Photo forensics in the dark

A few years ago I was asked to investigate claims of scientific fraud in which it was suspected that several images published in a scientific journal had been inappropriately manipulated. These images of cells were devoid of the usual type of content that you see in a photograph (people, objects, lighting, shadows, reflections, etc). As a result it wasn’t immediately clear how many of the forensic techniques which I had developed could be useful. Shown below, for example, is the type of image that I was asked to analyze.

Over the years one of the things that I have learned is that, due to noise and compression artifacts, areas of an image that appear to be black are rarely perfectly black. And that these dark regions can be very informative when enhanced. This enhancement can be done using any standard photo-editing software. For example, in Adobe Photoshop select Image -> Adjustments -> Levels. The Levels panel shown below allows you to adjust the image lookup table that controls the mapping from pixel values to brightness.

The first (0) and last (255) fields, outlined in red, control the minimum and maximum pixel value that are mapped to black and white. As the maximum value is decreased, the lower pixel values begin to span a larger part of the intensity range, and higher pixel values become saturated at white. In so doing, detailed differences in the dark areas become enhanced. Shown below is an animation showing this effect as the maximum value is reduced from a value of 255 to 5 in steps of 5.

At the end of this animation the pixel values between 0 and 5 are mapped to the entire intensity range. As a result, small differences become visible and it can be seen that a region in the center of the image has most likely been erased.

When a part of the image is digitally erased, everything is erased including subtle levels of noise. Small differences in dark regions cannot usually be seen with the naked eye. By expanding the intensity range of the dark region, these areas can easily be seen.

Of course, this forensic technique only applies in very specific cases. This, however, is the nature of forensic analysis in which a suite of complementary tools are needed to analyze and authenticate an image.

[Cells photograph (C) istockphoto.com/dra_schwartz]

Comments

[Petter]

A very simple technique, but often useful!