Coaxial Lighting versus Ring Light

The ring light has been the chioce of evidence photographers for years, when circumstances have dictated the need for even, shadowless subject illumination.

However, ring light illumination of subjects such as fingerprints on highly reflective surfaces will be unsuccessful due to the nature of the reflections produced on the substrate.

Shown is a representation of a semi-transparent subject being illuminated with a ring light at a short camera to subject distance.

The illumination, although quite homogenous, will still display a slight drop in intensity toward the center due to the close range from the subject to the light.

This drop in intensity will be minimal, and will have little effect on a non-reflective surface. However, its presence, due to the lack of illumination from the center area of the optic, will prevent the successful imaging of the subject.


This ring light illuminated fingerprint on a mirror surface demonstrates the downfalls encountered when using this technique.

What may be a valid method of recording evidence on a non reflective surfaces has suceeded in recording only a small portion of the fingerprint, along with a nice shot of our camera and flash reflecting back at us.

 

The coaxial prism allows the photographer to "fill in" that black hole that the camera lens presents us with when using the ring light to record subjects on reflective surfaces..


Shown is a representation of coaxial illumination of a reflective surface.

The light source (A) passes through a condenser. The homogenous output (B) strikes the semi-transparent prism (D) and is directed down onto the substrate (C) bearing the
subject..

The camera's lens axis passes through the sem-transparent prism and the reflected light path onto the subject.

In essence, it is as though the light being cast onto the subject is originating from within the camera and being projected through its lens onto the subject.

Only an interruption of the light beam will be seen by the film. That interruption will be caused by materials present on the reflective surface of the substrate.

 


Having filled in the "black hole" by using the coaxial prism to illuminate our substrate, the fingerprint present on the reflective substrate is recorded successfully.

Here is a brief lesson on the proper method of positioning your subject for optimal results.

 

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