Part 4 in a 5-part series on color measurement fundamentals
Density is one of the earliest metrics used to quantify the physical absorption properties of an object. It was originally developed for the photography industry, to control the manufacturing and use of photographic films and papers. It was later adopted by the print industry for controlling film development and quantifying printed ink film thickness. It’s important to understand that density is not a “color” metric. It describes the amount of light absorbed by an object, in other words, the opposite of reflectance. This paper describes density, how it’s used in the graphic arts, and how to interpret the density data you might encounter.
Overview of Density
Density is one of the oldest color metrics still in use. Originally developed for use in photography, film and paper exposure and development processes, it eventually became a staple of the commercial printing industry. The strict definition of density is given in Eq. 1. Density is the inverse log of reflectance and increases exponentially as reflectance decreases.
The relationship between density and reflectance is illustrated in Figure 2.
The density metric became popular in photography and printing because it relates linearly to the quantities of exposure and development in photography and ink film thickness in printing, as illustrated in Figure 2. Here we’ll focus on the relationship between density and ink film thickness.
Colored objects both reflect and absorb light. Light that is not reflected is absorbed. Consider the illustration in Figure 3. White light, comprised of red, green, and blue energy, is incident upon a white object. White reflects all wavelengths and therefore absorbs little energy. Black, on the other hand, absorbs almost all the incident energy and reflects very little. The energy absorbed by the black object is dissipated as heat, explaining why you feel warmer when standing in the sun wearing a black shirt than when wearing a white shirt.
Yellow, as discussed previously and illustrated in Figure 3, reflects red and green light and absorbs blue light. A yellow object’s green and red reflectance is almost equal to that of the substrate and changes little when the amount of ink is increased or decreased. The yellow spectral reflectance curve is much lower in the blue region, where light is absorbed. As that yellow becomes more like a substrate, either by reducing the tone value or the amount of ink, the blue reflectance will increase. The same relationship is true for all colors: as the amount of ink decreases, reflectance in the absorption region of the spectrum will increase until there is no more ink and only the substrate remains.
The specific absorption region of an ink is the result of the pigments used to create the ink. Cyan, magenta, yellow, and black process colors are highly standardized and tend to have similar spectral reflectance within their color group. This is true even when inks from different processes are compared.
Density can only be analyzed by focusing on the absorption region of a spectral curve. This is accomplished by filtering out the unwanted light. Older devices called densitometers, commonly used before portable spectrophotometers and handheld color measurement instruments became widespread, had actual filters that could be rotated and placed for the measurement of specific process colors. White light would reflect off the object and pass through the filter to a detector. Yellow inks are measured using a blue filter, magenta inks using a green filter, cyan inks using a red filter, and black inks using a light greenish filter, called the “Visual” filter, shown in Figure 4. Reflectance is calculated relative to the measurement of a white calibration tile and density is calculated from reflectance.
The specific filters used in densitometers are standardized, referred to by their “Status”. Status T filters are common in the US and Status E filters are common in Europe. The physical filters themselves can be described in terms of spectral transmittance; the amount of energy transmitted through each filter at each wavelength. The only difference between Status T and Status E is in the blue filter used to measure yellow colors. The spectral transmittances for Status T and Status E filters are shown in Figure 5, color-coded as the process color for which they are used.
There are very few traditional densitometers still in use today in the printing industry. Most printers use a color measurement spectrophotometer that can calculate density. Density is calculated from spectral reflectance using Eq. 2,
where wλ is the spectral transmittance of a particular density filter, Rλ is the spectral reflectance, and Dw is the density for filter w.
The relationship between density filter transmittance and spectral reflectance is further illustrated in Figure 6, in which cyan, magenta, yellow, and black density filter transmittance curves are overlaid on respective spectral reflectance curves. The filters are all seen to align with the absorption region of each ink.
Similarly, Figure 7 and Figure 8 show the density filters overlaid on plots of process color tints. Each spectral reflectance curve is the measurement of a different halftone patch, beginning with a solid and ending with the substrate. It’s clear from the illustration that the density filters align with the spectral reflectance region that changes the most as a function of tone value.
Figure 7. Spectral transmittance of density filters overlaid on plots of cyan and magenta tint ramp spectral reflectance.
Density values for the cyan and magenta tint ramps are plotted in Figure 9, and for the yellow and black tint ramps in Figure 10. The density in both figures increases as a function of tone value, from substrate to solid.
Density as a Process Control Tool
Standardization plays a key role in the commercial printing industry. The entire printing process is engineered so that printers across the country can print to a common standard. The standardization of process color inks was mentioned previously. Paper types are also standardized in several key categories, as are the density aim points for ink printed on that paper. One standard, for example, called the Specification for Web Offset Publications (SWOP), sets aim points for the printing of magazines and other publications. The density aim points are used as targets during production. In the SpectraLab color measurement system, as in Clarios, AVT offers the ability to monitor production against density aim points, whether from a standard or a value set by the printer. The difference between the target density and the measured density is referred to as density difference, or ΔD.
This is the fourth article in a series of five articles covering color measurement fundamentals. The next article, “Understanding Tone Value” will explain various industry terms related to tone value including the difference between Nominal and Target TV and how they impact the printing process.
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