Evaluate Prints Under the Right Lights

PHOTO Techniques, Sept/Oct 2007

By Timothy Edberg Back to

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Creating a color print involves making trial prints and adjusting color and density for your next attempt. This is as true for digital printing as for darkroom printing—digital profiles and color management only take you so far, and then it’s adjustment by visual inspection. But are you examining your trial prints under the right illumination? The nature of the illumination can alter the appearance of colors, so using a poor viewing light can lead to poor choices.

The illumination affects the look of the print because we view color by the light it reflects. A lemon looks yellow because it reflects only yellow light and not other colors. If the light source is devoid of yellow, that lemon has no yellow light to reflect—the only light it can reflect—and will appear black. Black lemons? Not a good light for viewing a photo!

So what is the best choice for print lighting that won’t skew the colors? If you know where the print will hang and what lighting it will be under, the best viewing light for evaluating the print is that known illumination, but how often do you know the exhibition site beforehand? Lacking knowledge of where a print will hang, I don’t know what else to do but balance my print colors under a light that recreates the outdoor light of a sunny day when the sun is high in the sky. This is the illumination that our species has evolutionarily defined as white, a light that shows true colors.

Selecting a suitable light bulb for viewing prints under “white light” requires an acquaintance with the topics of color temperature and something called color rendering index (CRI).

Color temperature

Every object emits electromagnetic radiation in a characteristic distribution of wavelengths that is determined solely by its temperature. (Temperature in this case is measured on the Kelvin scale, abbreviated K. One degree Kelvin is the same as one degree Celsius, but zero on the Kelvin scale is absolute zero,-273 ̊ Celsius.) Something at room temperature, for example, emits most copiously at a wavelength of 10,000 nm (nanometers, one billionth of a meter). Since our eyes are only sensitive to electromagnetic waves with wavelengths between about 400 nm and 700 nm, we don’t see the radiation from room temperature things. We see them only by the light they reflect.

Electromagnetic radiation in the wavelength band from 400 nm to 700 nm is visible light. This radiation is no different from radiation at other wavelengths, except that our eyes happen to be sensitive to it. The red end of the visible spectrum is at 700nm and the blue end is at 400 nm. All the rainbow colors of orange, yellow, green, and cyan fall in between the red and blue extremes.

As an object heats up, the emitted wavelengths get shorter. If it’s hot enough, some of its emission falls within the range of visible light and we see it glow. As the temperature marches up, the emitted wavelengths march down, first entering the visible range at the 700 nm (red) end, then moving more and more into the rest of the visible range. This is why things heating up first glow a dull red, then cherry red, then orange, then yellow, and eventually white.

Figure 1 shows the electromagnetic emission of objects at various temperatures. A plot that shows the emission intensity vs. wavelength, such as this one, is called a spectrum. The color we perceive is determined (speaking loosely) by the weighted average of wavelength intensities within our visible spectrum.

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Figure 1 shows that something heated to 2,000 K, for example, will emit most intensely at 1,500 nm, which is still outside the window of visible wavelengths, but it will emit a small fraction of its radiation at wavelengths shorter than 700 nm, concentrated at the red end of the visible range. This 2,000 K item will glow red-hot. An object heated to 3,200 K— oh, say the tungsten filament of a photoflood bulb—has even more of its emission within the visible range. The bulb’s peak emission is still just beyond the red end of our visual range, but there is still a lot of red light emitted, somewhat less yellow, and even a little green and blue. We perceive this combination of colors as yellow, so these light bulbs are yellow-hot. Anything at 5,000 K has emission peaked right smack in the mid- dle of the visible spectrum, and we see it glowing white-hot. Something even hotter would glow blue-hot, although this is beyond our everyday experience.

Since every temperature produces a unique color, these colors can be identified by the temperature that produced them. There’s a one-to-one correspondence. In other words, a color can be matched to its creation temperature, the “color temper- ature” for that color. A red might have a color temperature of 2,000 K, a yellow might be around 3,200 K, white is in the vicinity of 5,000 K, and blues are 6,000 K or higher.

(Although every temperature has a corresponding color, not every color has a corresponding temperature. The color temperature concept only works for things that are glowing hot, and nothing glows “green-hot” or “magenta-hot.” Color temperature is a useful concept for reds, yellows, whites, and blues, but not for every color in the crayon box.)

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We’re at the end of our tour through color temperature. The main point is that we perceive light with a 5,000 K color tem- perature as white. This is just what we’re seeking for our daylight-equivalent print-viewing light source. The white light of a sunny day has a color temperature of 5,000 K to 5,500 K, so for indoor viewing we want a lamp with a color temperature in this range. Such a light bulb is said to be “daylight balanced.”

Fluorescent lights

So for white light illumination, you only have to run out and buy some 5,000 K incandescent bulbs, right? Wrong. Incandescents create their light by heating a filament to a given temperature; the difficulties in making bulbs and fixtures for the intense heat of 5,000 K—not to mention the baking you’d undergo in using them—make incandescent lighting impractical for producing white light.

Instead, you have to use a fluorescent bulb. Unlike incandescents, fluorescents don’t create their light by making a filament glow, so they run much, much cooler. They work by passing a high-voltage discharge through mercury gas, which excites the atoms to emit ultraviolet light. A phosphor coating on the inside of the glass bulb converts this ultraviolet light into visible light. There are myriad phosphors available, so there is great variety in the spectra of the many bulbs on the market. Most phosphors don’t provide anything close to white light. If you doubt this, check out Figure 2. We must be picky in our choice, and opt only for a daylight-balanced phosphor to get white light. Unfortunately, some “white” fluorescents are better than others.

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CRI (color rendering index)

How can some white fluorescents be better than others? Doesn’t “5,000 K” or “5,500 K” tell the whole story of a bulb? If only working with fluorescents were that easy. The problem is that a phosphor can create 5,000 K light in a variety of ways. Ideally, the light would provide a smooth, featureless spectrum like those I showed for glowing objects in Figure 1. All wavelengths would be represented in the light illuminating the print, so all colors would be revealed— no black lemons. But a bulb with a very “spiky” spectrum can look white because of how it triggers the eye’s color sensors, yet still lack many spectral colors. A spiky-spectrum white bulb can thus misrepresent colors, and perhaps even create black lemons. Examples of white and spiky, and white and smooth spectra of marketed lights are shown in Figure 3.

What this boils down to is that you need a fluorescent light that is not only 5,000 K or 5,500 K, but also one that has as smooth a spectrum as possible. A bulb that has a relatively smooth spectrum is called a “full spectrum” bulb.

The degree to which a bulb’s spectrum is spiky or smooth is measured by the color rendering index, or CRI. CRI falls on a scale of 0 to 100, where 100 corresponds to a perfectly smooth spectrum and 0 is lousy beyond belief. A CRI 100 daylight-balanced bulb would perfect- lymimic the white light of a sunny day; in practice, a CRI above 90 is considered very good for rendering colors accurately. Figure 3 shows spectra for two 5,000 K bulbs with very different CRIs.

Although CRI gets less press than color temperature, you must pay attention to it every bit as much as color temperature when selecting a fluorescent light for print viewing.

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Final word

You’ve seen that to view prints under simulated white daylight you should use illumination that is daylight balanced and has a high CRI. This lighting will of necessity be fluorescent, not incandescent. Daylight-balanced light calls for a color temperature of 5,000 K to 5,500 K; the CRI should be above 90. (Slide shoot- ers take note: this discussion of lighting for accurate colors applies to light tables as well.)

In my case, I use Sylvania Design 50 bulbs: these are 5,000 K with a CRI of 90. There are several other excellent bulbs on the market. Figure 4 shows a MacBeth ColorChecker Chart viewed under both my Sylvania lights and out- door daylight. The two images are not identical, so my lighting is not absolutely perfect, but the differences between them are so slight that they might not show up in magazine repro- duction. Short of working outdoors on sunny days only, lights such as these are the way to simulate the white light of sunny daylight.



About the Author

Timothy Edberg
timothy edberg
Timothy Edberg is an award-winning photographer and teacher whose photographs have been published in numerous photography and nature magazines. Before he heard the siren song of professional nature photography, Timothy Edberg was a Ph.D. research physicist. Now he searches the nation for spiritually uplifting photographs of the natural world.