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Other Lighting Metrics

Many of the metrics we'll cover here have to do with blackbody radiation, so we'll start there.  Planck's law describes the amount of radiation over range of wavelength that an object emits based on its temperature: 

Eλ is radiated per unit volume by a cavity of a blackbody in the wavelength interval λ to λ + Δλ (Δλ denotes an increment of wavelength) can be written in terms of Planck’s constant (h), the speed of light (c), the Boltzmann constant (k), and the absolute temperature in Kelvin (T).

Lamps, candlelight and incandescent bulbs were the earliest light sources and all these sources approximate black-bodies.  Therefore, much of the early color descriptions are based on these principles.  Objects that are warmer emit more radiation and the spectrum is shifted towards shorter wavelengths (blue light).  Cooler objects emit less radiation and have spectrum shifted towards longer wavelengths (red light).  The sun can be approximated as a black-body radiator with temperature around T = 5900K.  However, sunlight around the world varies based on geographic location, time of day, season, local landscape and local weather.

Correlated Color Temperature (CCT) approximates the closest black-body radiation temperature to the spectra of a light source.  Note that most modern light sources have no underlying mechanism connecting them to black-body radiators and their spectra may look very different.  However, the CCT connects these spectra to black-body spectra by curve fitting various color functions.  You may have seen "cool white" fluorescents which have CCT around 6500K.  Similarly, "warm white" fluorescents have a CCT of 3000K.

Color Rendering Index (CRI) characterizes the closeness of a white light source spectra to natural daylight or a black-body radiator (click on the link for the details of the measurement procedure.  Note that the spectra of the source is not directly measured.  The spectra is measured after the light bounces off 8 standard color targets).  Perfect agreement is CRI = 100 and values go down from there, some being negative.  Light sources that are very close to black-body radiators like incandescent lights have high CRI, whereas lights with "peaky" spectra like common fluorescents have CRI around 60.  However, special phosphors can be incorporated into fluorescents to get them to the upper 90's.  Typical LEDs have CRI around 80; however, again special phosphors can increase CRI into the 90's but this is often at great expense to efficiency. 

Although CRI and CCT are useful metrics for lighting situations in photography, cinema and other areas, their use is limited when talking about photosynthesis.  High CRI may indicate a relatively broad spectrum; however, if the CRI is altered the expense of other key metrics like PPE then the light source may be less efficient and useful for horticulture.

Foot-candles specify illumance and specify the same quantity as lux.  They are the equivalent of the amount of illumance experienced by a point source emitting 1 lumen 1 foot away.  Some orchid growers still specify light requirements in terms of foot-candles.  1 foot-candle is equal to 10.76 lux or inversely 1 lux = .0929 foot-candles.

Candela is a unit of luminous intensity (with units of luminous power per solid angle).  We're talking about the optical engineering definition of intensity here weighted by the luminous efficiency function.  See how intensity is defined in our section on Grow Light Metrics

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