Page 105 - Graphic Design and Print Production Fundamentals

4.3 Measuring Devices

Alan Martin

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.

Densitometre

To measure only the volume of light, we use a densitometer. The densitometre 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.

Colorimetre

Colorimetres 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.

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