CCFTs make light by striking and maintaining an arc through a noble gas and exciting a phosphor layer on the inside of the tube. The CRI (Color Rendering Index) of these tubes is high, but the light output tends to be 'peaky' in that it has lots of output in some narrow bands and next to none in other bands of the visible spectrum. Manufacturers can manipulate this characteristic to some extent, but Figure 2 is representative of a typical CCFT. Note how the manufacturer has peaked the output in the green/yellow portion of the spectrum to produce more apparent brightness; this works well since the eye is more sensitive in this portion of the visual spectrum.
Figure 2: Spectral output of a typical CCFT backlight
LEDs make light by using a blue die to excite a phosphor, and the resulting light has a much smoother spectral curve (see Figure 3
). Today's LED phosphors benefit from continued development in light output, efficacy, and consistency of color. The CRI of the LEDs equals that of the CCFT, but because the light output is higher over a broader range of the spectrum, LCD modules with LED backlights can have a 'richer' look to their color palette since their intermediate colors are somewhat more saturated. However, good overall colorimetry depends on where the manufacturer’s backlight LEDs are sourced; some have good performance in the red-orange region of the spectrum, and others do not.
Figure 3: Spectral output of a typical LED backlight