The densitometer is the most widely used instrument in the newspaper industry for quantifying and communicating color. To get the most out of your densitometer, you should make sure that it is calibrated as precisely as possible. You should also understand how it works.
Optical density can be defined as the ability of an object to stop or absorb light. Density values are obtained by measuring the proportion of the original light value to the reflected or transmitted light value.
The eye is a very sensitive comparison device. It can perceive density variations and compare them to a known standard that resides in the brain. It cannot, however, assign precise numerical values to those variations.
A densitometer, on the other hand, can assign numbers to density variations by quantifying the amount of light that is reflected from the surface of a printed sheet. A densitometer cannot be used to measure color per se--it is strictly a device to measure the density of colors.
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A densitometer produces white light from a stabilized source. The light passes through a lens, where it is focused on the printed surface. Color filters in the light path restrict the light to the wavelengths of the color being measured. Depending on the film thickness and the pigmentation of the ink, part of the light is absorbed. The non-absorbed light is reflected back by the printed surface. A lens then captures the reflected light and focuses it into a photo diode. |
The light received by the photo diode is converted into an electric current. The densitometer's electronics then compare this current with a reference value for white light. The difference is used to calculate the absorption characteristics of the ink film, and the results are shown on the densitometer's display.
The densitometer is designed to correspond to the human eye, which evaluates optical stimuli on a logarithmic scale. For example, if an observer is looking at a light table, where the glass top is illuminated by a fluorescent bulb, he perceives a certain intensity of light. If a second fluorescent bulb of equal brightness were turned on, the observer would perceive the new brightness as less than twice the original. In other words, the more the light energy is increased, the less the increase is perceived.
The ink-density values on a densitometer's display are therefore always expressed as logarithmic numbers. As the amount of available light increases, the logarithmic density value decreases. For example, if 1 percent of the light falling on a sample is reflected, the densitometer reads 2.00. If 10 percent is reflected, the densitometer reads 1.00. If 100 percent is reflected, the reading is 0.00.
These logarithmic differences are extremely important to understand when setting color on press. One example is how ruboff is affected by print density. The amount of ink that rubs off on a reader's hands is highly dependent on the ink-film thickness, which in turn corresponds to density. Ruboff varies exponentially with film thickness and becomes particularly bad when the density exceeds 1.10.
In addition to ruboff, a densitometer can help a press operator control color balance, setoff, pipe-roller build up and tracking or marking, all of which are related to print density.
Dennis Cheeseman is director of customer services for US Ink Corp. Phone is (201) 935-8666; fax, (201) 933-2291.
©1997 Newspaper Association of America. All rights reserved.
