# What do all these terms mean??????



## Mutt (Mar 8, 2006)

Well, After reading many many posts. It has come to my attention that a list of lighting terminology needed to be put down. It gets a little technical but the jist is there. I will add to it over time. This was just one of my documents dealing with light. It helped me understand why certain kinds of lighting were better then others when shopping. The charts at the bottom are just for generic reference. Each bulb and light manufacturer would need to researched to have an accurate chart.

CRI (Color Rendering Index): 
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. 

The higher the CRI, the better the color rendering ability. Light sources with a CRI of 85 to 90 are considered good at color rendering. Light sources with a CRI of 90 or higher are excellent at color rendering and should be used for tasks requiring the most accurate color discrimination.

SPD (Spectral Power Distribution): 

The visible part of the electromagnetic spectrum is composed of radiation with wavelengths from approximately 400 to 750 nanometers. The blue part of the visible spectrum is the shorter wavelength and the red part is the longer wavelength with all color gradations in between.

The picture on the Left Is the full spectrum of light measured in nonometers.

The picture on the right is Daylight Spectrum after passing through the Atmosphere (which filters, reflects and refracts certain spectrums of light.


CCT (Color Correlated Temperature):
Color temperature is a simplified way to characterize the spectral properties of a light source. While in reality the color of light is determined by how much each point on the spectral curve contributes to its output, the result can still be summarized on a linear scale. 

Standard unit for color temperature is *Kelvin (K)*. 

Low color temperature implies warmer (more yellow/red) light while high color temperature implies a colder (more blue) light. _Daylight_ has a rather low color temperature near dawn, and a higher one during the day.

The picture on the bottom shows the typical color temperature for standard bulbs.

Lumen:

The unit of luminous flux in the International System, equal to the amount of light given out through a solid angle by a source of one candela intensity radiating equally in all directions. 

PAR (Photosynthetically Active Radiation):

is the range of light that can be photosynthesized by plants. It coincides with the range of visible light.


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