GB1592037A – Process and apparatus for printing transparencies
– Google Patents
GB1592037A – Process and apparatus for printing transparencies
– Google Patents
Process and apparatus for printing transparencies
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Publication number
GB1592037A
GB1592037A
GB9667/78A
GB966778A
GB1592037A
GB 1592037 A
GB1592037 A
GB 1592037A
GB 9667/78 A
GB9667/78 A
GB 9667/78A
GB 966778 A
GB966778 A
GB 966778A
GB 1592037 A
GB1592037 A
GB 1592037A
Authority
GB
United Kingdom
Prior art keywords
density
limit
original
exposure
printing
Prior art date
1977-03-11
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
GB9667/78A
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Gretag AG
Original Assignee
Gretag AG
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
1977-03-11
Filing date
1978-03-10
Publication date
1981-07-01
1978-03-10
Application filed by Gretag AG
filed
Critical
Gretag AG
1981-07-01
Publication of GB1592037A
publication
Critical
patent/GB1592037A/en
Status
Expired
legal-status
Critical
Current
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Classifications
G—PHYSICS
G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
G03B—APPARATUS OR ARRANGEMENTS FOR TAKING PHOTOGRAPHS OR FOR PROJECTING OR VIEWING THEM; APPARATUS OR ARRANGEMENTS EMPLOYING ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ACCESSORIES THEREFOR
G03B27/00—Photographic printing apparatus
G03B27/72—Controlling or varying light intensity, spectral composition, or exposure time in photographic printing apparatus
G03B27/73—Controlling exposure by variation of spectral composition, e.g. multicolor printers
G03B27/735—Controlling exposure by variation of spectral composition, e.g. multicolor printers in dependence upon automatic analysis of the original
Description
PATENT SPECIFICATION ( 11) 1 592 037
( 21) Application No 9667/78 ( 22) Filed 10 Mar 1978 ( 19) A o ( 31) Convention Application No 3097/77 ( 32) Filed 11 Mar 1977 in d ( 33) Switzerland (CH) Ck ( 44) Complete Specification Published 1 Jul 1981 ( 51) INT CL 3 G 03 B 27/73 ( 52) Index at Acceptance G 2 A 105 108 110 112 116 119 BJ C 15 C 23 C 30 C 3 C 5 C 6 ( 54) PROCESS AND APPARATUS FOR PRINTING TRANSPARENCIES ( 71) We, GRETAG AKTIENGESELLSCHAFI, a body corporate organised according to the laws of Switzerland, of 8105 Regensdorf, Switzerland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it
is to be performed, to be particularly described in and by the following statement: 5
Field of the invention
This invention relates to a method of and apparatus for printing of colour transparencies.
In by far the majority of cases, paper prints of transparencies are conventionally indirectly produced using an interneg on negative paper.
More recently, reversal papers of satisfactory quality have become commercially 10 available which make it possible to print transparencies directly and thus obviate the intermediate interneg step.
Theoretically, reversal prints from transparencies can be produced similarly and by similar equipment to prints from negatives An important consideration, however, is that the gradation pattern and density range of transparancies differ considerably from the 15 corresponding characteristics of negatives, the gradation curves being much steeper and the density range being greater The gradation curve of a transparency can be compensated for by choosing a reversal paper with the right gradation to achieve a good overall gradation, but the density range has to be dealt with by the exposure system and exposure control of the printer 20 The exposure system is subject to physical limitations making it difficult to design it in any way that differs considerably from the system used for printing negatives of comparable format.
Consequently, problems arising from the differences between the density range of transparencies and that of negatives must be solved virtually in their entirety by exposure 25 time control These problems are that very underexposed transparencies need very long exposure times and very overexposed transparencies need very short exposure times In both cases colour anomalies may occur due to reciprocity failure (Schwarzschild effect).
Since the Schwarzschild exponent may differ for each colour component, there are unwanted deviations from the neutral colour balance, a defect known to the skilled 30 addressee as “clipping” of the emulsion Very long exposure times impair the performance of the printer, while when the conventional colour shutter systems are used in which filters are pivoted into and out of the path of the beam from the light source, very short exposure times may result in non-uniform exposure of the paper print due to variations in the dwell time of the shutter filter above each dot of the image These small differences in dwell time 35 become more important in proportion as the required exposure time is shorter.
In an endeavour to reduce these unwanted effects, one commercially available apparatus has features for restricting the exposure time range; a circular graduated filter of neutral colouring is provided in the exposure beam path and at the beginning of each exposure starts tg rotate through 360 from its maximum-density position Consequently, the printing 40 light in the plane of the paper is relatively weak to start with and increases as the exposure proceeds i e the printing light is at a minimum at the beginning of exposure and at a maximum at the end of exposure Consequently, variations in the density of the original do not have to be compensated for just by variations in exposure time but can be dealt with to some extent by varying the intensity of the printing light 45 2 1 592 037 2 There are two disadvantages of this system First, the outlay on apparatus for graduated filters, a motor drive and control are considerable, and secondly since the available light can only be reduced by the filter the average exposure times become longer and the printer therefore becomes less efficient Another disadvantage is that, since the intensity of the printing light varies in time, automatic printers using such a system can operate only with a 5 measuring system providing time integration of a value proportional to printing light intensity This limitation reduces flexibility.
It is an object of this invention to provide a process and an apparatus which obviate very short and very long exposure times in the reversal printing of transparencies.
It is the intention to fully utilize the available printing light and achieve optimally short 10 exposure times and therefore high printer efficiency Another object is to improve the colour quality of the prints prepared from originals of extreme total density, and to provide all these advantages with a reduced financial outlay.
The invention relates to a process for controlling exposure in the direct printing of transparencies on to a print material comprising determining the difference in neutral 15 density between an original transparency to be printed and a reference transparency; establishing a first positive limit and a second negative limit for the determined density difference; in the case where the density difference exceeds the first positive limit, increasing the time during which the original transparency is exposed to printing light for printing on a reversal print material at a predetermined rate relative to the density 20 difference which is less than the rate of increase in exposure time VS density difference applied to positive density differences below said first positive limit and/or; in the case where the density difference is less than the second negative limit, decreasing the time during which the original transparency is exposed to printing light for printing on a reversal print material at a predetermined rate relative to the density difference which is less than 25 the rate of decrease in exposure time VS density difference applied to negative density differences above the second negative limit.
The invention also relates to an apparatus for performing the process The apparatus comprises a printing light source, a support for the original to be printed, a support for photo-sensitive reversal material, and imaging lens, servo-operated colour control filters 30 and an automatic exposure control acting thereon, the control comprising: first means for determining the colour densities of the original in the three primary colours; second means for determining the neutral density of the original to be printed; third means which lengthen or shorten the exposure times in the three primary colours from the values for a reference original in dependence upon the variations of the neutral density and of the colour densities 35 from the corresponding values of a reference original; and fourth means which respond to the difference between the neutral density of the original to be printed and the neutral density of the reference original and which reduce the rate at which exposure time varies with said neutral density difference when such difference exceeds a first positive limit and/or when such difference falls short of a second negative limit.
The advantage of the invention in printing under-exposed transparencies is that with effect from a particular density of the original the exposure time remains substantially constant, so that printer efficiency can be higher than in the conventional process Tests have shown that this feature causes very little impairment of print quality as regards density and, indeed, improves print colouring 45 So far as the printing of over-exposed transparencies is concerned, the invention provides the advantages that the minimum exposure time can be at a value which is still not critical as regards the non-uniform dwell time of the shutter filters in and out of the beam path, and that the avoidance of very short exposure times does not cause untrue colouring of neutral low-density shades and pastel shades.
A preferred embodiment of the invention will be described in greater detail hereinafter with reference to the drawings wherein:Figures 1 to 4 are various diagrams which help to explain a process according to the invention.
Figure 5 is a simplified sketch of a printer according to the invention; 55 Figure 6 is a block schematic diagram of the exposure control system of the printer of Figure 5; and Figure 7 is a diagram which helps to explain the operation of the exposure control system.
Referring to Figure 1, Dv represents the overall density of an original or part thereof and is calculated in accordance with the formula: 60 D, = log (I/T) = log T ( 1) from the mean transmission T of the complete original or the part thereof defined as important to the image In Anglo Saxon literature the density Dv according to ( 1) is called 65 1 592 037 3 1 592 037 3 LADT (large area transmission density).
In the diagram of Figure 1 the logarithm of the exposure time, log t E, is plotted as ordinate.
So that a constant printing density Dp may prevail on the printing material independently of the density D, of the original, there must be compliance with the formula: 5 Dp = y (log E log I) =const ( 2) which applies to the linear part of the characteristic H and D curve This formula is illustrated in draft form in Figure 2 where: 10 y denotes the so-called gradation or gradient, which is a constant for a particular material; I denotes the so-called inertia representing that E-value which (theoretically) leads to the positive density Dp = 0, and E denotes the so-called exposure as a yardstick for the quantity of light incident on the printing material 15 E is given by the formula:
E = BO T t E = IE t E ( 3) where BO denotes the light intensity illuminating the original, T denotes the mean 20 transmission of the original as defined above and t E denotes the duration of effect (exposure time) of the resulting printing light intensity IE.
It follows from formulas ( 2) and ( 3) that:
DP = const = y llog (B, T t E) log Il ( 4) 25 or log B O + log T + log t E const + log I or 1 30 log t E =-const + log I log B O log T ( 5) i.e.
log t E = Cl + D, ( 6) 35 with Cl const + log I log B O ( 7) Cl is a system constant depending on the printing material (y, I) and the lighting system (B.) As will be apparent from formula ( 6) variations of the density D, can be compensated for 40 by proportional variations of the exposure time log t E, as shown in graph form in Figure 1.
An exposure time t EN corresponds to the normal of an original having the density DVN.
Figure 3 shows how the process according to the invention affects the exposure time.
With effect from a predetermined original density Dvu greater than the normal density DVN and representing an under-exposed transparency, there is only a minor increase in the 45 exposure time t E In other words, there is deliberate non-conpliance with the reciprocity formula ( 6) in the density range Dv>Dvu It is possible to speak here of a density under-correction (sub-proportional density compensation, YU
