We measure light to provide the data needed to manage colour in a graphic production environment. There are three ways to measure light and three corresponding tools available to take those measurements: densitometer, colorimeter, and spectrophotometer.
To measure only the volume of light, we use a densitometer. The densitometer provides a known volume of light and then records what remainder of that light is returned to the device. A transmissive densitometer records how much light gets through a semi-transparent material such as camera film, and a reflective densitometer measures how much light has bounced back. The majority of densitometers in the print environment are reflective.
How does measuring the volume of light help us? Maintaining a consistent thickness of ink in printing is a very good way to control consistency and quality, and measuring the amount of light absorbed by the ink is a very accurate indicator of ink thickness.
Since our eyes have to function over a very wide range of brightness, we have a non-linear response to increasing volumes of light. That means it takes approximately 10 times the amount of light for us to experience one step in our perception of brightness. To match this behaviour of our eyes, the density scale is based on powers of 10, with each larger whole number representing one-tenth the volume of light of the preceding number. A density reading of 1.0 means that 1/10 of the original light has been reflected back. This is a typical reading for a process Yellow patch in offset lithographic printing. A density reading of 2.0 indicates that 1/100 of the original light is returned, while a density reading of 3.0 shows only 1/1000 coming back. Black ink is usually in the 1.7 density range, with cyan and magenta at 1.3 to 1.4.
Scanning or hand-held densitometers are typically found in the viewing station by a press. Densities are recorded when the printed sample matches the desired result and then ongoing adjustments to maintain the target densities keep the printing on target.
Colorimeters mimic the three-colour response of our eyes by using red, green, and blue filters to measure the amount of light present in each third of the spectrum. They have built-in software to calculate Lab values based on what volume of red, green, and blue is returned from a sample. Colorimeters are particularly useful for calibrating and profiling monitors. Some well-known examples of colorimeters are the X-Rite ColorMunki or i1 Display devices.
Spectrophotometers measure slices of the spectrum to produce a spectral ‘map’ of the light reflected back from a sample. Spectrophotometers are typically more expensive than densitometers and colorimeters but are employed because they can more accurately do the jobs of both devices. They work by recording the light at specific wavelengths over the wavelength range of visible light, and then by converting this spectral data to colorimetric and densitometric values.
While we are talking about measuring spectral values, it is important to note that we do not depend on identical spectral values to achieve matching colour experiences. Different spectral values can trigger the same volume of colour signals in our optic system and lead to matching colour perception. In fact, we depend on this phenomenon in graphic production in order for proofing devices to simulate the colour output of a printing press or for any two devices to be colour aligned. The ability of the CMYK (cyan, magenta, yellow, black) process colour set to mimic most of the colours in the world is also based on the fact that we can achieve a colorimetric match without having identical spectral values.