LED Library - Lighting Terms, Luminous Efficacy & Luminaire Efficiency

Lighting Terms

- Luminous flux - unit : lumen(lm)
Luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light.
- Illuminance - unit : lux(lx)
Illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. Similarly, luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance.


- Luminous efficacy - unit : lm/W
Luminous efficacy is a property of light sources, which indicates what portion of the emitted electromagnetic radiation is usable for human vision. It is the ratio of emitted luminous flux to radiant flux. Luminous efficacy is related to the overall efficiency of a light source for illumination, but the overall lighting efficiency also depends on how much of the input energy is converted into electromagnetic waves (whether visible or not).
- Luminous intensity - unit : cd
Luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye.
- Color temperature - unit : K
Color temperature is a characteristic of visible light that has important applications in lighting, photography, videography, publishing, and other fields. The color temperature of a light source is determined by comparing its chromaticity with that of an ideal black-body radiator. The temperature (usually measured in kelvin (K)) at which the heated black-body radiator matches the color of the light source is that source's color temperature; for a black body source, it is directly related to Planck's law and Wien's displacement law.Counterintuitively, higher Kelvin temperatures (5000 K or more) are "cool" (green–blue) colors, and lower color temperatures (2700–3000 K) "warm" (yellow–red) colors. Cool-colored light is considered better for visual tasks.Warm-colored light is preferred for living spaces because it is considered more flattering to skin tones and clothing. Color temperatures in the 2700–3600 K range are recommended for most general indoor and task lighting.

Luminous Efficacy

Luminaire Efficiency

The majority of applied light source are directional light source, during the conversion from non-direct to direct light source there will be a loss of around 40~60%.

Brightness vs. Height

IP Ratings

IP (INTERNATIONAL PROJECTION) followed by two numbers, e.g. IP65. The first digit in the rating is the protection against contact and foreign bodies. The second digit in the rating is the water protection factor.