Datasheet

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CHAPTER 1: BEST PRACTICES ■
Valid Forensic Procedures
As a general rule, we can use valid procedures to adjust the quality of the image but
not to change the content of the image. But, as with most general rules, there are
exceptions. Some techniques are intended to make a qualitative change but result in a
change of content. And there are instances when changing the content is necessary.
Overprocessing an image so that image artifacts distort or alter the image content
is an example of making changes that result in a change of content. Strong adjustments
of contrast or brightness values can result in objects changing size or shape or blending
into each other. Caution must be used in making any image adjustments so that quali-
tative changes are not overapplied.
As to necessary changes of content, examples include adding annotations to an
image (as in a court chart) or changing the backgrounds of pictures in a photo lineup
so that one individual does not stand out from the rest. In cases where the addition of
text, lines, or other data is very obvious, the change stands for itself. In cases where an
unusual background in a photo lineup is made to better match the individuals, docu-
mentation should be included in a report so that all interested parties are aware of
the change.
Many image adjustments can be done in a nondestructive manner with adjust-
ment layers. When you use this feature in Photoshop, the unchanged image resides
as the base layer and the image adjustment layer is a separate layer that includes the
parameters set by the technician. To make corrections and enhancements to images,
one can turn off the adjustment layers and see the unchanged image and then turn on
each adjustment layer to display what changes each has made to the image quality. The
adjustment layer icon can be accessed to see the parameters used for each adjustment.
Masks can be included for each adjustment layer to apply the adjustment to specific
portions of the image.
Valid forensic methods are repeatable with similar results, are applied to groups
of pixels, and provide explainable and predictable results. Image adjustments should be
applied to a copy of the original, and an audit trail (either the procedure itself or notes)
should be a part of any valid forensic workflow.
Repeatable Processes
It is frequently necessary to make adjustments to an image. Perhaps the image has a
green cast from fluorescent lights or has very little contrast from being photographed
on an overcast day. Or, perhaps a fingerprint image is obliterated by a stain and needs
enhancement to see fine details. In all of these instances, changes need to be made to
the image to improve its quality.
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