RHEM Light Indicators

The Printing Industries of America RHEM Light Indicator is a small adhesive patch with a unique printed design. It is attached to color proofs, color copy, color reproductions, or press sheets to indicate whether or not they are being viewed under a 5000K (degrees Kelvin) standard light source. Stripes will appear on the patch if a nonstandard light source is used.

The use of the RHEM Light Indicator can prevent costly and time-consuming changes. Clients, agencies, photographers, trade houses, color labs, printers, and manufacturers of ink, paper, and film all can benefit from consistent use of the Printing Industries of America RHEM Light Indicator.

Design Of The Printing Industries of America RHEM Light Indicator
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.

How The Light Indicator Works
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

Purpose Of The RHEM Light Indicator
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.

Published on Friday, October 10, 2008 (updated 08/27/2014)

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