色温测试纸是用于观察光源是否标准的标贴，可以有效察觉照明设备是否影响产品或印品的外观颜色。
光源标贴显示一样的棕红色这是表示色温5000K的标准光源。
如看见光源标贴有两条呈现深棕色，这表示色温较低，如2600K的偏黄光源。
如看见光源标贴有三条呈现蓝色，这表示色温较高，如8000K的偏蓝光源。

The PIA/GATF RHEM Light Indicator is a tool for the visual control of color viewing. This adhesive patch can be attached to the border of color proofs, copy, reproductions, or the press sheet. The Light Indicator uses metamerism to “change color” and alert the viewer if the light source is not 5000°K, the graphic arts lighting standard.

Under a 5000°K standard light source, stripes do not appear. Stripes appear when the RHEM Light Indicator is viewed under nonstandard light sources, such as incandescent and fluorescent light （see simulation）.

RHEM Light Indicators are sold in books of 50 stickers. Each sticker measures 2.0 x 0.75 inches （51 x 19 mm）.