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On the left is the long wavelength, low frequency microwaves. We then progress to infrared (IR) radiation. Although humans cannot see IR radiation with the unaided eye, many nocturnal animals have the capability to see in this range. Much like a night scope, this type of vision is monochromatic. At the opposite end of the spectrum are short wavelength, high frequency ultraviolet (UV) and X-Ray spectrums. Many insects and birds are able to see UV light. Flora use UV sensitive pigments to guide potential pollinators to the reproductive area of the flower. X-Rays are common in the medical and security field as this high-energy radiation is able to penetrate objects easily. In the middle is the human visible spectrum. This represents the gamut of color we can interpret, and is only a very small slice of the entire EM range. Cameras are designed to record in the visible spectrum, however, they can "see" the entire EM range. Film cameras are simply loaded with film sensitive to the part of the spectrum we wish to capture, whether this be IR sensitive film, UV film, X-Ray film, or standard daylight (visible) film. Digital cameras, however, cannot easily change the characteristics and response of the imaging surface - they can only modify its output. They are, however, sensitive to a wider range of the EM spectrum than the human eye can visualize. Digital cameras can pick up part of the IR and UV spectrum. This is both a positive and negative. It allows us to use digital cameras for IR and UV photography, capturing a world we cannot see. However, this stray radiation can affect the image quality and color balance when recording in the visible spectrum. As such, digital cameras incorporate UV and IR cut filters to reduce the amount of stray light in the exposure.
Should the color temperature of the actual light source differ from that baseline, a color cast occurs. For example, standard incandescent lighting has a color temperature around 3000K. While our eyes and brain automatically adjust to this allowing colors to appear true, a camera will have difficulties. At 3000K the light source is biased toward the red-green end of the spectrum, and as such, a photo taken without factoring in this bias will appear to have an yellow cast. Conversely, a photo taken under 20,000K metal halide lighting will have a blue cast. The higher frequency, shorter wavelength photons from this light source skews the distribution of the light toward the higher end of the visible spectrum. A camera will try feebly to mimic the sophisticated supercomputer we refer to as our brain. Its dismal attempt is referred to as "auto" white balancing. It tries to judge the temperature of the light source through a photodiode that rests under a translucent white cover. This allows it to guess at the proper balance of light, and adjust the RGB channels accordingly. In practice, however, auto white balance works best only under simple lighting conditions, such as under the sun, the flash as the main source of light, or using the flash to fill outdoor scenes. Once lighting moves from the sun into the shade or indoors under non-standard temperatures, auto white balance chokes.
As pictured here, have the subject hold, or place the card next to the subject, and take a snapshot as the first frame of each photo session. The important thing here is for the card to receive the same light as the subject. When the light source changes, a new reference photo should be taken. This reference photo also allows us to check exposure, lighting angles, ratios, and shadows prior to starting the actual shoot. During post processing, the reference photo will allow us to check color values by measuring the color of individual patches. If a slight pink cast appears in a portrait of a child, we can go back and check the reference photo. If the skin tone patch matches the published values, the skin tone is accurate. If the child's actual skin tone in the reference photo also appears pinkish, the child may have simply been blush.
Gray cards vary in quality. Some less expensive ones are not truly neutral in color. They still reflect 18% of the light (think black and white), which is its purpose. Using a gray card not neutral in color as a reference will result in a color cast.
Again, as with all other white balance methods, a new reference photo should be taken whenever the lighting conditions change. Expodiscs, like the GretagMacbeth chart, are carefully calibrated and checked to assure neutrality. This is important as we are using these devices as references to assure our color rendition is faithful.
The best I have found is the Pringles lid method. It works on the same principle as a $100 Expodisc. The only downside is that we've got to eat those chips. You can also use the stuffing in the can to attract potential subjects. Works well on: Ducks, birds, squirrels, kids. Doesn't seem to work well on: Hot chicks. Coffee filters over the lense as an expodisc, and copy paper as a white card are not very good choices, in fact, they are poor choices and should only be used as last resort. The issue is that trees are naturally brown (source), and the purity of the white depends a lot on the bleaching methods used by different paper producers. Remember that our eyes adjust to see white as white even though there may be a slight color cast. Take two different pieces of paper of the same brightness from two different manufacturers and place them side by side. We may notice a slight variance in color between the two - one will appear more yellow than the other. This difference in color is not a good thing to have when trying to use it as a reference standard. Although there are slight differences between different lots of Pringles cans, they remain fairly consistent. For the best results, however, use a device specifically designed for color balancing. UPDATE: I have been told that Pringles lids are now clear. Bad move for Pringles, in my opinion. I think they will lose a bulk of their sales now that us photographers are no longer buying them.
From the above example, we can see that there is a blue cast resulting from the light source. The counter top appears to be painted blue and the reference card colors are all shifted towards the blue end of the spectrum. Roll the cursor over the image and examine the color corrected sample. The counter is now white and the color patches are accurately rendered color-wise. This sample was corrected in post-processing from the original RAW file by sampling the middle gray patch on the GretagMacbeth chart.
In order for the final product to have accurate color, it is important to have consistency through the entire photographic workflow, from snapping the photograph to printing. White should not only appear white to the camera, but it should also appear white on the monitor and when printed. Similar to how we tell the camera what temperature the light source is, we have to calibrate the monitor and printer to a known reference sample. Monitor: For photoediting, although LCDs are sharp and clear, CRT monitors are actually a better choice. CRTs, having a wider dynamic range, are able to represent colors more accurately than an LCD display. CRTs warm up fairly quickly, and stay at pretty consistent operating temperatures. Thus the color shift that occurs with increasing/decreasing temperature is minimized. LCDs, being more energy efficient, run much cooler and are more prone to color shifts from ambient temperature changes. Both types of devices tend to color shift with age, and it is best to color calibrate the monitor on a regular basis. As an added benefit of color calibrating our monitor, web images will appear truer in color. Printer: There exists tools to calibrate the printers also, but for the most part, color does not shift significantly since it is a function of the ink, toner, or pigments and the transfer medium. However, each printer has its own unique color characteristics. To get a faithful rendition of color, the computer, once again, needs to be told how to render color information. The International Color Consortium (ICC), established in 1993, is a group of manufacturers who developed an open standard for specifying color information. This allows a particular color to be printed accurately regardless of printer manufacturer or technology. With each printer ships a disc that contains an "ICC Printer Profile." When loaded into the operating system, applications are able to tailor output data so color is rendered accurately in the final print. Notice this ICC profile is provided by the manufacturer. Printer manufacturers create this profile using those expensive calibration tools available, and they do so with only their inks. Some manufacturers also provide different profiles for use with different papers they produce. What this means is with the use of third-party inks or paper, these ICC profiles are useless and will result in inaccurate colors. Third-party manufacturers use different ink formulas than the Original Equipment Manufacturer (OEM). As a result, their inks often appear either washed out or too dark, are more prone to bleeding, and, of course, do not match ICC profiles. For the best results, stick with OEM inks.
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