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.