T.O. 33B-1-1
2-76
2.7.4.1.3
Sources.
2.7.4.1.3.1 Incandescent and Carbon Arc Systems.
Electric current heating a tungsten element to incandescence is the typical visible light bulb familiar to everyone. The
wavelength of the associated electromagnetic radiation is generally in the visible and infrared range. It is characterized
by large amounts of heat (infrared) and visible light. Electric current arcing between two carbon electrodes generates a
high quantity of electromagnetic radiation in the carbon arc lamp. The radiation spans a range of wavelengths from
about 10 nanometers to over 10 micrometers. This covers the entire ultraviolet and visible light ranges and a portion of
the infrared range. However, little if any useful ultraviolet radiation is produced. In addition, the lamps require a high
electrical power supply and are very bulky or large due to the need for electrode drive mechanisms. Incandescent and
carbon arc systems are not used for fluorescent penetrant inspection.
2.7.4.1.3.2 Low Pressure Fluorescent “BL” Bulbs.
Low pressure, fluorescent bulbs are similar to standard fluorescent tubes. However, instead of an inert gas, the tube
contains metallic mercury. When an electric current is applied, the mercury vaporizes and emits hard (deeply
penetrating) ultraviolet radiation with a wavelength of about 254 nm. This wavelength is not useful for fluorescent
penetrant inspection. Therefore, the inside of the tube is coated with a phosphor that is activated by the hard ultraviolet
and emits black and visible light in the wavelength range of 320 to 440 nm. The amount of useful black light at 365
nm is relatively small. However, there is a large amount of both harmful short wavelength black light, (below 320 nm)
and visible light, (above 400 nm) that is emitted through the phosphor. Some of these undesirable wavelengths are
removed by the use of filters. While this reduces the unwanted radiation, it also reduces the already low amount of
useful 365 nm black light. In addition, fluorescent black light bulbs, because of their configuration, cannot be easily
focused and their intensity per unit area is below that of other type of bulbs. Thus, fluorescent “BL” black lights
SHALL NOT be used for detecting fluorescent indications.
2.7.4.1.3.3 Fluorescent “BLB” Bulbs.
Most fluorescent black lights do not produce an output sufficient to meet the requirements of ASTM 1417.
2.7.4.1.3.4 Mercury Vapor Bulbs.
High pressure, mercury vapor bulbs are the most common sources for black light. They are preferred for fluorescent
penetrant inspection because they have an acceptable output at a reasonable distance from the bulb. They can be
focused to increase their intensity over a localized area. They are available in a wide range of sizes from a 2-watt
pencil type to a 400-watt floodlight. The smaller sizes, less than 100 watt, SHALL NOT be used for penetrant
inspection unless specifically authorized. The most frequently used size is the 100-watt bulb that is mounted in a
variety of fixtures or housings and is fairly portable. The bulbs are purchased from a lamp manufacturer, and the
fixtures or holders are provided by the penetrant supplier. Figure 2-28 shows a typical 100-watt black-light source; the
base contains a transformer
