In the old days (read: "Dragnet") paint was simple. All GM cars used lacquer, and all Fords used enamels, or something like that. Cut and dried. In our more modern age, paint exists in a spectrum of colors and compositions. Formulas are jealously guarded and change every year as manufacturers experiment and shop around for good deals.
The standard equipment for paint analysis is a Fourier Transform Infrared Spectrometer. This machine sends a laser beam of light through a sample of paint which has been flattened to the point of transparency (or through a thicker sample using reflectance, or an ATR crystal-it really gets quite complicated). The sample might be one or two millimeters square-use a stereomicroscope unless you want to go blind. The point is, the FTIR produces a spectrum, which is a measurement of how much light is absorbed by the paint at different frequencies. It looks something like an EKG. The peaks and valleys on this spectrum tell the scientist what functional groups are present in the compound, say an -OH group, or a carbon-carbon double bond, or titanium dioxide.
The computer attached to the FTIR can then compare the spectrum produced by your paint to a library of spectra of other, known, samples. It will give you a list of spectra which most closely match yours-something like a computerized fingerprint system. Just like a fingerprint system, however, the computer is just a tool. The scientist must study the spectra and decide if they are similar.
Paint is applied to a car in layers. Starting at the metal, the first coat is called an electron coat, or e-coat. This is a very thin layer of dark gray. Then there is one or two coats of primer, usually a grainy gray or red-brown color. Then there is the color coat, the color you actually see when you look at the car. In most modern cars, there is a clearcoat over the color coat, which is, literally, clear. (This will turn white when you flatten it for the FTIR.) If the car has been repaired or repainted, there may be yet another layer of color and another layer of clearcoat. The more layers your unknown chip has in common with your known sample (say, from the suspect's car), the more compelling the evidence.
Go To Page: 1 2
