GB2096336A

GB2096336A - Controlling contrast

Info

Publication number: GB2096336A
Application number: GB8208111A
Authority: GB
Original assignee: Eskofot AS
Current assignee: Eskofot AS
Priority date: 1981-03-31
Filing date: 1982-03-19
Publication date: 1982-10-13
Also published as: DK146381A; DE3211059A1; DE3211059C2; DK147954B; JPH0318540U; GB2096336B; JPS5816238A; NL8201204A

Abstract

A densitometer for the control of the contrast in the reproduction of a half-tone picture, by means of a main exposure and an auxiliary exposure, initially measures the density in both the lightest and the darkest field of the picture by scanning across the picture, a memory (1A, 1B) remembering both the greatest and the smallest value of the reflection, and the ratio of the auxiliary exposure to the main exposure required in the reproduction being calculated after the scanning. The densitometer comprises a photodiode and at least one amplifier, said amplifier communicating with an analog-digital-converter, the outlets of which feed both a memory registrating the lowest reflection value and a memory registrating the highest reflection value. After a scanning and through the assistance of an operator, these two values may be transmitted to a calculating unit adapted to calculate the exposure ratio. The densitometer may scan at a velocity of 20-200 cm/sec.

Description

SPECIFICATION A method and a device for controlling the contrast by reproduction The invention relates to a method of controlling the contrast by reproduction such as screen reproduction, e.g. screen reproduction of halftone pictures by means of a main exposure and an auxiliary exposure, whereby the density both in the lightest and in the darkest field of the picture and consequently the contrast range of the halftone picture are first measured by means of a densitometer, whereafter the ratio of the lightest spot of the background chosen to the highlight of the original is measured and the main exposure is adjusted in response thereto, and an exposure of a color wedge including the screen is performed and the maximum contrast range chosen to be used is measured, whereafter the auxiliary exposure is adjusted in response to the contrast and screen range of the half-tone picture, the percentage of the auxiliary exposure in proportion to the main exposure depending on the contrast range and the screen range of the half-tone picture in an unambiguous manner according to the formula --10-8+10-R flash percentage= 1 00% --10-8+1 whereafter the half-tone picture or the original in question may be exposed, the main exposure, however, being adjusted in such a manner that the sum of the main exposure and the auxiliary exposure remains substantially unchanged.

Such a method is known from German patent application No. 2,815,886.

According to the invention the measuring of the density both in the lightest and the darkest field of the half-tone picture is performed by scanning the densitometer across the half-tone picture, a memory in a calculating unit remembering both the greatest and the smallest value of the reflection, i.e. the transmission, and the desired exposure parameters such as the flash percentage, are calculated when the scanning is finished. In this manner a quick scanning of a halftone picture is made possible, although the calculations are performed by means of a relatively slow and inexpensive calculating unit, which previously restricted the scanning velocity.

According to the invention it has been utilized that only one calculation should be performed after the scanning by means of the greatest and the smallest value of the reflection.

According to an advantageous embodiment, an analog-digital-converter is provided before the memory.

The invention also relates to a densitometer for carrying out the method and comprising a photodiode and at least one amplifier. The densitometer is characterised by the amplifier being in connection with an analog-digitalconverter, the outlets of which feed both a memory registrating the lowest reflection value, i.e. the transmission value, and a memory registrating the highest reflection value, i.e. the transmission value, said two values after a completed scanning and through assistance of an operator being able to be transmitted to a calculating unit adapted to calculate the desired exposure parameters such as the flash percentage. As a result, a densitometer is provided which is able to scan very quickly, possibly 20 200 cm/sec.

The densitometer may furthermore be adapted to read a code, possibly a bar code on the wrapping of the light-sensitive material in question and to transmit information thereof to the calculating unit. A grey-tone field may optionally serve as a space indication.

The invention will be described below with reference to the accompanying drawing, in which: Fig. 1 illustrates a diagram of a densitometer for carrying out the method according to the invention, and Fig. 2 illustrates an example of a bar code for the wrapping of the light-sensitive material.

The method according to the invention is a further development of the method described in DE-OS No. 3,114,684, said publication being part of the present specification. According to this publication, the maximum contrast range R is read (i.e. the contrast range chosen to be used in connection with the printing technique in question), and the auxiliary exposure, i.e. the flash percentage, is adjusted in response to the contrast range B of the half-tone picture in question, since the percentage of the auxiliary exposure in proportion to the main exposure depends on the contrast range B and the screen range R of the half-tone picture according to the formula --10-B+10-R flash percentage= - x 100% -10-B+1 In this manner the auxiliary exposure may be determined in a simple manner in response to the sensitivity of the film and the form of the screen, whereby the entire available contrast range is utilized and all the information is maintained during reproduction. Furthermore, the ratio of the highlight of the chosen background to the highlight of the original is measured and both the main and the auxiliary exposure are adjusted in response thereto.The invention thus illustrates how to perform this procedure automatically.

Both the highlight and the shadow value are measured by means of a densitometer, either by reflection or by transmission, and a voltage corresponding to the difference may be read out, e.g. by performing a zerosetting by means of the highlight. The measuring of the density both in the lightest and the darkest field of the half-tone picture is performed by scanning the densitometer across the half-tone picture, a memory 1 A, 1 B in a calculating unit remembering both the greatest and the smallest value of the reflection, i.e. the transmission, and the flash percentage may be calculated by means of the calculating unit after the scanning. An advantage of this method is that it is quicker and it may be performed by a weak-sighted person. The densitometer can scan at a velocity of 20-200 cm/sec.One of the reasons why the scanning may be performed so quickly is that the minimum and maximum values are not read into the calculating unit until the scanning is finished.

The densitometer comprises a photodiode, from which the signal is transmitted to an analogdigital-converter through one or several amplifiers. The digitalized values of the lightest and the darkest spot are transmitted to their respective memories 1 A and 1 B. From these memories, the signals are not transmitted until the scanning as previously mentioned is finished.

The scanning does not, of course, start until the respective exposure means has reached its maximum intensity. The densitometer may furthermore be adapted to read a code, possibly a bar code on the wrapping of the light-sensitive material in question and to transmit information thereof to the calculating unit. The bar code reading may for instance be initiated by three bars per se forming a code. The information in question may be information concerning the material, the screen, and preferably the shutter.

Fig. 2 illustrates an example of such a bar code.

This code permits 23=eight different numerals, but may in principle be extended indefinitely.

Parity bits may be added. The grey fields are utilized as a space indication.

The calculating unit may be composed of a microprocessor Intel, model 8085.

The logarithmic function may be encoded in the calculating unit, whereby the time-delay caused by a noise-reducing feed-back capacitor is avoided. The scanning range must, of course, be encoded.

The densitometer may be adapted to the color wedge in question.

The method and the densitometer according to the invention may be varied in many ways without thereby deviating from the scope of the invention.

Claims

1. A method of controlling the contrast by reproduction such as screen reproduction, e.g.

screen reproduction of half-tone pictures by means of a main exposure and an auxiliary exposure, whereby the density both in the lightest and in the darkest field of the picture and consequently the contrast range of the half-tone picture are first measured by means of a densitometer, whereafter the ratio of the lightest spot of the background chosen to the highlight of the original is measured and the main exposure is adjusted in response thereto, and an exposure of a color wedge including the screen is performed and the maximum contrast range chosen to be used is measured, whereafter the auxiliary exposure is adjusted in response to the contrast and screen range of the half-tone picture, the percentage of the auxiliary exposure in proportion to the main exposure depending on the contrast range and the screen range of the half-tone picture in an unambiguous manner according to the formula -1 0-B+ 1 0-R flash percentage= x 100% --10-8+1 whereafter the half-tone picture or the original in question may be exposed, the main exposure, however, being adjusted in such a manner that the sum of the main exposure and the auxiliary exposure remains substantially unchanged, characterised by the measuring of the density both in the lightest and in the darkest field of the half-tone picture being performed by scanning the densitometer across the half-tone picture, a memory in a calculating unit remembering both the greatest and the smallest value of the reflection, i.e. the transmission, and by the desired exposure parameters such as the flash percentage, being calculated when the scanning is finished.

2. A method as claimed in claim 1 characterised by an analog-digital-converter before the memory.

3. A densitometer for carrying out the method of claim 1 and comprising a photodiode and at least one amplifier, characterised by the amplifier being in connection with an analog-digitalconverter, the outlets of which feed both a memory registrating the lowest reflection value, i.e. the transmission value, and a memory registrating the highest reflection value, i.e. the transmission value, said two values after a completed scanning and through assistance of an operator being able to be transmitted to a calculating unit adapted to calculate the desired exposure parameters such as the flash percentage.

4. A densitometer as claimed in claim 3, characterised in that the densitometer is adapted to read a code, possibly a bar code on the wrapping of the light-sensitive material in question and to transmit information thereof to the calculating unit.

5. A densitometer as claimed in claim 4, characterised by a grey-tone field as a space indication in the code.

6. A method substantially as described above with reference to the accompanying drawing.

7. A densitometer substantially as described above with reference to the accompanying drawing.