Understanding Spectral Reflectance
Part 1 in a 5-part series on color measurement fundamentals
Spectral reflectance is the percentage of incident light reflecting off an object and is the primary unit of color measurement for color spectrophotometers. This section will give an overview of spectral reflectance and provide examples of spectral reflectance values as well as explain how to “read” spectral reflectance data.
Overview of Spectral Reflectance
The human visual system can perceive light with wavelengths roughly in the range of 400-700 nm. The shorter wavelengths (around 450 nm) correspond to blue-ness, the middle wavelengths (around 550 nm) correspond to green-ness, and the longer wavelengths (around 650 nm) correspond to redness (see Figure 1).
Spectral reflectance is a set of values, each corresponding to the amount of light reflecting off an object (e.g. a color target) at different wavelengths relative to a perfect white. Perfect white objects reflect all light incident upon them. Reflectance values are usually provided for wavelengths from 400-700 nm, in 10 nm increments. The reflectance vector, the group of reflectance valuesfor a perfect white object, will have a value of 1.0 at each wavelength (or 100%, depending on how the numbers are scaled). Most real objects measured by a spectrophotometer will have spectral reflectance values of less than 1.0.
The reflectance at each wavelength depends on the color. Objects with a warmer appearance reflect more reddish light than bluish light, and thus have higher reflectance values at longer wavelengths and lower reflectance values at shorter wavelengths. Likewise, objects that appear cooler reflect more bluish light than reddish light, and thus have higher reflectance values at shorter wavelengths and lower values at longer wavelengths.
Visualizing the approximate color of a patch from reflectance data is a useful skill. Reflectance data can be thought of in the context of additive color mixing. Imagine you are shining a red light on the wall. You then shine a green light on the wall next to the red light. Individually, they appear red and green yet when brought together, the color becomes yellow. The red color is mostly long wavelength light. The green color is mostly medium wavelength light. Yellow is a combination of both long wavelength and medium wavelength light. By extension, the spectral reflectance of yellow colors is high around the medium and long wavelengths, and low around the short wavelengths. Similarly, magenta is a combination of long wavelength red and short wavelength blue light, with little medium wavelength green light. The specific color perceived can be adjusted by changing the amounts of each wavelength. For example, a yellow becomes orange when the amount of medium wavelength greenish light is reduced.
The color mixing diagram in Figure 2 is a common tool for understanding color mixing and the relationships between different colors. It also relates well to spectral reflectance and aids the process of learning how to visualize the color of objects using spectral reflectance.
The most common visual representation of spectral reflectance is the plot of spectral reflectance as a function of wavelength. The spectral reflectance curves of four common printing inks are shown in Figure 3. As mentioned above, yellow contains high reflectance values at medium and long wavelengths. Magenta contains high reflectance values of long wavelengths and moderate values of short wavelengths. Cyan contains high reflectance values of short wavelengths and moderate values of medium wavelengths. Black contains low reflectance values across the spectrum because little light of any type is reflected.
One of the most important spectral reflectance curves to identify is the substrate. Substrate color is tightly controlled during manufacturing, but variations in substrate do exist. Substrate spectral reflectance curves can help reveal information about the dyes used in the substrate manufacturing process and whether the substrate is fluorescent or not. The plot in Figure 4 shows the spectral reflectance curves of several different white substrates. These curves represent paper, white film, white ink on clear film, and clear film over different white backings. The common characteristics are the high reflectance values across all wavelength. None of these substrates are perfect reflectors, so the reflectance values are less than one, except the section of the curves for paper with optical brightening agents (OBAs). OBAs are dyes or pigments added to most paper pulp that absorb UV energy and emit blue light. This makes the paper appear brighter. In this case, the paper is not reflecting more than 100% of the light, but the spectrophotometer cannot separate the reflected light from the fluorescent emission. Therefore, measurement is greater than 100% at the few wavelengths where light is emitted from the OBAs.
Another common feature of some white materials is a sharp drop in reflectance at 400-410 nm caused by the presence of titanium dioxide, TiO2, in the material. TiO2, a common filler in paper and white inks, reflects light well at all wavelengths except between 400 and 410nm. Most ceramic white backing materials and white films do not contain TiO2 and therefor will have higher reflectance values at 400- 410 nm.
While we “see” the color of light reflected from objects, it is also important to consider the light that objects absorb. The mixing of inks is a subtractive process (unlike the additive process of mixing colored lights). As a printer increases ink film thickness, the printed color appears darker or more chromatic because the ink absorbs more light. Rather than reflecting green and red light, one can think of yellow as absorbing blue light. The less yellow ink printed (either as a thinner film or as a halftone), the less blue light is absorbed. The plot in Figure 5 shows the spectral reflectance of a substrate (black line) and several solid colors printed on that substrate. If we think of the substrate as having the highest possible reflectance in a print, and ink as absorbing light, then it is understood that the reflectance of an ink will not be higher than the substrate (in most cases). Vice versa, the substrate will have the highest reflectance of all colors in a print, as illustrated in Figure 5.
In addition to spectral reflectance, all spectrophotometers also report color values in the L*a*b* color space, which is modeled after human color perception. However, spectral reflectance still provides the most information about the physical nature of objects: fluorescent, absorptivity, and pigment composition. When comparing the color of two objects (such as a print reference and a sample), L*a*b* and ΔE are great tools for determining whether colors are perceptually different, but spectral reflectance reveals how they are physically different and informs what actions are needed to make adjustments.
This is the first in a series of five articles covering color measurement fundamentals. The next article, “Understanding Light and Color Vision,” will help prepare you for a discussion about CIE L*a*b* and ΔE. It reviews the basis of human color vision and how these concepts were used to develop the standard observer and other standardized colorimetry systems. Illumination is also discussed, and how the science of light measurement was used to derive CIE illuminants.
AVT Color Measurement Fundamentals Series: