T.O. 33B-1-1 2-89 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. 2.7.6.4.6 Lighting. 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. 2.7.6.4.7 Light Meters. 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. 2.7.6.4.8 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). Defect Description Black Light Intensity (µW/cm²) Lens Opening Time(sec) Large and bright 1100 F11 16 Large and bright 500 F11 60 Average  (turbine blades) 1200 F11 16 Very small (20µm, cracked chrome plate panel 1100 F8 120