Light emitting diode and colour vision

AN2042

6/30

Figure 2.

The electromagnetic spectrum and visible region of light

The red extreme of the visible spectrum, 700 nm, requires an energy release of 1.77 eV to

provide the quantum energy of the photon. At the other extreme, 400 nm in the violet, 3.1 eV

is required.

The human vision efficacy is not constant in the entire visible region, but decreases near the

edges, as shown in Figure 3 featuring a peak value for a wavelength of 555 nm (green-

yellow).

Figure 3.

Human relative vision curve

Wavelength can be defined in terms of dominant wavelength and x-y chromaticity

coordinates, which define the color as perceived by the human eye. The dominant

wavelength is derived from the C.I.E.

(Commission Internationale de l'Eclairage - International Commission on Illumination)

Chromaticity Diagram, as shown in Figure 4 This is an international standard for primary

colors established in 1931. Based on the fact that the human eye is able to separately sense

three different portions of the spectrum (we identify these peak sensitivities as red, green

and blue), the eyes response is best described in terms of such primary colors. All the other

colors are defined as weighted sum of them.