Monochromatic light sources (AX ^ 0) are located on the perimeter of the chromaticity diagram. Light emission from LEDs is monochromatic (single color) to the eye but LEDs are not monochromatic in the strict physical sense since LEDs have a spectral linewidth of about 1.8 kT. Owing to the finite spectral linewidth of LEDs, they are not located on the very perimeter of the chromaticity diagram but are located close to the perimeter. When a source emits light distributed over a range of wavelengths, then the chromaticity location moves towards the center of the diagram.

The location of different LEDs on the chromaticity diagram is shown in Fig. 17.10. Inspection of the figure reveals that the location of red and blue LEDs is on the perimeter of the chromaticity diagram. That is, their color purity is very high, close to 100%. However, blue - green and green LEDs are located off the perimeter closer to the center of the diagram due to the finite linewidth of the emission spectrum and the strong curvature of the chromaticity diagram in the green wavelength range.