Figure 1.2.1 depicts basic parameters of LED light source compared with the most common traditional light sources. LED shows better or at least comparable numbers for all important parameters than traditional light sources.
Figure 1.2.1: Various light sources versus LED.
Efficacy of LED luminaires
The efficacy (energy efficiency) of LED lighting fixtures is ratio between net lumen output (in lumens) and input power (in watts) of a luminaire, or lm/W. LEDs with the highest efficacy are the coolest whites – 5000 K and above.
Color rendering index – CRI
Color rendering index measures the ability of a light source to render colors of illuminated objects faithfully in reference to an ideal light source – sun or incandescent bulb.
Figure 1.2.2: Standardized color samples set.
Correlated color temperature – CCT
Color temperature of a light source is the temperature of an ideal black-body radiator (solid object with certain properties heated up to point of incandescence) that radiates light of comparable hue to that of the light source, and its temperature is expressed in Kelvins (K). As a black body gets hotter, wavelenght of light emits progress through a sequence of colors from red to blue (Figure 1.2.3). Sequence of colors is described by curve (Planckian locus) within a CIE 1931 color space (Figure 1.2.4).
Figure 1.2.3: Example of LED color temperature correlation.
For colors based on black body theory, blue occurs at higher temperatures, while red occurs at lower temperatures. This is the opposite of the cultural associations attributed to colors, in which “red” is “hot”, and “blue” is “cold”.
Figure 1.2.4: The black-body curve (Planckian locus) defines the range of color temperatures, from warm (reddish) to cold (bluish), within the CIE 1931 color space.