced one slit width at a time, is sensed by 
the next measuring stripe until results for every area of the film are 
stored in the processor 18. 
A series of lenses that project images of the liquid-crystal components 12a 
onto the photocells 15, 16, and 17 for each color can also be employed 
instead of positioning the photocells directly below or downstream of 
these components. When the light penetrates the spectroscope at an angle, 
or when the spectrum of the light from the source 1 is unevenly 
distributed and every color in the negative is accordingly not illuminated 
or evaluated at the same intensity, this situation can be taken into 
consideration when controlling the translucencies (attenuation) of light 
valves 12a. Specifically, any irregularities in the wavelength 
distribution of the measuring light and in the collection of light behind 
the light valves are compensated by controlling the attenuation of the 
light valves in accordance with these irregularities. This can be done by 
a calibration process that involves measuring, adjusting, and storing the 
intensities of the wavelength ranges associated with all the 
liquid-crystal pixels with no film at the slit 6. This procedure also 
takes into account not only the spectral sensitivity of the print medium 
but also any external interference. A negative is now advanced past the 
slit 6 to establish the relationship between the intensities with and 
without the film for all colors as a standard for the transparency of the 
negative to each color. These results are then converted logarithmically 
into densities. 
Bi-stable liquid-crystal cells (per FIG. 1b) can also be employed instead 
of the light valves which are continuously controllable by an analog 
signal (per FIG. 1a) in dependence upon the desired weighting of the 
intensities of the spectrum. Such bi-stable cells will, like a light 
shutter, alternate between being completely transparent and completely 
opaque in response to a pulse train having a duty cycle that depends upon 
the sensitivity curve. The result is a modulation of translucency with 
respect to time. 
FIG. 1b illustrates a modification of the apparatus of FIG. 1a whereby the 
light valves are bi-stable liquid-crystal cells that receive pulse trains 
36 with variable duty cycles on the respective control lines 13. The 
amount of light passing through the light valves is thus dependent upon 
the duty cycles. 
The embodiment illustrated in FIG. 1 can sense only one area of the 
negative at a time. In the embodiment of FIG. 4, the light valves 12a are 
distributed along the image of the slit into the same number n of rows of 
individual controllable subsidiary cells 12a as there are test areas of 
the negative at the slit 6, for example ten test areas can be sensed for 
the three colors at once by the ten photocells 15', 16' and 17'. 
The overlap of the blue and green curves at 470 to 510 Nm and of the green 
and red curves in the narrow range around 600 Nm can reduce the accuracy 
of the evaluation process. This disturbance can be taken into account as 
illustrated in FIG. 2b by providing a separate row 19, 20, and 21 of 
liquid-crystal light valves for each color with color-detection photocells 
15", 16", and 17" under them, such that the pairs that sense wavelengths 
in the overlap regions will be next to each other. The distribution within 
the spectrum being sensed ensures that the cells 19, 20, and 21 will be 
subject to the lines of the spectrum to the same extent. A total result 
for each of the three colors will then be available at the photocells 15", 
16", and 17" below the rows 19, 20, and 21 of liquid-crystal light valves, 
the translucencies of which are controlled in accordance with the graph in 
FIG. 2a. 
Another way of taking into account the overlap of the sensitivity curves in 
FIG. 2a in determining the results is to provide a single straight row 12' 
of light valves 12a' as illustrated in FIG. 3. Each valve is controlled 
by a driver 14' through a line 13' in accordance with the spectral 
sensitivity illustrated in FIG. 2a. Light valves 12a' are assigned to 
wavelengths in the manner illustrated in FIG. 2b. Ocular lenses 22, 24, 
26, and 28 each produce a continuous image of the opposing area of 
liquid-crystal light valves 12a on a photocell 23, 25, 27, 29, and 31 
below it. The image from the lens 22 represents the range of wavelengths 
to which only the blue-sensitive layer is sensitive. The lens 24 images 
the range of overlap between blue and green onto the photocell 25. The 
lens 26 accounts for the pure green range, the lens 28 the overlap between 
green and red, and the lens 30 the pure red. 
This embodiment is intended for the sequentially double exploitation of the 
sensors for the overlapping ranges. The apparatus operates in the 
following manner: The translucency of the light valves 12a' above lenses 
22 and 24 is controlled in accordance with the sensitivity curve B, with 
photocells 23 and 25 together issuing the weighted blue. The result is 
stored in the processor 18'. At the same time, the valves above the lenses 
28 and 30 are controlled in accordance with the red sensitivity curve, and 
photocells 29 and 31 supply the weighted red to the processor. 
Subsequently thereafter, the light valves 12a above the lenses 24, 26, and 
28 are adjusted to the green curve in FIG. 2a, and the photocells 25, 27, 
and 29 supply the total intensity weighted with the green component to the 
processor 18'. Although this system takes twice as much time for 
evaluation, it has the advantage that the light valves 12a can be ideally 
positioned in accordance with the intensity distribution of the spectrum. 
Another version, in accordance with FIG. 5, that differs slightly from the 
one illustrated in FIG. 3 has only one ocular lens 34 for the whole row of 
light valves and one photocell 35 intercepting all the light that travels 
through the light valves. The three measurement results for blue, green 
and red, respectively from each area of the negative are obtained 
sequentially in the following manner: The liquid-crystal light valves 12a' 
in the wavelength range of blue sensitivity are initially adjusted to the 
blue curve and the others are rendered opaque (switched off). The light is 
then passed through the light valves to the single photocell 35. 
Thereafter, the liquid-crystal light valves in the wavelength range of the 
green sensitivity curve are switched on and controlled in accordance with 
this curve to carry out measurements for the green light. Finally, the 
liquid-crystal light valves in the wavelength range of the red-sensitivity 
curve are controlled in accordance with that curve and the red light in 
the given area of the negative is measured. The same procedure is then 
carried out for each area of the negative in succession. 
The results are then processed as described in the aforementioned German 
Patent No. 2,840,287 (and corresponding U.S. Pat. No. 4,279,502). 
There has thus been shown and described a novel method and apparatus for 
measuring the optical density of a negative which fulfill all the objects 
and advantages sought therefor. Many changes, modifications, variations 
and other uses and applications of the subject invention will, however, 
become apparent to those skilled in the art after considering this 
specification and the accompanying drawings which disclose the preferred 
embodiments thereof. All such changes, modifications, variations and other 
uses and applications which do not depart from the spirit and scope of the 
invention are deemed to be covered by the invention, which is to be 
limited only by the claims which follow.