LED lighting technical support
1.3 ----Luminous efficiency
The efficiency of a light source is determined by the luminous flux [lm] emitted per unit of power consumed by the light source [W]: lumen/Watt [lm/W].
The luminous efficiency also takes into account the perception of light by the human eye. This perception varies with the wavelength.
1.3.1 Transition from radiometry to photometry
Radiometry studies the electromagnetic waves in general
Photometry pertains to electromagnetic waves visible to the human eye, that is, light.
1.3.2 Luminous flux perceived by the human eye for 1W of radiated energy
In day vision, a light source that radiates 1W of power will be perceived differently based on the emitted wavelength (color). In day vision, a wavelength of 555nm (yellow-green) is best perceived by the eye.
The International Commission for Lighting has defined the luminous flux perceived by 1W radiated at a wavelength of 555nm as being equivalent to a flux of 683 lumens.
Figure 5: Luminous flux perceived by the human eye for a radiation of 1W at different wavelengths under day vision and night vision (International Commission for Lighting)
In night vision(2) the 507nm blue-green wavelength is best perceived by the human eye (1700lm/W).
1.4 ----Chromaticity Diagram
The chromaticity diagram defined by the International Commission for Lighting in 1931 shows the chromatic coordinates of a light. The colors present on the periphery of the diagram are saturated pure colors (monochromatic), and inside the diagram we have multi-chromatic lights.
The black curve gives the chromatic coordinates of the heated black body at different temperatures.
Perfect white can be found at coordinates x=0.3333 and y=0.33333, corresponding to a black body temperature of 5600K.
Remark: in case of the white LED NICHIA NSPW300 BS given in figure 3, the standard chromatic coordinates are x=0.310 and y=0.320 (in dashed lines on the diagram) corresponding to a cold color temperature of 7000K.
Figure 6: Chromaticity diagram and evolution curve of black bodys chromatic coordinates at temperatures