How can a machine match a paint color perfectly?

To properly understand how a spectrophotometer works, we first need to understand how color works. This is a broad topic, so for our purposes, we'll do a brief overview. Color is made up of wavelengths of light and can be broken into six categories:

As you can see, visible light is somewhere in the middle, and this is what the spectrophotometer analyzes to match paint. The visible light spectrum is composed of good ol' ROY G. BIV, which is an acronym for red, orange, yellow, green, blue, indigo and violet. These seven colors make up all the different colors we can see with the naked eye. Black is the absence of color, and white is made up of all of these colors. This will come into play in a minute.

A spectrophotometer is actually a pretty simple piece of machinery, about the size of a shoe box. The basic components of a spectrophotometer are:

Because it's made up of all of the colors in the visible spectrum, white light is the illumination source used in a spectrophotometer, in the form of a tungsten bulb or in more advanced models, a Light Emitting Diode (LED). A clip located on the outside of the machine holds the paint sample to be matched, and white light is flooded onto the sample. This light is reflected off the sample back into the machine and onto a small wheel. The wheel is a highly efficient color analyzer, consisting of a number of interference filters and powered by a stepper motor, which is known for its precision.

Visible light is measured in nanometers, which is a tiny unit of measurement in the metric system, equal to one-billionth of a meter. Each filter is programmed to allow a specific wavelength of light to pass through it, and each wavelength is within a particular range of nanometers. The visible light spectrum ranges from the reds at 400 nm to the blues in the 700nm range, and the most efficient wheels have filters that handle intervals of 10 nm each.

Earlier spectrophotometers used transmission filters, which meant the wavelength that matched the color sample would be reflected through the filter, and all of the other wavelengths would be absorbed. An example of this is if you were to shine a white light on a red apple. The apple will absorb all of the other colors, but will reflect red back to you.

Nowadays, spectrophotometers in paint matching use interference filters, where only the color of the sample reaches the filters -- all other wavelengths are reflected away. This increases the accuracy of matching the sample because absorption can interfere with readings. The wavelength representing the correct color match passes through the correct filter and is then picked up with fiber optics and piped to a photo diode. The photo diode converts the information to an electronic signal, which is then sent to computer software that formulates the exact amount of pigments needed to make the match. And that's how ya do it, folks.

Though the technology may seem complicated, the process of using a spectrophotometer is not. It's hooked up to a touch screen computer monitor that asks all the questions, making the process user-friendly for the operator. You pick your paint base, and if you want the best match, it's wise to steer clear of the cheap stuff. Mid-grade and high-grade paint work best.

These machines are pretty adept at matching colors from a variety of sources, though the best matches happen with samples that don't have much texture or sheen. For example, glossy magazine clippings can cause a glare, and a busy wallpaper pattern or a high pile carpet present challenges in detecting the true color. Photos aren't great either because the colors aren't precise.

Older models of spectrophotometers are limited to samples that can fit in the little clip attached to the machine, but newer models have developed technology that doesn't require the sample to be in contact with the machine, so you could bring in a lamp or a door. These newer models use LED lights that project a hot, strong beam of light that easily reaches the sample while also rejecting ambient light in the room, which can get in the way of an accurate reading. 

Typically, color matching is about 90 percent accurate, and accuracy really depends on the model of spectrophotometer and the computer software that goes with it [source: House Beautiful]. Spectrophotometers typically have between 16 and 31 filters, with 31 being the most accurate. The smaller amounts you can break the wavelengths into to measure, the better. Computer software also determines accuracy. If there were 100 interference filters, but the computer could only read 15 of them, the information from the highly functioning machine wouldn't be interpreted as accurately.

If you have something you want to match that's too big to take to the store, you can buy a handheld spectrophotometer, which ranges in cost from about $225 to $299. These smaller units are less sophisticated than their countertop counterparts, and rather than coming up with a formula for an exact match, they choose the closest match to existing paint colors. Some paint manufacturers have handheld units that specifically match their selection of paints, and other units offer the full range of 13,000 colors offered by the leading paint manufacturers. Certain models even help you choose complementary colors that will coordinate with the color you’re matching, taking the guesswork out of decorating.