Requirements analysis of standard light source in

Analysis of the requirements of standard light sources in the printing industry

for the printing industry, the requirements of standard light sources are mainly measured from the following aspects

first, color temperature (or related color temperature)

color temperature is an interesting indicator. The displayed value is the value of temperature, but the evaluation is the color of light. There are many similar indicators in physics, such as viscosity expressed in time (seconds), light year is actually a unit of length, and so on. Color temperature refers to that when the absolute blackbody is heated to a certain temperature, the absolute black will emit light, and the color of this time is expressed by the current temperature. Absolute blackbody is a purely physical concept. A simple example can be given. Workers in steel mills usually determine whether the molten steel reaches the appropriate temperature by observing the color of the molten steel. The lower the temperature, the redder the color; The higher the temperature, the bluer the color. Eat charcoal "Ernst Siebert repeatedly stressed that this phenomenon can also be found when burning hot pot. When the color of the fire is red, the temperature is not high, and when the fire light is blue, the temperature of the fire is the highest

the printing industry used to use the standard light source with 6500k color temperature, but now it has changed to use the standard light source with 5000K color temperature. The color of 5000K color temperature is standard white, which is equivalent to the lighting effect of the North window at noon. The relative spectral energy distribution is more uniform, so it is very suitable for the printing industry

but for fluorescent tubes, it is fluorescent stimulated luminescence, which is inconsistent with the luminous characteristics of blackbody, so the index of relevant color temperature is usually used when evaluating color. The relevant color temperature refers to the temperature at the time of blackbody radiation, which is the most similar to the color with the same brightness stimulus, expressed in k-temperature. Generally, most of the standard light sources used in the printing industry are fluorescent tubes. It is said that its color temperature is actually the relevant color temperature, which is equivalent to the color of 5000K blackbody radiation. Fluorescent tubes are all stimulated. The most remarkable feature of the spectral energy distribution of this kind of light source is that pulses will suddenly appear in a certain band, where the energy will increase, and unlike tungsten filament lamps, the spectrum is continuous. When measuring the color temperature of fluorescent tubes, it usually refers to the relevant color temperature, which is equivalent to the color temperature value of blackbody under the same brightness. This does not affect the actual use

second, the color rendering index

color rendering index can be understood as the similarity between the light source and the ideal standard light source under the same color temperature. If the value is 100, it means that 100 is close; If the value is 90, it means that 90 is close. For the printing industry, the higher the color rendering index, the higher the degree of color restoration observed in it; The lower the color rendering index, the lower the degree of color restoration

therefore, the printing industry requires the color rendering index to be at least 90 Because many fluorescent tube manufacturers no longer make detailed classification for those with a color rendering index of more than 90, if the color rendering index of the lamp is higher than 95, it is a very high-quality observation light source

third, illuminance

illuminance is the intensity of light. The higher the illuminance, the brighter the light source; The lower the illumination, the darker the light source. For printing applications, there are mainly two kinds of use cases. One is to use high illuminance light source for lighting, so that every detail of printing (looking forward to the development trend of the printing industry) can be found. It is used in strict comparison of printed matter, which is easy to protect, but has a great impact on the level of high brightness part, which may cause losses; Another case is to use relatively appropriate illumination, which is used when daily inspection and printing will achieve a total output value of 5billion yuan in 2025

fourth, uniformity

poor uniformity will make the printed matter look different in depth, so the observation light box will be equipped with a light fence. A good light box surround plate will have a special design to ensure that the light can be evenly illuminated on the sample viewing table, so that the brightness of the entire sample viewing table is very uniform. In addition, some standard light source light boxes use a diffusion system, that is, a shading plate is installed under the lamp tube, and the light passing through the shading plate will shine more evenly on the sample viewing table

for the observation conditions of the printing industry, ISO3664 has made clear provisions, which involves the previously introduced indicators (color temperature, color rendering index, illumination and uniformity) and the light source of the surrounding environment, and also stipulates the color temperature and brightness of the display, as well as the environmental requirements for soft proofing

standard light source plays a crucial role in the application of quality control and color management. But people are often the easiest to ignore it, so that the whole color management process eventually fails. In practice, the author often finds that some customers don't care about the use of standard light sources, and their reason is: printing and proofing are viewed under the same light source, and the color should be the same! But in fact, they ignored that the color presentation methods of digital proofing or screen soft proofing are different. Even if the printing sheets and signature samples are printed matter, there is also the phenomenon of same color and different spectrum due to the different inks used to produce more than 60million building blocks last year alone. The so-called metamerism means that the colors look the same, but the spectra of colors are different, so the results must be observed and compared under a specific light source. Metamerism is one of the fundamental principles of printing technology. It is a double-edged sword. On the one hand, it enables printing reproduction, on the other hand, it will bring limitations to the comparison of replicas. About metamerism, we will talk about it in a later article. In short, in order to use color management reasonably, standard light sources must be used. Because the whole color management is based on the color measurement with D50 illuminant and 2 standard observer. If the observation light source is not D50, the observation error will occur, rather than the error of the system itself