How can do you express colors in numbers by measured spectrum?

It is possible to express colors in numbers by measured spectrum?


In general, the color can be expressed by the color coordinates CIE L* a* b* and Y x y.

Y x y (D65/10) : 62.2, 0.292, 0.388



If you know the color distribution as shown the picture, you can imagine what color of this object is by knowing the value of x(0.292), y(0.388) color coordinates which can be calculated  by using transmittance or reflectance spectrum.

So, If we can measure the transmittance or reflectance spectrum, We can calculate the color coordinates and compare them exactly by numbers.

Above color coordinates value is calculated with transmittance spectrum of the actual green glass (thickness 12 mm).


L* a* b* (D65/2) : 83.0, -13.62, -1.80




L* a* b* is another way of expression of color.

L* a* b* (D65/2) : 83.0, -13.62, -1.80 values are calculated by transmittance spectrum for the same green glass as above.

+a* : red
-a* : green
+b* : yellow
-b* : blue

a*, b* values are meaning of the three primary colors (object : red, yellow, blue, light : red green, blue).

Y, L* values range is 0 to 100 which is meaning of bright or dark colors.

Then, What does it mean (D65/2) or (D65/10)?
This means the standard light D65 and observer angle 10 degree.

at first, look at the standard light,

Since the color of an object depend on which light it looks under. In principle, the color of an object should be measured under the reference sunlight.

However, as long as we live on Earth, the sunlight cannot always be the same strength because of the atmosphere.

Solar energy depends on it condition on the weather, such as cloudy and clear day, and its altitude and geographical location.

Therefore, in order to calculate the colorimetric values on the same basis, various kinds of standard illuminate were defined as shown in the table below.



The second reason is that colors look different depending on the viewing angle.

Humans feel different colors depending on the size of the picture in their retina.
This is because the number of cells that can recognize color decreases as they move away from the center of the retina.

When you look at object painted in the same color, the color of a large area, such as a wall, and the color of a small object, such as a coin, look different.

Therefore, when reporting measured values of Y, x, y or L* a* b*, the sort of standard illuminate and viewing angle shall be recorded together.