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 ΔEab≤5.00～6.00, Fine Product ΔEab≤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
NBS Unit Color Difference
Perception Degree of Color Difference
The Tiny Color Difference: Travel
The Smaller Color Difference: slight
The Small Color Difference: Noticeable
The Big Color Difference: appreciable
The Bigger Color Difference: much