Color Temperature & Color Rendering Index DeMystified

http://lowel.tiffen.com/edu/color_temperature_and_rendering_demystified.html

"What is color? No object of itself alone has color. We know that even the most brightly colored object, if taken into total darkness,loses its color. Therefore, if an object is dependent upon light for color, color must be a property of light. And so it is." Paul Outerbridge, Photographer 1896 - 1958

Color Temperature Defined

Color temperature has been described most simply as a method ofdescribing the color characteristics of light, usually either warm (yellowish)or cool (bluish), and measuring it in degrees of Kelvin (°K). That's a little too simple to be of more than introductory value. A more technical definition assigns a numerical value to the color emittedby a light source, measured in degrees of Kelvin. The Kelvin ColorTemperature scale imagines a black body object--- (such as a lamp filament)being heated. At some point the object will get hot enough tobegin to glow. As it gets hotter its glowing color will shift, moving fromdeep reds, such as a low burning fire would give, to oranges & yellows,all the way up to white hot. Light sources that glow this way are called"incandescent radiators", and the advantage to them is that they have acontinuous spectrum. This means that they radiate light energy at allwavelengths of their spectrum, therefore rendering all the colors of ascene being lit by them, equally. Only light from sources functioning thisway can meet the truest definition of color temperature.

Note - the light spectrum is wider than our ability to see it. Light valuesfalling beneath the visible part of the spectrum are referred to asinfrared, and above the spectrum as ultraviolet. Each can adverselyaffect an image, and you may need to add some filtration to removethem. Light sources that are not incandescent radiators have what is referredto as a "Correlated Color Temperature" (CCT). It's connotations to anypart of the color temperature chart are strictly visually based. Lights witha correlated color temperature do not have an equal radiation at allwavelengths in their spectrum. As a result, they can have disproportionatelevels (both high & low) when rendering certain colors. Theselight sources are measured in their ability to accurately render all colorsof their spectrum, in a scale is called the Color Rendering Index (CRI).Incandescant radiators have a CRI of 100 (the max.) More on thisbelow.

Color Temperature in Imaging

The above is not a true Color Temperaturechart. Instead it is a hybrid, showing the colortemperatures of light sources most commonlyencountered in professional imaging. In ourscale, tungsten-halogen has a color temperatureof 3200°K. Household fluorescents areaccepted to be around 4500°K, depending onthe lamp. (They are shown for reference, butwould not be part of a true Color Temperaturechart, for reasons described below).	Sunlight is 5600°K, with shade & skylight hitting highertemperatures. These are basically averageswhich became standards when they wereselected, back at the beginning of color filmmanufacturing, as the choices for variousemulsions to be made sensitive to (daylightfilm, tungsten film, etc).	There are variations on these standards, butthis is a good start to understanding the relationshipbetween different colored lightsources. It's important to keep in mind thateven tho' reddish light has a technically lowercolor temperature, its frequently described aswarm. Bluish light, which has a higher colortemperature, is described as cool. In thisinstance, warmer & cooler describe color, nottemperature.

The greenish color of 4500°K fluorescent would not appear in a trueColor Temperature chart because a fluorescent lamp does not get itscolor by heating a black body object to the point of glowing, it usesgases & phosphors instead. Household quality fluorescent lamps can have either too much greenor magenta rendered in their color. The degree to which this occurswill affect the lamps CRI rating. See Color Rendering Index (CRI) below for more information.		Tungsten incandescent, most common in household lamps, has aslightly lower color temperature at 2900°K than tungsten-halogen(aka quartz) at 3200°K, so its output will be slightly warmer.Incandescant lamps also shift their color, growing warmer as theyage, something tungsten-halogen lamps don't suffer from. Cinematographer Tom Robotham has a theory that we prefer thewarm color of tungsten in our living environments because of ourlong pre-historic practice of sitting around campfires, and our pre-electrichistory of lighting early dwellings with fireplaces & candles.How many people have a wall dimmer in the dining room to set anintimate mood? Notice that when you dim tungsten lights, they getwarmer in color, closer to flames & candles. Their color temperatureis shifting lower when this happens.
Daylight is not the same as sunlight. Sunlight is the light of the sunonly, where daylight is a combination of both sunlight & skylight. Outdoors, shadows are lit by skylight, since sunlight is being blockedto create them. This is why shadows in exterior day-lit shots arebluish in color.		Sunlight changes itscolor as it crosses thesky (or more accurately,as the Earth rotatesin relation to it). At dawn & sunset thesun appears more reddish,due to the filteringnature of thedenser atmosphericlayer it's rays are passingthru at that angle. Ithas a correlated colortemperature of approximately2000°K at sunrise/ sunset, and5600°K when directlyoverhead. Sunset Photo by Marrike Van Irsel

Color Rendering Index

Color Rendering Index (CRI) Defined

A simple definition of Color Rendering Index (CRI) would measurethe ability of a light source to accurately render all frequencies of itscolor spectrum when compared to a perfect reference light of a similartype (color temperature). It is rated on a scale from 1-100. Thelower the CRI rating, the less accurately colors will be reproduced.Light sources that are incandescent radiators have a CRI of 100since all colors in their spectrum are rendered equally. As stated earlier,light sources that are not incandescent radiators will haveCorrelated Color Temperatures.

Examples of light sources with Correlated Color Temperatures, havingCRI levels that are less than 100 would include: HMIs, and also mostphoto quality fluorescent lamps, as well as LEDs. With lower CRI ratingsthese sources may also have too much green or magenta intheir spectrums. An acceptable Color Rendering Index level for professionalimaging is considered to be 90 or above.

Tungsten-halogen 3200K The lamp formerly known asQuartz has a more stable colortemperature throughout the lifeof the lamp, than tungsten-incandescant. These lamps get hotand have shorter lifespans thansome others. CRI is 100.

Fluorescent (Photo Quality) 3000-3200K, or 5000-5600K Fluorescent lamps made for photouse are available in tungsten-halogenor daylight colors, with CRI's over 90.High frequency ballasts are flickerfree even when shooting slo-motion.

HMI 6000K HMI stands for HydrargyrumMedium-arc Iodide lamp. Thisdischarge lamp has a very highoutput of daylight color (usually6000K), normally with a CRI of95+.

LED (for imaging) 3000-6000K LED stands for Light EmittingDiode. A semi-conductor basedlight source, that is energy efficient,with a long life. CRI is normally70 - 90+, but subject tocontroversy.

Comparison of High & Low CRI Fluorescent Lamps These 2 images are worth really studying. Both were shot with daylight colored fluorescent lamps and a camera with daylight presetwhite balance. The image on the left was shot with Lowel 27w day-flo lamps (CRI 92+), and the image on the right side was shot with ahousehold day-flo lamps (CRI not listed, but assumed to be aprox. 80). Compare the details of each image, noting where the colorsare pretty similar, such as the red & orange peppers, and the radishes. Then look at the items with colors rendered differently, such asthe floor, cutting board, carrots, cabbage, and lettuce leaf, for example. A low Color Rendering Index does not mean all colors will shift,and no 2 lamps with the same low CRI rating will necessarily have the same errors in rendering.