Color rendering describes how a light source makes the color of an object appear to human eyes and how well subtle variations in color shades are revealed. The Color Rendering Index (CRI) is a scale from 0 to 100 percent indicating how accurate a "given" light source is at rendering color when compared to a "reference" light source. CRI is rated on a scale from 0 to 100 and only Sunlight is classified as having a CRI of 100. Colors look exactly like they should underneath a light scoring CRI of 100. So the simple concept: the higher the CRI, the better colors will look and if the lower the CRI, the worse colors will look.

  CCT (correlated color temperature)

  The correlated color temperature (CCT) is a specification of the color appearance of the light emitted by a lamp, relating its color to the color of light from a reference source when heated to a particular temperature, measured in degrees Kelvin (K). The CCT rating for a lamp is a general "warmth" or "coolness" measure of its appearance. However, opposite to the temperature scale, lamps with a CCT rating below 3200 K are usually considered "warm" sources, while those with a CCT above 4000 K are usually considered "cool" in appearance.

  The correlated color temperature (CCT) designation for a light source gives a good indication of the lamp's general appearance, but does not give information on its specific spectral power density. Therefore, two lamps may appear to be the same color, but their effects on object colors can be quite different. Examples of the CCT of some common light sources are:

  Efficiency

  Efficacy of radiation measures the fraction of electromagnetic power which is useful for lighting. It is obtained by dividing the luminous flux by the radiant flux. Light with wavelengths outside the visible spectrum reduces luminous efficacy, because it contributes to the radiant flux while the luminous flux of such light is zero. Wavelengths near the peak of the eye's response contribute more strongly than those near the edges. In SI, luminous efficacy has units of lumens per watts (lm/W). Photopic luminous efficacy of radiation has a maximum possible value of 683 lm/W, for the case of monochromatic light at a wavelength of 555 nm (green). Scotopic luminous efficacy of radiation reaches a maximum of 1700 lm/W for narrowband light of wavelength 507 nm.

  Lux

  The lux (symbol: lx) is the SI unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square meter. Illuminance is a measure of how much luminous flux is spread over a given area. One can think of luminous flux (measured in lumens) as a measure of the total "amount" of visible light present, and the illuminance as a measure of the intensity of illumination on a surface. A given amount of light will illuminate a surface more dimly if it is spread over a larger area, so illuminance (lux) is inversely proportional to area when the luminous flux (lumens) is held constant. One lux is equal to 1lumen per square meter.

1lx = 1lm/m2 = 1cd.sr/m2

  Lumen

  Light measurements can either be radiometric or photometric. Radiometric measurements measure all the wavelengths of a light source, both visible and invisible. Photometric measurements measure only the visible wavelengths of light. The total electromagnetic energy that a light source emits across all wavelengths is known as radiant flux, and is measured in watts. The total energy that a light source emits across the visible wavelengths of light is known as luminous flux, and is measured in lumens. One lumen is the amount of light emitted in a solid angle of 1 sr, from a source that radiates to an equal extent in all directions, and whose intensity is 1 cd. One lumen is the equivalent of 1.46 mill watt (1.46 x 10-3 W) of radiant electromagnetic (EM) power at a frequency of 540 terahertz (540 THz or 5.40 x 1014 Hz). Reduced to SI base units, one lumen is equal to 0.00146 kilogram meter squared per second cubed (1.46 x 10-3kg multiplied by m2 / s3).

  Candela

  The candela (abbreviation, cd) is the standard unit of luminous intensity in the International System of Units (SI). It is formally defined as the magnitude of an electromagnetic field, in a specified direction, that has a power level of 1/683 watt (1.46 x 10-3 W) per steradian at a frequency of 540 terahertz (540 THz or 5.40 x 1014Hz).