Carbon arc system. This system uses two carbon rods which are electrified individually and brought
almost together at the tips. As the tips are brought close together, an intense, hot, bright arc is formed. Filters on the
containers exclude visible light.
Low pressure fluorescent bulb. This is a tubular electric lamp which has a coating of fluorescent
material on its inner surface. The vapor inside the bulb is bombarded by electrons from the cathode and produces
High pressure mercury bulbs. The most common black lights currently in use are 100and 400-watt
quartz mercury arc lights. They are sealed into a housing which includes reflectors, and which is covered with a special
filter glass. The filter transmits black light, but excludes nearly all visible light and is opaque to the short wave length
ultraviolet light. The 100-watt lights are furnished in two types of reflectors, either spot or flood, but only the spot type is
recommended for Inspection work.
Generation of black light. Black light is a form of electromagnetic radiation. As electromagnetic radiation
is absorbed from a source that has a greater unit energy, light is produced in both the visible and invisible ranges. Filters
are then used to exclude light with specific wave lengths and allow only the black, or nearly invisible, light to pass.
Black light fixtures. Fixtures used with black light lamps are holders, filters, and on occasion, constant-
voltage transformers. The holders can be similar to desk-type fluorescent lights (i e, movable, adjustable arms or hood)
or to service station trouble lights (i.e. portable).
Filters. The glass filter used separate out the 3650A wave-length is a dark red-purple color. It is selected
to effectively remove practically all visible light from the energy given off by the mercury arc. At the same time, it also
removes all radiation of wave length below 3000A that is, it eliminates all the harmful short-wave ultraviolet. It passes
near ultraviolet radiation in the range from 4000A (the lower edge of the range of visible violet) down to 3200A. The
radiation passed by the filter peaks at 3650A, the optimum for energizing most of the fluorescent dyes used in liquid
penetrants and magnetic particle inspections. The black light filters shall be kept clean. Dust and dirt collecting on the
filter glass will reduce the black light output significantly. Care shall be taken against breakage of the filter. The glass
gets very hot from the heat of the mercury arc and even though the glass has been heat treated to minimize the effect of
thermal shock, a splash of moisture or contact with a cold object can crack it.
Intensity. The intensity of the black light radiation used to energize a fluorescent object determines the
amount of visible light which the object emits. Doubling the intensity of the black light at an indication will double the
brightness of the indication. Therefore, black light available at the indication must not fall below a safe minimum.
Approximately 800 microwatts per square centimeter is adequate in darkened inspection areas, but for critical work, it
should be greater than this. This intensity is obtainable with the 100 watt spot light over a six inch diameter circle when
the lamp is held approximately 15 inches from the surface of the part. However, testing of black light intensity with the
lamp at a specific distance is used only for procurement of lights or bulbs. When a hand-held portable light is being
used, much greater intensities are obtainable by holding the light closer to the work, or a small part can be held closer to
the lamp The operator should remember that a very little black light, 200 microwatts per square centimeter, is sufficient
to light up gross indications In the dark, but without sufficient intensity, a fine Indication may go entirely unseen.
The black light filter looks deceptively cool; however, it becomes extremely hot once the light is
turned on. The operator must exercise caution not to touch the filter or bump into It with any
exposed part of the body; otherwise, a severe bum may result. Flammable items must also be
kept away from the filter surface.
Dark adaptation. Fluorescent indications are viewed in darkness or dim light where the sensitive eye
mechanisms function. The dark adaptation of the eyes (pupil dilation) necessary to see fine fluorescent indications
requires from five to ten minutes in the dim light before it is well attained. Dark adaptation occurs slowly. The reverse,
eye accommodation to light, occurs rapidly. This means the eyes must readjust each time the operator leaves the
inspection booth. Once the eyes are dark adapted, minute light sources too small to be seen in a bright light appear
relatively brilliant and easily seen. The ability to perceive small light sources, such as fluorescent indications, is
increased by the fact that the eye is drawn to any source of light in a dark background