Color Temperature & Color Rendering Index DeMystified

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
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	Color Temperature Defined

Color temperature has been described most simply as a method of describing 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 emitted by a light source, measured in degrees of Kelvin. The Kelvin Color Temperature scale imagines a black body object--- (such as a lamp filament) being heated. At some point the object will get hot enough to begin to glow. As it gets hotter its glowing color will shift, moving from deep 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 a continuous spectrum. This means that they radiate light energy at all wavelengths of their spectrum, therefore rendering all the colors of a scene being lit by them, equally. Only light from sources functioning this way can meet the truest definition of color temperature.

Note - the light spectrum is wider than our ability to see it. Light values falling beneath the visible part of the spectrum are referred to as infrared, and above the spectrum as ultraviolet. Each can adversely affect an image, and you may need to add some filtration to remove them. Light sources that are not incandescent radiators have what is referred to as a "Correlated Color Temperature" (CCT). It's connotations to any part of the color temperature chart are strictly visually based. Lights with a correlated color temperature do not have an equal radiation at all wavelengths in their spectrum. As a result, they can have disproportionate levels (both high & low) when rendering certain colors. These light sources are measured in their ability to accurately render all colors of their spectrum, in a scale is called the Color Rendering Index (CRI). Incandescant radiators have a CRI of 100 (the max.) More on this below.

Color Temperature in Imaging

The above is not a true Color Temperature chart. Instead it is a hybrid, showing the color temperatures of light sources most commonly encountered in professional imaging. In our scale, tungsten-halogen has a color temperature of 3200°K. Household fluorescents are accepted to be around 4500°K, depending on the lamp. (They are shown for reference, but would not be part of a true Color Temperature chart, for reasons described below).

Sunlight is 5600°K, with shade & skylight hitting higher temperatures. These are basically averages which became standards when they were selected, back at the beginning of color film manufacturing, as the choices for various emulsions to be made sensitive to (daylight film, tungsten film, etc).

The greenish color of 4500°K fluorescent would not appear in a true Color Temperature chart because a fluorescent lamp does not get its color by heating a black body object to the point of glowing, it uses gases & phosphors instead. Household quality fluorescent lamps can have either too much green or magenta rendered in their color. The degree to which this occurs will affect the lamps CRI rating. See Color Rendering Index (CRI) below for more information.		Tungsten incandescent, most common in household lamps, has a slightly 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 they age, something tungsten-halogen lamps don't suffer from. Cinematographer Tom Robotham has a theory that we prefer the warm color of tungsten in our living environments because of our long pre-historic practice of sitting around campfires, and our pre-electric history of lighting early dwellings with fireplaces & candles. How many people have a wall dimmer in the dining room to set an intimate mood? Notice that when you dim tungsten lights, they get warmer in color, closer to flames & candles. Their color temperature is shifting lower when this happens.
Daylight is not the same as sunlight. Sunlight is the light of the sun only, where daylight is a combination of both sunlight & skylight. Outdoors, shadows are lit by skylight, since sunlight is being blocked to create them. This is why shadows in exterior day-lit shots are bluish in color.		Sunlight changes its color as it crosses the sky (or more accurately, as the Earth rotates in relation to it). At dawn & sunset the sun appears more reddish, due to the filtering nature of the denser atmospheric layer it's rays are passing thru at that angle. It has a correlated color temperature of approximately 2000°K at sunrise / sunset, and 5600°K when directly overhead. Sunset Photo by Marrike Van Irsel

Color Rendering Index (CRI) Defined

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

Examples of light sources with Correlated Color Temperatures, having CRI levels that are less than 100 would include: HMIs, and also most photo quality fluorescent lamps, as well as LEDs. With lower CRI ratings these sources may also have too much green or magenta in their spectrums. An acceptable Color Rendering Index level for professional imaging is considered to be 90 or above.

Tungsten-halogen 3200K The lamp formerly known as Quartz has a more stable color temperature throughout the life of the lamp, than tungsten-incandescant. These lamps get hot and have shorter lifespans than some others. CRI is 100.		Fluorescent (Photo Quality) 3000-3200K, or 5000-5600K Fluorescent lamps made for photo use are available in tungsten-halogen or daylight colors, with CRI's over 90. High frequency ballasts are flicker free even when shooting slo-motion.		HMI 6000K HMI stands for Hydrargyrum Medium-arc Iodide lamp. This discharge lamp has a very high output of daylight color (usually 6000K), normally with a CRI of 95+.		LED (for imaging) 3000-6000K LED stands for Light Emitting Diode. A semi-conductor based light source, that is energy efficient, with a long life. CRI is normally 70 - 90+, but subject to controversy.

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 preset white 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 a household day-flo lamps (CRI not listed, but assumed to be aprox. 80). Compare the details of each image, noting where the colors are pretty similar, such as the red & orange peppers, and the radishes. Then look at the items with colors rendered differently, such as the 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.

Human Eye vs Camera White Balance

| We humans are lucky. Our eyes adjust automatically to subtle changes in brightness\, as well as differences in the color of light sources we may find ourselves in. For example\, if you walk indoors to a warm-lit living room from a dusk exterior where everything was lit cool blue\, your eyes will adjust to the differences in brightness & color almost instantly. The change will be largely imperceptible to you when it happens. So our eyes adjust to these differences automatically\, but how do todays digital cameras deal with them? While continuing to get better\, they still aren't quite so lucky. Most digital video & still cameras have the ability to automatically set both exposure and white balance\, self-adjusting them on the fly as lighting conditions change. If a camera is set to Auto-White Balance\, it will search the image for something it perceives to be white\, lock on it\, and then adjust all other color renderings in the image based on that decision. And yes\, in this mode cameras can be fooled. So their auto-exposure / auto-white balance imaging sensors & circuitry do a clumsier job of adjusting to both brightness & color changes in lighting conditions than the human eye. And if the adjustments happen while you are shooting video\, those auto-adjustments can be recorded & visible to the viewer in playback.
A less potentailly distracting option than auto-white balance is available\, but it isnt as versatile in dealing with the subtleties of real world lighting. We are talking about the camera's Preset White Balances\, which are factory default settings for the accepted standards of the most common lighting environments. | These installed presets are created based on the color reflection information of a white (or neutral gray) object\, lit by the selected color source (exdaylight\, or tungsten-halogen).

Note - Film stocks are created using the same concept, manufactured with emulsion color balances that anticipate being used in standard tungsten or daylight environments.
Using Camera Preset White Balances
All digital video & still cameras will have Daylight & Tungsten presets\, some will also have Fluorescent\, and Cloudy Sky settings. Most still cameras will have a preset for Electronic Flash\, which is basically the same as daylight. Selecting a preset white balance that fits the type of light being used can give generally pleasing results\, and can be a popular choice when you are working quickly. However using a preset that is different from the color temperature of your lighting can give pretty inaccurate results\, as some of the following photos will show. |

Cameras and Colors of Light
4 photos to show how camera white balance presets deal with color temperatures. Notice the color of the white card in each image.

Outdoors on a sunny day with the daylight preset on the camera. Generally accurate, and a great choice if you're working fast.			Outdoors on a sunny day, but with the camera set to tungsten white balance. The camera expects the warmer 3200K but is getting the cooler 5600K.

Indoors, in a tungsten light filled room. The camera is set on tungsten white balance.			The ambient lighting in the room is still tungsten, as is the white balance setting. However, the dominent lighting of the shot is now the daylight color of the camera's electronic flash.

Storing Camera White Balances
The real world doesn't always offers us ideal scenarios. We can often find ourselves in lighting environments that fall outside the confines of the camera presets, either for practical or creative reasons. Such as:
* The color temperature of the light source you are shooting in is different than the color of the preset (a scene lit only by blue sky with no direct sun may have a color temperature closer to 6500°K than the Daylight preset of 5600°K).* You are shooting in a room with several different light sources\, thus a mixture of color temperatures.* The creative needs of your production may require a different color mood than the standard rendering that the camera presets will provide.To move outside the confines of the Presets\, you will need to store your own white balance. When storing a white balance\, you should place a white or neutral gray card in the key (main) light of the scene\, place the camera on auto exposure\, and fill the image frame with the lit card before hitting the white balance button. Note - it is important that the camera be set to auto exposure\, to avoid overexposing the white card. If overexposed\, the camera will not read the color of the light on the card accurately. Note - Some cameras have a 'tunable' white balance where you can dial in the desired color temperature you want the camera balanced to. For an experienced user\, this can bring increased creative options.

Filtering Cameras, Filtering Lights

Filtering Cameras
Film can be used with light it was not designed for, by using color correction filters on the camera lens. Another option is matching the various light sources with color correction gels, to create the color scheme the film was designed to work with. The Tiffen filters shown above offer cameras the following color corrections:
(A) - 5500K (day) to 3200K (tungsten)
(B) - 3200K (tungsten) to 5500K (day)
(C) - FLD for using fluorescents with daylight film or digital camera setting.
Filtering Light That We Cannot See
Light from beyond the visible spectrum can effect imaging. At the low end of the visible spectrum is Infrared light. While not visible to the eye\, digital sensors are very sensitive to IR. | The effect on the image is most notable in areas that should be neutral black but reproduce with a reddish tint. It can also exaggerate colors with red in them. This is partly caused by Far Red\, which is at the very edge of visible light just above Infrared. You may hear of these effects referred to as IR pollution or corruption. Not all sensors are the same so the effect can vary in intensity. (D) IR or Hot Mirror filters prevent infrared from striking the sensor\, so the blacks and reds appear truer to what the human eye sees when reproduced. A custom white balance should be done after an IR filter is added. A white (or neutral grey) reference can be shot to allow adjustments later in the processing or editing portion of the workflow. | At the high end of the visible spectrum is Ultra Violet light. Film is very sensitive to ultra violet light and many a spectacular landscape shot is rendered with a contrast-lowering bluish haze caused by the build-up of UV in the atmosphere\, over distance or high altitude. (E) A UV or Haze filter removes this extra blue for better color and normal contrast. The filters do not remove haze or color shifts due to mist, fog, or smoke. They may be clear to the eye or have light pink tint that can subtly enhance skin tones or have a slight warming effect.
Digital camera sensors are usually not overly sensitive to UV and don't require a filter; however a clear UV filter is still very often used to protect the front lens element against rain\, dust and scratching. |

Filtering Lights with Gels

Color correction gels work in much the same way as camera filters, except their effect is applied only to the output of the light they are placed in front of, instead of filtering all light into the camera. In the example to the right, the first image shows our model lit only by the daylight from a nearby window, with a camera balanced for daylight. We wanted to fill in some of the shadows so we added a 500W Omni-light, near our camera position, with just some diffusion to soften its output. The difference in color temperatures is apparent in the second image, with the tungsten color of the Omni now dominating. We added some daylight blue gel to the light which corrected its tungsten-halogen output to match both the daylight window and the white balance set in the camera. The result, in the third image, is a more natural feel to the lighting. For more information on using gels, seeLight Controls DeMystified

Daylight window only, no fill	Tungsten-halogen fill, no gel	Tungsten-halogen fill, daylight gel

	Mixing Sources With Different Color Temperatures

While our eyes quickly and constantly adapt to make a scene appear normal to us, even when it contains multiple light sources of mixed color temperatures, film and digital cameras can only be balanced to one color temperature at a time. This can create some problems, but but at the same time it can also open up creative possibilities.

Daylight Preset White Balance	Tungsten Preset White Balance

It is most convenient if all the light that falls on our scene has the same color temperature, for obvious reasons. One color means a white balance sets the camera to render the whole shot accurately. The examples above were shot in a room with daylight coming in a window, mixing with tungsten from a table lamp. The only thing changing between the images is the white balance of the camera. In tungsten lit locations with daylight windows, if you balance to the daylight part of the mix, the tungsten elements will fall warmer than they should.	This creates a certain look & feel. Or, if white balancing for just the tungsten sources, adding the daylight from the windows will make their effect look cooler, while the tungsten sources look more accurate. Daylight will usually be considerably stronger than ambient tungsten, just due to the relative strengths of the sources, so the mix will rarely be 50-50. At a daytime location, with windows & tungsten lamps, getting one color temperature might be achieved by simply blocking the windows to	remove the daylight, and then balancing for tungsten, Or switching off the tungsten lamps lamps, using just the available light from the windows & balancing for daylight. However, often we have to make images in environments that contain light sources of different color temperatures. This should not necessarily be viewed as a problem. Having different color temperature sources lighting a location scene reinforces the feeling of reality. Creatively used, it can enhance your image.

Here is an example where changing the color temperature of a light source can bring contrast and depth to an image. The key light & hair lights being used on our model are tungsten-halogen, and they do an effective job of lighting and also separating her from the background. But, notice the effect in the second image where her hair light has been gelled with 1/2 day blue gel. It instantly has more visual interest. When storing a custom white balance for a shot like this, turn off the gelled light to keep its color from interferring.

Using Lights with Selectable Color Outputs

As we have seen, the real world we want to light & shoot rarely offers us environments that are all daylight or all tungsten. Earlier in this lesson, we saw the results of choosing daylight or tungsten color in these settings. While most light souces have traditionally offered one color temperature, recent developments in lamp techology and fixture design have brought some new possibilities for color mixing. This can help find a middle ground in mixed source locations, or give you instant access to either day or tungsten color.

Photo-quality fluorescent lamps are available in daylight or tungsten color temperatures with high color rendering indexes (CRI). Mixing the 2 colors, as shown in this 4 lamp Caselite, can help light a mixed color environment more evenly.		LED's also are being manufactured in Daylight or Tungsten color, with quality suitable for photographic imagery. Some products such as Lowel Blender can mix the combined output of daylight & tungsten LED's, allowing you to find the right blend to match a mixed source environment.

Blender LED fill, mixing with daylight		Blender, mixing with household fluorescent		Blender, mixing with tungsten incandescent

Color of Light Meets Color of Skin
Certain colors of light are more complimentary to certain colors of skin tone. In general, tungsten-halogen color is more flattering to a caucasion skin-tone. it gives a warm healthy glow, especially in winter or other times when skin tones can be pale. Using daylight creates a pale look, more washed out. In certain situations, this is a creative choice that can have a lot to offer.
The opposite is often true for African skin-tones. The yellow tones of tungsten can get absorbed, and make facial details look too warm and murky. Daylight color, however, provides an effective contrast, giving strong contrasting highlights to facial features.

Tungsten-halogen	Daylight