1. The Color of Objects

There are three major elements to perceive color: light, object and observer. Any change of these three elements will influence the observer’s color perception. When the light source and the observer don’t change, then the object will determine the observer’s color perception.

Objects can affect the final color perception because the reflectance spectra (transmittance spectra) of the object have modulated the light source spectrum. Different objects have different reflectance spectra (transmittance spectra). The light source spectrum modulates the reflectance spectra (transmittance spectra) of different objects to obtain different results. The observer is the same, so it shows different colors. The theory is shown in the figure below.

2. Human Eyes to Distinguish Colors

The color difference unit, NBS, is derived from the unit of color difference formula which is established by Judd-Hunter. In 1939, the American Bureau of Standards adopts this color difference formula and calculates the color difference according to it. When the absolute value is 1, it is called “The NBS Color Difference Unit”.

Since then, people would consciously adjust the later color difference formulas to be similar with the NBS units. The formulas such as Hunter Lab, CIE LAB and CIE LUV are generally similar to NBS (not exactly the same),therefore, do not mistake the color difference units calculated by other color difference formulas are all NBS.

In national standards GB7705-87 (lithograph), GB7706-87 (letterpress), GB7707-87 (gravure) which promulgated by the National Bureau of Standards, the color printings rating in the same batch and same color according to color difference are defined as follows: General Product ΔE*ab≤5.00～6.00, Fine Product ΔE*ab≤4.00～5.00. Meanwhile, this quality standard is considered as a qualification for state enterprises promotion.

Table: The Perception Degree between NBS Unit and Color Difference