The adequacy of a black light source for fluorescent penetrant inspection is determined by measuring the intensity of
the black light at a distance of 15 inches from the front or outside surface of the black light source filter. This intensity
SHALL be at least 1000 microwatts/square centimeter (mW/cm2), and sources providing less than this intensity SHALL
NOT be utilized. The actual intensity needed at the surface of the part will vary depending upon the ambient light
conditions and size of the suspected indication. Table 2-6 indicates the intensity of black light required under varying
ambient light levels. Refer to paragraph 126.96.36.199.2 for measuring the ambient light. When performing portable
fluorescent penetrant inspection, a dark colored canvas or photographers black cloth SHALL be used to darken the area
during the examination. Values of 3,000 mW/cm2 can be achieved with acceptable black light sources by moving the
source closer than 15 inches to the part, yet leaving sufficient space to observe the specific area of interest. The part
has been moved closer to the black light to increase the intensity. Modern fluorescent penetrant testing has improved
significantly due to the increase in black light intensities, as well as the formulation of brighter fluorescent penetrants.
This has greatly improved the sensitivity and reliability of the penetrant process.
Table 2-6. Empirical Black Light Intensity Requirements at Various Ambient Light Levels.
Minimum Intensity (µW/cm2)
Ambient Light (Lumens)
(1 Lumen = 1 Foot-Candle)
(20 microns wide)
(50 microns wide)
0.01 to 2
Fully darkened inspection booth
2 to 10
Dim interiors such as
warehouses or storage areas
20 to 40
Well lighted (bright) interiors
900 to 1000
Outdoors, cloudy day
More than 20,000
3000 to 7000
Outdoors, direct sunlight
More than available
Measurement of Black Light Intensity.
188.8.131.52.6.1 Measurement Devices.
Ultraviolet light is electromagnetic energy and is measured in units of energy; hence the watts per square meter or
microwatts per square centimeter, where one watt per square meter (W/m2) equals 100 microwatts per square
centimeter (µW/cm2). For measurement purposes, the ultraviolet spectrum is divided into three bands: UV-A, UV-B
and UV-C (see paragraph 184.108.40.206.1). Care must be exercised to assure the instrument is designed for the black light
(UV-A) or 365-nm band. This meter is filtered to respond to 365 nm and comes with a multiplier screen to extend the
scale. Digital radiometers are also available. The digital radiometers are easier to use than the meter instrument.
Examples of digital radiometers are shown in Figure 2-31.