28
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
NBS Unit Color Difference
Perception Degree of Color Difference
0
．
0
～
0.50
0.5
～
1.51
1.5
～
3
3
～
6
Above 6
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
