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2025-02-27
D50 and D65 are two commonly used artificial sunlight simulations in standard light boxes. They play an important role in simulating natural light lighting effects, especially in color evaluation and printing industries. However, the printing industry generally uses D50 light sources, and rarely uses D65 light sources. So why do we use D50 for printing color matching? Can D65 replace D50? The following is a brief introduction.
Printing Sample Table
Because in traditional printing, according to the main transmission or reflection characteristics of the sample to be evaluated, it can be divided into two different types of observation conditions: transmission medium observation conditions and reflection medium observation conditions. For transmission media, such as color separation films in the traditional printing industry, most of them are neutral gray, and tungsten filament lighting with a color temperature of 2800K to 4000K is a more appropriate observation condition; for reflection samples, the best observation condition is under natural light. The spectral power distribution of natural light is not uniform nor eternal, and it changes with time. According to research, the D65 light source with a color temperature of 6500K represents the average daylight in the northern hemisphere (or northern window light), which is suitable for replacing natural light for color evaluation or comparison; therefore, the best color temperature for observing perspective films is 3000K, and the best color temperature for observing reflection samples is about 6500K. It is obviously inappropriate to use two light sources with different color temperatures in the same industry, and people have reached a compromise and uniformly adopted an average observation color temperature of 5000K. This compromise is necessary because the human eye's visual adaptation requires a stable reference condition; directly comparing two images with different "white points" will produce significantly different perceptions, so that printing plants do not have to install light sources with different color temperatures in prepress and pressroom. Other industries, such as the textile or automotive industries, do not have these problems and have always used D65 as a reference light source.
Printing D50 light source
After years of standardization work in the printing industry, D50 has been integrated into all aspects, whether it is international and domestic standards, factory lighting, software configuration, or the thinking of technical managers. Large-scale conversion of D50 to D65 is a huge system project, which is time-consuming and labor-intensive, and the effect is not obvious. Specifically:
1. If you want to change the reference lighting source in the printing industry, you must first change the standard, whether it is an international standard, such as the "ISO12647 Printing Technology Halftone Color Separation Film, Sample and Printed Product Process Control" series of standards, "ISO3664 Printing Standard Observation Conditions", "ISO13655 Printing Technology Printed Graphics Image Spectral Measurement and Colorimetric Calculation", etc., or a national standard, such as "GB/T18722 Printing Technology Reflection Density Measurement and Colorimetric Measurement Application in Printing Process Control" "GB/T30329 Printing Technology Four-color Printing Ink Color and Transparency" and dozens of other standards must be modified.
2. Now most companies use D50 light sources in factory lighting, observation boxes for print evaluation, etc. If all of them are updated, the funds required will be huge, which is undoubtedly a further blow to the printing industry, which is already fully competitive and has increasingly lower profit margins.
3. Now many certifications, such as G7, PSD, PSO, GMI, etc. are based on previous standards. Once the standards are modified, the basic documents and data of the certification must be changed, which requires a lot of time to re-test, revise the documents, obtain new characterization data, etc.
4. The upstream and downstream industries of printing services, such as food packaging, cigarette packaging, paper, ink, etc., have also adapted to the D50 light source of the printing industry. If modifications are required, the upstream and downstream industries need to modify them together, and the workload is also huge.
From the relative spectral power distribution curves of D50 and D65, it can be seen that D65 contains more ultraviolet light (UV light) than D50, which also brings great uncertainty to the printing industry where fluorescent brighteners are widely used.
Therefore, from a realistic point of view, it is not feasible for the printing industry to switch the reference light source from D50 to D65. However, in some specific segments, printing companies can use light sources flexibly according to customer needs and should not be rigid and dogmatic.
D65 light source
1. Color temperature difference
D50, with a color temperature of 5000K, is the standard color temperature (ICC standard) recognized by the world printing industry and is usually used in the fields of printing or pre-publishing. D65, an international standard daylight light source, with a color temperature of 6500K, is a light source with a slightly cooler color tone, simulating the lighting effect of average northern natural daylight (or northern window light), and is the most commonly used standard light source for color matching in China.
Most printing factories are equipped with two standard light source color matching light boxes, D50 and D65. D50 and D65 usually refer to two standard light sources, and their technical parameters include color temperature, color rendering index, metamerism index, illumination, etc. The main difference between the two is the color temperature, where D50 has a color temperature of 5000K (actually 5002K), which corresponds to the color of the morning sun; while D65 has a color temperature of 6500K, which corresponds to the color of the midday sun. Therefore, D50 is slightly yellowish, while D65 is slightly blue.
2. Differences in spectral curves
CIE standard illuminant D50 represents typical daylight with a correlated color temperature of 5003K. On the CIE1931 chromaticity diagram, its chromaticity coordinates are x=0.3457, y=0.3586. CIE standard illuminant D65 represents typical daylight with a correlated color temperature of 6504K. On the CIE1931 chromaticity diagram, its chromaticity coordinates are x=0.3127, y=0.3291. The figure below shows the relative spectral energy distribution of D50 and D65 light sources.
Relative spectral energy distribution diagram of two different standard light sources D50 and D65
As can be seen from the figure, the relative spectral energy distribution of the D50 light source is flatter than that of the D65 light source. The relative spectral energy of the D65 light source reaches a peak at around 470nm and then shows a downward trend. Theoretically, the same color measured using the D65 light source is slightly bluer than the result measured using the D50 light source, which also shows that the D50 light source is a light source with a slightly warmer color, while the D65 light source is a light source with a slightly cooler color. This may be the reason why ISO recommends the use of the D50 light source. In the ISO3644:2009 "Observation Conditions for Printing Technology and Photography" standard, the D50 light source is considered to be the standard light source for observing colors in the true sense.
Regarding the selection of standard light sources, our country recommends the use of D65 light sources. Although its relative spectral energy distribution curve is not as smooth as that of D50 light sources, the D65 light source is closer to the color temperature of normal sunlight, and therefore has a color rendering that is closer to actual conditions.
3. Difference in usage
D50 is the standard light source commonly used in international printing. The ICC color specification stipulates that all color measurements must be performed under the D50 light source, and the PCS (color connection space) characteristic file also uses D50 by default. If a non-D50 light source white point is used, it will be automatically converted to D50 through chromatic adaptation. Therefore, the basic color space of many design software is based on the D50 standard light source, especially traditional image design. Therefore, the ideal standard light source in the printing industry is D50.
However, the application effect of D50 light source is not very good at present, because no lighting company claims to be able to manufacture D50 light source 100% and ensure that the intensity in different bands is consistent with D50, that is, the color rendering index under 100% standard light source. As early as 1965, D65 was defined as the standard light source for color evaluation by the International Illumination Association CIE, and the most commonly used in China is the D65 standard light source .
