The standard sources term explaining in detail

1.Spectrum requirements for light source

According to ISO3664:2000, ordinary fluorescent tubes definitely cannot provide lighting for color observation. Only special fluorescent tubes that meet the following technical requirements can be used.
- Color temperature of the light source* must be 5000K or 6500K, i.e. observing colors under this lighting is similar to that under natural light between 8 to 10 a.m. and 3 to 5 p.m. in most Chinese areas. 
- Color rendering index of the light source**, Ra>90, i.e. under this light source, more than 90% colors can be observed correctly. 
Please refer to the top of page 2 for detailed explanation of “color temperature and color rendering index.”

under natural light

observing effects

under artificial light

2.Brightness of the light source

Observing the color of an object needs certain amount of light. We can image that nothing can be seen inside a completely dark room. According to ISO3664:2000:
- Light intensity on the illuminated surface of the observed object due to reflection should reach 2000Lux (+/-500 Lux). Within the 1mX1m area on the illuminated surface, light intensity of any point should not be less than 75% of the central light intensity.
- Light intensity on the illuminated surface of the observed object by transparency should reach 1270cd/m2 (+/-320cd/m²)
- Light intensity of monitor should be greater than 75cd/m².

suitable brightness

low brightness

3.Surrounding environment of the light source

Observing colors not only needs a standardized light source and appropriate brightness, but also has critical demands for the surrounding environment.
- Other light sources near the viewing light source should neither directly nor indirectly influence the viewing surface. 
- Walls, ceilings and grounds (including the clothes of the observer) near the viewing light source should not contain colors other than matt grey with reflection degree <60%. 
- When observing color transparency, their frame must be of 50mm or more in width and darkened to a brightness level >90%.

observation condition for transmissive objects

observation condition for 

reflective objects

4.Evenness of the light source

It is impossible to shine light evenly and modestly on the object surface with only a few tubes. In other words, standardized viewing light source must have scientifically designed light-containing system in order to ensure that light from the fluorescent tubes can be shone as evenly as possible on the object surface without glare or shine.

5.*Color Temperature (K)

Color temperature refers to the temperature to which one would have to heat a theoretical black body to produce light of the same visual color. Its unit is Kelvin (K). Higher K value implies colder color (more blue); lower K value implies warmer color (more red).

5000k (D50)
A type of standardized tube of warmer color (more yellow) is widely used in graphic arts and prepress industry. But based on ISO3664:2000, such light source is mandatory for the prepress area and other color management processes. 
6500K (D65)
A type of standardized tube of colder color (more blue) was previously used in press industry in America and European countries. But according to ISO3664:2000, such light source is gradually replaced by D50 light source. In China, however, D65 light source is still the most common standardized light source used in the press area.

Color temperature diagram

6.**Color Rendering Index (Ra)

Ra is a measure of the average appearance of eight standardized colors chosen to be of intermediate saturation and spread throughout the range of hues. 
Ra=color under one particular light source / color under natural light source
Ra=100% indicates that colors under one particular light source are similar to those under natural light source.
Attention: Color temperature and Ra value are two different physical concepts. Color temperature only shows the color of the light source, warm or cold. Ra value indicates the variety of colors under the light source. High color temperature does not necessarily mean high Ra value and vice versa.