D50燈管測試紙是用于觀察光源是否標準的標貼，可以有效察覺照明設備是否影響產品或印品的外觀顏色。
光源標貼顯示一樣的棕紅色這是表示色溫5000K的標準光源。
如看見光源標貼有兩條呈現深棕色，這表示色溫較低，如2600K的偏黃光源。
如看見光源標貼有三條呈現藍色，這表示色溫較高，如8000K的偏藍光源。

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）.