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:
- Visible light
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:
- Illumination source
- Interference filters powered by stepper motor
- Detector or photodiode
- Readout device
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.