A Functional Comparison of the Densitometer and Spectrophotometer
In deciding on the appropriate color measurement instrumentation, it is essential that you understand the operational characteristics, functional comparisons, and intricacies of both the densitometer and the spectrophotometer. This is best communicated in the in-depth instrument descriptions below.
Density is a measure of the absorbance of an ink at specific wavelengths of light. As an example, yellow ink has a maximum absorbance in the blue region, so its density is calculated using a filter with a blue response. For CMYK inks, the corresponding filters are red, green, blue and “visual”, which is slightly wider than the green filter. Beyond measuring specific wavelengths of light, the densitometer also transforms the reported result according to the following relationship:
Density = – Log10 ( Remission )
Remission is a measure of the reflected light, so 1.0 is perfectly reflective and 0.0 is a perfect absorber. Following the equation above, paper that has a remission of 0.85, would yield a density of 0.07 D. A saturated ink with a remission of 0.1 would yield a density of 1.0 D. Density is useful because it corresponds closely with relative ink film thickness of most printing inks. Most observers would agree that it is very difficult to see the difference between yellow ink at 0.8 D and 1.0D and yet they would have used 125% of the ink to arrive at the 1.0D result. As a result of those characteristics, the densitometer quickly eclipses the capabilities of even the best trained eye when used on the task of monitoring ink film thickness. Establishing correct masstone density is important as the separations are done based on an assumption of a specific ink film thickness.
Once density is measured, additional data can be calculated using standardized equations. Those metrics are:
- Apparent dot area
- Apparent trap
- Dot gain and print contrast
- Slur and doubling
While Density is useful for controlling the masstones, Dot area or Dot gain / contrast monitors the halftones. Those metrics monitor tone reproduction quality which varies due to press printing conditions. Trapping evaluates ink performance as it applied directly to a substrate, as would be the case in a masstone and as it is applied to wet ink, as would be the case in an overprint. Slur and Doubling can detect defects in imaging where dots are stretched in one dimension or repeated due to a phenomenon of contamination of the blanket from previous impression.
It’s important to note that there is a significant difference between filter spectral characteristics used in various applications and locations for density measurements. The commercially available variety of accepted filter sets conforms to ANSI established standards.
- Status A and M standards are used in photographic applications.
- Status T standards represent wide band filters typically used in North America.
- Europeans use Status E filters.
- There is yet another set of narrow band filters referred to as Status I.
For calibration, color process control, and quantitative color communications it is critical that the Status response of the filter set be identified.
The previous section has identified the fundamentals of print quality measurement as provided by a traditional three filter densitometer. It’s fair to say that this technique, as useful and practical as it is for monitoring printing variables, does little to qualify the materials used during printing. To qualify the ink and paper, a spectrophotometer is more useful. The modern spectrophotometer for the press room and for ink and paper manufacturers is a cost effective, fast, and useful measurement instrument for capturing discreet spectrum amplitude values at typically 10 nm or 20 nm intervals. This instrument is typically used
- in the proofing area
- in the press room
- by paper and other substrate manufacturers
- by ink and colorant manufacturers
In the printing industry, a spectrophotometer is most often used to measure reflected light from a printed sample and compute light absorption as well as several other important parameters. As in the case of the densitometer, a stable white light source perpendicular to the printed sample is used for illumination. The main difference is that in the optical path the reflected light is separated by very narrow band filters into spectral samples spaced 10 nm to 20 nm apart. The resultant spectral reflectance curve is an excellent identifier of a color sample somewhat analogous to how fingerprints can be used to identify individuals. With such a spectral fingerprint, one can quickly see whether the magenta ink delivered to the press is formulated with rubine or rhodamine pigment, which although quite similar visually, would achieve dramatically different color across the resulting gamut. One of the most humanly intuitive metrics that can be computed from this spectral data is the Lightness (L*), Chroma (C*), and hue angle (h°) of a color sample. This type of metric has its roots in early work done by Alfred Munsell and others that attempted to organize the color that they saw in nature.
- L* – the value of the sample relating to lightness or darkness without regard to color.
- C* – a measure of the saturation or intensity of a color.
- h° – the quality of color in terms of the primary colors red, yellow, green and blue.
Computing metrics that correlate to human perception is typically called colorimetry. So, in a sense most spectrophotometers also qualify as a spectrocolorimeters as they also can provide colorimetric data based on the measured spectral data.