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Standard light source in the field of printing

Release Time:2019-9-5 Click:35

The International Organization for Standardization (ISO) made provisions for the observation environment in 2000, namely ISO3664:2000:
1. Spectral requirements of the light source
According to the requirements of ISO3664:2000, ordinary fluorescent tubes are absolutely not used as a light source for observing color. Special fluorescent tubes that meet the following technical requirements must be used:
(1) The color temperature of the light source must be 5000K-6500K. The effect of observing color under the color temperature of this light source is basically similar to that observed in natural light from 8:00 to 10:00 in the majority of China and from 3:00 to 5:00 in the afternoon.
(2) The index of the light source is Ra>90
2. Light source brightness requirements
(1) The brightness of the light source irradiated on the surface of the object to be observed by reflection should reach 2000 Lux (/-500 Lux). The surface to be illuminated is in the range of 1mX1m, and the brightness of any point shall not be lower than the brightness of the center of the illuminated surface.
(2) The brightness of the light source on the surface of the object to be observed by transmission should reach 1270 cd/m2 (/-320 cd/m2)
(3) The brightness of the display should reach >75cd/m2
3. Requirements for the environment around the light source
(1) Observing other illumination sources around the source cannot directly or indirectly affect the surface of the object being observed.
(2) Observe the wall panel around the light source. The top and bottom (including the observer's own clothes) cannot use other colors. Only neutral gray with a reflectance <60 can be used.
(3) When observing a color transmission photograph, a border of 50 mm or more must be left around the photograph, and the color of the border must be black with a blackness of >90.
4. The softness requirements of the light source
The printed viewing source must have a scientifically designed enveloping system to ensure that the energy of the light from the fluorescent tube is illuminated as much as possible on the surface of the illuminated object as much as possible without the glare of light or the shadow of light.
Definition of standard light source
Because it effectively solves the problem of product quality caused by the metamerism effect, it is highly praised for the fields where printing, printing and dyeing, photography, advertising, etc. have certain requirements for color. So what is it, how is it defined? To answer this question, we must first introduce the relevant indicators about the light source.
Photometric index
Luminous flux, in lumens (lm). The sum of the amount of light emitted by the light source per unit time is called the luminous flux of the light source.
Luminous intensity, in Candela (cd). The luminous flux emitted by a light source in a unit solid angle of a given direction is referred to as the luminous intensity of the light source in that direction, referred to as light intensity.
Illuminance, unit: lux. It is the luminous flux that the light source illuminates on the unit area of ​​the object to be illuminated.
Luminance in units of candela per square meter (cd/m2). The brightness of the light source in a certain direction is the ratio of the light intensity of the light source in the same direction to the projected surface area of ​​the light emitting surface in that direction.
Luminous efficacy of light source, unit: The ratio of the total luminous flux emitted by a lumen/watt (lm/W) source to the electrical power (watts) consumed by the source is called the luminous efficacy of the source.
Average life, in hours (h). Refers to the number of hours of ignition of a batch of lamps, when 50% of the lamps are not damaged.
Chroma index
Color temperature (K). When the color of the light emitted by the light source is the same as the color of the black body radiated at a certain temperature, the temperature corresponding to the black body is called the color temperature of the light source, and is expressed by the absolute temperature K (Kelvin).
Color rendering index (Ra). We refer to the degree to which the light source represents the true color of the object as the color rendering of the light source, expressed by the color rendering index.
That is, Ra = the color that can be seen under the illumination of a certain light source. The color that can be seen under natural light.
The closer Ra is to 100%, the smaller the difference between the color developed and the color developed under natural light illumination. We can compare the color difference of the items in the following set of colors under different color rendering indexes.
According to the requirements of ISO3664:2000 established by the International Organization for Standardization (ISO), (including the observation environment) should meet the following conditions:
●The color temperature of the light source is 6500K or 5000K, and the color rendering index (Ra) is >90%.
● Illumination should be 500-2000Lux
●The uniformity of illuminance is >85%, that is, when the illuminance at the center with the strongest illumination is 100%, the illuminance of the edge region with the weakest illumination shall not be less than 85%.
●The background color of the light source should be light-absorbing neutral gray
● Observe the environment from other light or color interference.
Standard light source in the field of printing
Color and density measurements are a common method of controlling the quality of an item's reproduction in printing. In printing, we usually compare the reproduced image with the original image to determine the degree of color reduction, so as to obtain the degree of faithfulness of the original image. Although in the printing process, we try to take quantitative indicators to avoid the influence of human factors on the degree of image reduction, but the observation of the human eye still plays an important role in image restoration, so it controls the human eye in the printing field. The error in color observation is especially necessary.
Because of the different light source and related observation conditions, there are also differences in the color development of each material, which directly affects people's judgment on the final effect of the image. In order to avoid misunderstanding of color reproduction effect, it is stable during product copying. The observation conditions are very important to the printing industry.
Since observing the color and brightness of the objects around the environment and observing the surface of the object greatly affects the viewer's perception of the color of the object, as well as the popularity of printed objects worldwide, to ensure that the printed items are in the copying process and the final observation. A unified observational condition is imperative to implement an internationally accepted observation. In 1975, the International Organization (ISO) formulated "ISO3664 to observe the lighting conditions of color transparency and reproductions." In 1989, ANSI developed the observation conditions for PH2.30-1989 prints, photographs, color prints, and projections of projected laser imaging. 》. In 1994, ISO3664 was revised. In 1999, ISO3664 introduced a new version. The current international observation environment is ISO3664:2000.
Although these are different in specific details, the definition of the key factors is consistent. According to these, the key factors are mainly concentrated on:
1. Spectral energy distribution of the light source
2. Luminous intensity and uniformity of the light source
3. Observe environmental conditions (including observation of the environment and lighting environment)
4. Stability of the lighting environment.
In simple terms, the following requirements should be met:
●The color temperature of the light source is 6500K or 5000K, and the color rendering index (Ra) is >90%.
● Illumination should be 500-2000Lux
●The uniformity of illuminance is >85%, that is, when the illuminance at the center with the strongest illumination is 100%, the illuminance of the edge region with the weakest illumination shall not be less than 85%.
●The background color of the light source should be light-absorbing neutral gray
● Observe the environment from other light or color interference.