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FLEXO Magazine : June 2014
areas of greatest concern, such as the overall balance of the image, the perception of near neutral or pastel tones—such as Caucasian skin tones—and advertiser’s brand equity colors. During the development of the ANSI CGATS standard characterization dataset, TR001, it was observed that the yellow ink was on density at 1.0 ± 0.07 units, but the color would vary by as much as 10 CIE Lab units. To put this value into context, a differ- ence in CIE Lab units of between 0.5 and 0.6 is generally only perceivable by highly trained observers making judgments in a controlled viewing environment. Differences greater than 5 CIE Lab units are considered the upper limit for moderate size color differences and differences above 8 CIE Lab units are no longer considered color differences, but rather different colors. This article documents the origins of ISO reflection density, how it is used to control print and where its shortcomings arise. ISO STATUS DENSITY ISO 5 is one of the oldest international standards and is still in use today. It was developed for the photographic industry by ANSI and then expanded and adapted to be used in color process printing. In photo- graphic films, the amount of emulsion on the acetate was determined by transmittance and the transmittance was converted to density. Following the laws of Lambert and Beer, the absorption of light is proportional to the concentration of the colored dye and to the thickness of the emulsion layer. Mathemati- cally, this is shown as: A=abc Where A is the absorbance, a is the absorp- tivity property of the dye, b is the thickness of the layer and c is the concentration of the dye in the emulsion. In this case: A = -log(T) This concept was extended to reflection prints and hence to printing by replacing the transmittance (T) with the reflectance factor (R), such that density (D) is expressed as: D = -log(R) So density is the reflection equivalent to absorbance. In the days when this was being done, spectral instruments were large, slow and expensive. It was also noted that colorants, be they dyes or pigments, have broad absorption bands and did not require high spectral res- olution to characterize their absorption. Be- cause absorption is based on the logarithm, a perfectly, non absorbing material—clear water for example—will have a transmittance of 1.0 and hence an absorbance of zero. So in parts of the spectrum where the dyes have little or no absorption, there is no contribu- tion to the absorbance and those regions can be ignored. Therefore, the ANSI committee selected some broadband filters, made from Kodak’s Wratten gelatin filters, which would isolate certain spectral regions: • For yellow colorants, they built a blue filter • For magenta colorants, they built a green filter • For cyan colorants, they produced a red filter Different ink sets required slightly different central or peak wavelengths and so the ISO 5 – Part 3 standard contains a number of differ- ent spectral filters known as Status Filters. From optics, they knew that if you put a filter in front of a light source, the final distribution of light will be the combination of the incident light and the filter transmittance. This infor- mation was calculated and tabulated and given the designation Spectral Product. ISO 5 – Part 3 documents six unique sets of spectral prod- ucts: One filter for each of the process colors (C, M, Y) and a fourth filter for the neutral key channel (K) based on the human visual effica- cy function Vλ. Since it is possible to produce a logarithmic response from an electric circuit without computation, the filter densitometers were very small and inexpensive instruments consisting of only a light source (incandescent JUNE 2014 | FLEXO 39