Standard viewing conditions for evaluating color on proofs and press sheets are critical as the metamerism effect of inks will cause some color to appear differently under different lighting conditions. By using standard viewing conditions color will appear the same when viewed at different locations. The importance of lighting goes through the whole printing facility, the viewing conditions need to be the same in the customer viewing area, pre press and also in the pressroom to eliminate time consuming changes between the different areas.
The standard for viewing color prints is ISO 3664-2009. This document specifies the lighting and viewing conditions. The light or illuminate is D50, which is a specific spectral curve between 300 and 780 nanometers. D50 is 5,000K, but there are many lights that will claim to be D50 or 5,000k that will not conform to the specification. The fluorescent bulbs that most closely match the D50 spectrum in the specification are those made specifically for the Graphic Arts industry and sold by viewing booth manufactures. There are other 5,000K bulbs available, but they are not D50 and many of these would not be considered to be acceptable for ISO viewing as they fall outside the tolerance. One would need the spectral curve measured every 5 nm to determine their compliance to the D50 spectrum. A requirement is that the bulbs have a Color Rendition Index (CRI) of greater than or equal to 90. The amount of light is another criteria for viewing compliance, for P-1 viewing (critical color) the ISO requirement is 2,000 lx, ±250 lx, (approximately 183 foot candles). The number of bulbs and distance from the viewing surface determine the amount of light or lx. To obtain the correct lighting intensity it is not practical to put D50 bulbs in overhead light fixtures in a suspended ceiling and light an entire room to P-1 viewing standards. All color critical viewing should be done in a viewing booth.
Another overlooked requirement of ISO 3664 is that amount of light (non D50) that illuminates the print being examined in the viewing booth. This means that the viewing booth should be in a dark or dimly lit room. Viewing booths with backs and sides would help reduce room light contamination. Most press consoles would have significant room light contamination and would not qualify for P-1 viewing.
The surrounding walls should be a light neutral gray such as Munsell N8 (L, 81.257, a, -0.638, b, -0.335). No decorations should be hung on the viewing booth walls, and even consider light gray aprons or smocks be available to cover any customer’s brightly colored cloths.
To help identify differences and deviations from standard lighting conditions RHEM light indicators can be used. The RHEM Light Indicator patch is 2×3/4-in. with a nonpermanent adhesive backing. The patches come in booklet form and can be easily peeled from the carrier sheet and attached to the border of a color sample.
The RHEM Light Indicator is printed with broad stripes of two magenta metameric colorants. Since the two colors are metamers and specially formulated, the stripes will not appear with a 5000 K standard light source, as shown in in Figure 1. However, stripes will appear when viewed under many common nonstandard light sources, such as cool white fluorescent, as shown in Figure 2.
Figure 1 – Appearance with 5000 K standard light source
Figure 2 – Appearance with flourescent light source
The curves in Figure 3 describe how the metameric pair of the patches function: the 5000 K light source represented by the blue curve with the two different colorants produce the same color with uniquely different stimuli. However, the incandescent light source represented by the red curve also produces different stimuli but different colors on the patch.
Figure 3 – shows sensitometric curves which show how the colorants function.
The stripes in the RHEM Light Indicator are printed with two different magenta colorants having very different spectrophotometric curves, as shown in Figure 4. View the RHEM Light Indicator under a standard 5000 K light source and no stripes will be seen. Then, view it under incandescent, cool, and warm white fluorescent light sources and see the stripes appear.
Figure 4 – Sensitometric curves of two magenta colorants
The RHEM Light Indicator is a simple, inexpensive tool that will identify many common nonstandard light sources. It will not show small color variations between light sources, such as the effects of lamp aging on 5000 K fluorescent lamps or deficiencies in light intensity. Refined photometric and radometric measurements may be necessary.