Editor’s note. This is Part 1 of a two-part series.
(Click here to read Part 2 of this article series.)

If the quality and repeatability of your prints could be expressed as a single number, that number would be density. Density is a measurement of the brightness and ink limit of a media; the higher the density, the greater the contrast. High contrast color “jumps off the page.” If you want to learn how to make your colors jump, Read on.

Ink density plays a key role in the creation of ICC profiles, as using the optimum amount of ink maximizes color without negatively affecting drying time. Profiling characterizes the color state of each ink and media to create a large color gamut without wasting ink, prolonging dry time, or creating artifacts. In fact, past the optimum ink level, density may actually go down with additional ink.

THE DENSITOMETER IS BORN
In 1963, Frank Preucil (see Figure 1), a researcher with the Lithographic Technical Foundation in Chicago (now the Printing Industries of America/Graphic Arts Technical Foundation in Pittsburgh), invented an instrument that measures contrast by shining a known quantity of light onto a surface, then measuring the percentage reflected back. The densitometer was born.

Preucil’s densitometer consisted of a light source and detector. The detector measured the percentage of light reflected by a subject, such as ink on paper. This was known as the percent reflectance.

% reflectance = reflected light ÷ incident light

Since before the second World War, measurements of density have been calculated that express light reflectance and transmittance on the logarithmic scale of the human eye. Many human senses, including hearing and seeing, operate on a logarithmic scale. This means, for example, that if the amount of reflected light is increased ten-fold, the eye perceives it as merely doubling. This gives the eye a tremendous range of sensitivity. Density is defined as:

density = log 1 ÷ reflectance

If an object reflects all (100 percent) of the incident light, it has a density of 0; if it reflects one tenth, density is 1; one one-hundredth, 2; and so on. In this equation, “log” means count the zeros, and the fraction means “count them in the denominator.”

ADDING COLOR
Preucil realized that he could more accurately measure process-color inks (cyan, magenta, and yellow) by reading them through a complimentary-colored filter, the one that was used to make separations for that color. Thus, the cyan color separation was made through a red filter, which made cyan appear black (see Figure 2) so that it could be measured more accurately.

Theoretically, each process color ink absorbs one-third of the spectrum (red, green or blue) and reflects the other two thirds. Of course, actual printing inks are not ideal. The problem with most inks is that they don’t reflect colors equally. For example, while red ink reflects blue and green (cyan), most cyan dyes and pigments don’t reflect as much green as blue (see Figure 3). This gives them a warm yellowish color and throws off all colors printed with cyan, including greens and blues. To combat this, Preucil compared cyan’s blue and green reflectance to find out how far out-of-balance they were, thus creating hue error, a number from 0–100 percent that describes how far off color a hue is.

Preucil’s measurements resulted in incremental improvements in ink formulations that noticeably improved print quality. Regardless of whether hue error is high or low, it is now compensated for by ICC profiles.

DENSITOMETERS TODAY
Densitometry first came into use in lithographic printing, where the color of ink on paper can be adjusted by manipulating the balance of ink and water. Although the eye is a good discriminator of color, the brain does not remember colors well. So visual color adjustments were subject to variability. This meant that press sheets might look different from run to run, from beginning to end of a run, or even across the sheet. Hence, the addition of densitometry as a mainstay of quality control in all forms of printing, including inkjet and digital.

Reflection densitometers measure light reflected from surfaces — generally printed press sheets or digital prints. Transmission densitometers measure the light transmittance of film—black-and-white densitometers for photo-imager film or screen-printing negatives, and color densitometers for transparencies.

Most densitometers sold today are actually spectrophotometers. Instead of measuring light through red, green, and blue filters, they measure the visible spectrum of light, which ranges from 380–720 nanometers (nm) in wavelength. Spectrophotometers generally measure reflectance every 10 nm., so they take 35 measurements in a single reading. Depending upon the model, spectrophotometers can calculate density using an onboard computer chip or through host software.

Large digital or conventional press operators that require process control most often rely upon pressroom densitometers (see Figure 4). These instruments have built-in LCDs that display density, and other variables calculated from density, including tone value and hue error.

Most large-format software RIPs have settings for ink density and linearization. Ink density can be set visually or with the aid of a densitometer. This establishes the amount of ink printed in solids. If too much ink is printed, the print will dry too slowly and printing will waste ink, and the colors will get “stuck in the mud.” If not enough is printed, colors won’t be as saturated as they could be.

Linearization must be done by measuring with a densitometer (see Figure 5 above). Once ink density is set, linearization evenly spaces all tones in between blank paper and solid ink, providing smooth images and consistent reproduction.

RIPs are generally set up to interface with commonly-available desktop spectrophotometers. These low-cost instruments have no onboard readouts and must be used with a RIP or color management application.

PRINTING SPECIFICATIONS
Densitometry provides the means of achieving print specifications. Commonly cited specifications include the Specifications for Web Offset Publications (SWOP: for magazines, catalogs, and other publications), Specifications for Non-heatset Advertising Printing (SNAP: for newspapers), and the General Requirements for Applications in Commercial Offset Lithography (GRACoL). Because of consistency demand by art directors, modern inkjet printers benefit from Preucil’s invention specifically when an image is run in several different regions through various digital print equipment, with color-matching output on every end.

(Click here to read Part 2 of this article series.)