When fluorescent materials are energized by ultraviolet radiation, visible light is emitted. The color of the emitted
light depends upon the material. Each type emits a specific wavelength ranging from violet (400 nm) to red (700 nm).
Factors in selecting a fluorescent dye are the color emitted and the intensity of emitted fluorescent light. The most
frequently used dyes emit a yellow-green light in the wavelength band of 510 to 560 nm. This color is chosen since the
human eye has its highest response to wavelengths in the 550 nm range. Figure 2-27 shows the relative response of a
typical human eye to various wavelengths of visible light under two different lighting conditions. Curve A at 100
lumens (100 foot-candles) is typical of a well lighted inspection bench. Curve B at 2 lumens (2 foot-candles) is the
maximum white light level allowed in a fluorescent penetrant inspection booth. Under the darkened condition, the
sensitivity of the eye increases about 30 times and shifts slightly to the blue region. At a light level of 2 lumens, it is
possible for the eye to see some light wavelengths below 400 nm and above 700 nm.
Figure 2-27. Relative Response of Typical Human Eye to Visible Light of Various Wavelengths.