Making a smooth clean background
for small objects that are photographed using magnification
In the Fall of 1995, one of my students, Sheila Donnelly, was doing an extended work study block at the Monroe County Crime Lab, located here in Rochester under the supervision of criminologist Harvey Van Hoven. While on there, she was asked to create a "better" "systematized" approach to be used in the making of photomacro/micrographs of small opaque evidence such as paint chips and fibers. The basic imaging problem revolved around the camera systems ability to eliminate the texture from the surface of the background materials that were being used at the time. Additionally, there was considerable variability of the colors produced when using a white card as a background color when photographing using color slide films. The transparencies seemed to exhibit subtle color changes as a result of the film emulsion, the background color/tone as well as the processing. Controlling these variation would have great value for making better comparisons.
In analyzing a variety of possible solutions, one simple approach that was considered incorporating methods found in several other techniques found in medical photography. The most promising solution was to make an "semi" shadow box using a clean and unused microscope slide. To do this, the bottom of a clean microscope slide should be spray painted (only on the bottom with black or white). It was speculated, that the very shallow depth of field traditionally found in this magnification range would work to our advantage. This shallow depth of field would allow all unwanted information from the texture of any background to be eliminated and to be outside the depth of field produced from the taking lens. This in turn would allow this sharpest focus to be placed effectively on the sample surface with no competing information/texture. By painting the bottom of the slide, the texture from the paint would out of focus and any shadows created from the texture lighting would be absorbed in the black background in the same way a shadow box works in gross specimen photography. Additionally, the smooth surface of the glass would serve as a very clean surface which would produce, few if any artifacts. We chose a matte paint and one that would be durable. Obviously the bargain basement paints would have little if any value for such work.
Our goal was to isolate the samples by minimizing any any texture data produced either from the surface of the background, or shadows produced from the sample being that would be cast onto the background.
As can be seen from the 3 pictures seen below, the technique works well. Cleaning is easy with a static master brush, however the glass surface is prone to scratches so make several as they will break or scratch when it is least desirable.
|Paint chip on metal base of a macro_camera stage|
|Paint chip a piece of dark grey mount board|
|Paint chip on the home made shadow box|
Michael Peres, MS, RBP, FBPA