GB2136588A - Colour printing - Google Patents

Abstract

In a colour projection printer using three pulsed flash tubes 4 having respective red, green and blue filters as in application 8306879, the colour balance of a negative is measured in a preliminary test procedure using respective sensors 6 and the measured information is modified to take account of different projection conditions, photosensitive material sensitivity, etc., stored and used to control the flash tubes in a subsequent printing process. <IMAGE>

Description

SPECIFICATION A photographic printer This invention relates to a photographic printer.

The invention provides a photographic printer comprising a light source containing different colour components; means for measuring the average density of the different colour components after passing through a photographic negative; and control means for using values derived from the said measurements to control the relative colour components of the light source.

Thus by employing the invention the colour characteristics of the light source can accurately be set to give the correct colour balance for a given negative or batch of negatives.

The apparatus preferably includes means for adjusting the colour component values by preset functions which may be set by the operator for a given batch of negatives depending on the particular characteristics of that batch.

The aforementioned colour component values are preferably also modified by an amount dependent on the current magnification to which the printer is set. This can be done automatically using a suitable distance measuring device such as an ultrasonic range finder to measure the distance between the lens assembly and a baseboard on which the photographic paper is supported. The colour component values are preferably also modified according to preset characteristics of the photosensitive paper being used.

In a preferred form of the invention the light souce comprises flash tubes designed, e.g. by the provision of colour filters, to provide light of different colours. In such an arrangement the control means is a digital processor designed to calculate the number of flashes of each tube required during an exposure period to produce a desired colour balance. The use of flash tubes in this way is particularly advantageous since it is readily compatible with the use of digital techniques to calculate the required amount of light of each colour.

One way in which the invention may be performed will now be described by way of example with reference to Figures 1 and 2 of the accompanying drawings in which: Figure 1 is a schemmatic block diagram of a photographic enlarger constructed in accordance with the invention; and Figure 2 is a schemmatic side elevation showing the more important mechanical parts of the enlarger.

Operation of the illustrated apparatus is commenced by placing a negative 1 in position and initiating a test procedure by operating a "start test" control 2. This causes a colour balance and exposure control circuit 3 repetitively to trigger blue, red and green flash tubes 4B, 4R and 4G in turn. These flash tubes form the light source for the printer and they illuminate the negative 1 through a diffuser 5.

Light from the negative is focussed in the normal way using a lens assembly shown on Figure 2 onto a sheet of photographic paper supported on the baseboard. The light also illuminates three colour discriminating photoconductors 6B, 6R and 6G which are rendered colour discriminating by appropriate blue, red and green filters. These photoconductors form part of a colour analysing circuit 6 shown in Figure 1 and their outputs, after current to voltage conversion at 61 B, 61 R and 61 G are passed to identical circuits 62B, 62R and 62G respectively, only one 62B, of which is illustrated in detail. In the circuit 62B, the amplitudes of pulses, derived from each flash of the blue tube are integrated at 63 and counted at 64 until the content of the integrator 63 is equal to or exceeds a reference voltage provided on line 65.A comparator 66 then sets a flip flop 67 so that it applies a zero input to AND gate 68 thereby discontinuing the counting process at 64. The integrator 63 and flip flop 67 are eventually reset by the subsequent test initiation signal from the control 2.

The reciprocal of the accumulated count in counter 64 is calculated at 69 and the output of the reciprocal calculator 69 represents a measure of the blue colour content of the negative. Similar outputs for the red and green colour contents are provided from circuits 6R and 6G.

The three outputs from circuit 6 are passed to a negative material correction circuit 7 where they are multiplied by respective numbers to correct bad colour balance which may have been discovered during a previous testing procedure. These colour balance correction numbers are derived from controls 8,9,10 (or 11,12,13) which represent yellow, cyan and magenta and can be manipulated by the operator. A further control 14 controls a multiplexer 71 so as to pass outputs either from 8,9 and 10 or from 11,12 and 13 (depending on the type of negative material in use) These yellow, cyan and magenta values are multiplied at 7B, 7R and 7G with respective blue, red and green colour content values from circuit 6. In this connection it will be understood that yellow can be considered to be the negative of blue, cyan the negative of red, and magenta the negative of green.

The settings of controls 8,9,10 and 11,12,13 can under most circumstances be left undisturbed, the control 14 being used when there is a change of make or type of negative being processed. However, over the long term there can be changes in the characteristics of negatives of a given make and type. This will result in a reduction in quality of the prints and will require re-setting of the controls 8,9,10 or 11,12,13. Variations with time in the characteristics of components of the illustrated circuit can also make such adjustments desirable from time to time.

The three outputs from the negative material correction circuit 7 are passed to a magnification correction circuit 15 where they are multiplied by a common scaling factor to compensate for the magnification setting of the printer. This scaling factor is calculated from a lens to baseboard measurement derived from a rangefinder 16; and from the focal length of the lens which is entered into the control circuit by the operator using a control 17. These two values i.e. of range and focal length, are used to perform a calculation (known peruse) in a circuit 151 to calculate a measure of magnification. The output of the magnification calculator 151 is multiplied at 15B, 1 SR and 15G with the colour correction values from the circuit 7.

The outputs of the magnification correction circuit 15 are passed to an aperture correction circuit 18 where they are multiplied at 18B, 18R and 18G with a common number representing the aperture size of the iens. This is generated by a control 19 which is set by the operator in this particular embodiment of the invention although it could alternatively be set automatically as the aperture size is adjusted.

The outputs from the aperture correction circuit 18 are passed to a paper correction circuit 20. The latter receives yellow, cyan and magenta correction values from controls 21,22 and 23 which are set by the operator according to the photographic paper being used. These correction values are normally specified by the manufacturer. In the paper correction circuit 20 the cyan, yellow and magenta correction values are multiplied at 20B,20R and 20G with respective outputs from circuit 18.

The outputs from circuit 20 are passed to a colour efficiency correction circuit 24 containing subtractors 24B,24R and 24G in which respective fixed constants KB,KR,KG are subtracted to correct the differences in the efficiencies of the different flash tubes and photoconductors and their associated filters.

The three outputs from the circuit 24 are stored at 25 and this completes the test procedure.

A print control 26 is now operated. This causes the colour balance and exposure control circuit 3 to read the stored values from the store 25 and to use them to control the flash tubes so that they are fired in such a way as to produce a sequence of blue, red and green flashes. The number of flashes of each colour is proportional to the output from the store 25 appropriate to that colour and the sequence is such that the flashes of each colour are distributed over substantially the whole exposure period. Alternatively the exposure period may include a central part containing a repeated sequence of blue, red and green flashes, this central period being preceded and succeeded by intermediate periods containing interleaved flashes of two of the colours; and initial and final periods containing flashes of only one colour.

The resulting print, after development, is inspected and any necessary adjustments to the controls 8,9,10 or 11,12,13 (depending on the type of negative in use) are made to provide an improved colour balance on subsequent prints.

Claims (9)

1. A photographic printer comprising a light source containing different colour components; means for measuring the average density of the different colour components after passing through a photographic negative; and control means for using values derived from the said measurements to control the relative colour components of the light source.

2. A photographic printer according to claim 1 in which the control means comprises means for calculating respective preset functions of the said values; means by which an operator may set the said functions depending on the type of material of the negative.

3. A photographic enlarger according to claim 2 in which the control means includes means by which the operator may set two or more groups of the said functions appropriate to respective different types of negative material and means for selecting which group is to be used for a given print depending on the type of negative material in use.

4. A photographic printer according to any preceding claim in which the control means comprises means for modifying the said values, derived from the colour density measurements, by amounts dependent on the current magnification of the printer.

5. A photographic printer according to claim 4 including means for producing a measurement of the magnification of the printer and for modifying the said values derived from colour density measurement according to the magnification.

6. A photographic printer according to claim 5 in which the said means for producing a measurement ofthe magnification includes an ultrasonic range finder for measuring the distance between a lens of the printer and a baseboard for supporting a photosensitive sheet.

7. A photographic printer according to any preceding claim in which the control means comprises means for calculating respective preset functions of the said values; and means by which an operator can set the lastmentioned functions depending on characteristics of a photosensitive sheet being used.

8. A photographic printer according to any preceding claim in which the light source comprises flash tubes arranged to provide light of respective different colours and in which the control means is a digital processor designed to calculate the number of flashes of each tube required during an exposure period to produce the desired colour balance.

9. A photographic printer substantially as described with reference to the accompanying drawings substantially as illustrated therein.