showing just enough of the part to adequately identify the location of the indication. When photographing penetrant
indications, a through-the-lens viewing system is preferred. Cameras with a separate viewing lens will not include the
exact area when making close-up photographs. This parallax problem must be compensated for by shifting the viewer
aiming spot the distance between the lens and viewer opening.
The maximum possible amount of black light energy must be used to reduce exposure time. The usual procedure is to
use two black light lamps placed at equal distances on each side of the indication and position the camera in the
middle. This procedure provides equal light intensity across the length of the indication. The black light lamps must
be positioned so that neither the direct beams, nor reflections from them, enter the camera. Tubular (fluorescent) black
light bulbs emit more visible blue light than high pressure, mercury bulbs. Therefore, a No. 2E filter will produce a
more natural photograph when fluorescent black lights are used.
Photographic light meters may be used to estimate exposure criteria when photographing fluorescent indications but
they are not precise. Normal photographic exposure meters respond to black light to a greater degree than does the
human eye. The exposure meter must be equipped with the same ultraviolet absorbing Filter that is used on the
camera. The level of light emitted by fluorescing indications is low and a sensitive meter must be used. A meter with a
narrow angle aperture is better than a wide angle type because most black light lamps are spot type sources, and there
are wide variations in intensity over the part surface. Meter readings will be influenced by the size of the fluorescing
indication. The meter readings will be correct or slightly over-exposed when the indications are large and emit
considerable light. On average size indications, the meter reading will be correct or slightly under exposed. In general,
it is wise to assume the meter reading is only a starting point. Light meters provide a more consistent and accurate
reading when photographing visible dye indications. White developer backgrounds may result in a meter reading
calling for a slight under exposure. This can be compensated for by slightly increasing the exposure.
Lens Opening, Exposure and Bracketing.
Close-up photography requires care in selecting the lens opening to obtain an acceptable depth of field. Depth of field
is the distance range that is in focus. Lens openings are called F-stops with larger numbers indicating a smaller lens
opening. As the lens opening increases (smaller F-stop numbers), the depth of field decreases. Always use the smallest
lens opening (largest F-stop number) possible to get an acceptable depth of field to keep the entire part in focus. The
lens opening number should be higher than F5.6 for most close photography of this type as stated. Stop numbers of F6
or smaller will result in portions of the picture being out of focus. Close-up photography of fluorescent indications may
require a number of exposures to obtain optimum results. Therefore, with black and white film, three exposures should
be made: The first at the meter indicated F-stop number; the second at two F-stop numbers under the meter reading;
and the third at two F-stop numbers over the meter indicated number. A fourth exposure may be required at an
intermediate setting. With color film, the same three exposure procedure should be used to obtain a usable quality
picture. However, it is recommended that the lens openings be adjusted at one stop intervals with allowance for
indication size as discussed above. With very large or very small indications, the optimum lens opening may be three
or four F-stop numbers off the indicated value. If an exposure meter is not available, the chart in Table 2-7 can be used
a guide to estimate the exposure starting point.
Table 2-7. Typical Photographic Exposure Settings for Fluorescent Indications
(Film Speed: ASA 64; Filter: Wratten 2B).
Black Light Intensity
Large and bright
Large and bright
Average (turbine blades)
Very small (20µm, cracked chrome plate panel