T.O. 33B-1-12-792.7.4.1.5 IntensityRequirements.The adequacy of a black light source for fluorescent penetrant inspection is determined by measuring the intensity ofthe black light at a distance of 15 inches from the front or outside surface of the black light source filter. This intensitySHALL be at least 1000 microwatts/square centimeter (mW/cm2), and sources providing less than this intensity SHALLNOT be utilized. The actual intensity needed at the surface of the part will vary depending upon the ambient lightconditions and size of the suspected indication. Table 2-6 indicates the intensity of black light required under varyingambient light levels. Refer to paragraph 2.7.4.2.2 for measuring the ambient light. When performing portablefluorescent penetrant inspection, a dark colored canvas or photographers black cloth SHALL be used to darken the areaduring the examination. Values of 3,000 mW/cm2 can be achieved with acceptable black light sources by moving thesource closer than 15 inches to the part, yet leaving sufficient space to observe the specific area of interest. The parthas been moved closer to the black light to increase the intensity. Modern fluorescent penetrant testing has improvedsignificantly 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) Inspection ConditionsFine Cracks(20 microns wide)Coarse Cracks(50 microns wide)0.01 to 2 Fully darkened inspection booth 1000 10002 to 10 Dim interiors such aswarehouses or storage areas1000 100020 to 40 Well lighted (bright) interiors 5000 1000900 to 1000 Outdoors, cloudy day More than 20,0003000 to 7000 Outdoors, direct sunlight More than available2.7.4.1.6 MeasurementofBlackLightIntensity.2.7.4.1.6.1 MeasurementDevices.Ultraviolet light is electromagnetic energy and is measured in units of energy; hence the watts per square meter ormicrowatts per square centimeter, where one watt per square meter (W/m2) equals 100 microwatts per squarecentimeter (µW/cm2). For measurement purposes, the ultraviolet spectrum is divided into three bands: UV-A, UV-Band UV-C (see paragraph 2.7.4.1.1). 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 thescale. 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.
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