The Crimescope CS‑16 Forensic Equipment for Philatelic Expertizing

By Ken Lawrence (American Philatelic Society)

In the spring of 1997, the American Philatelic Expertizing Service (APEX) acquired state‑of‑the‑art forensic equipment, of the type used by scientists of the U.S. Postal Inspection Service to evaluate evidence in criminal investigations, as a tool to enhance the APS Expert Committee's ability to detect altered and counterfeited stamps and covers, and to determine whether or not questioned material is genuine and in its original state.

The system consists of a Crimescope CS‑16 forensic light source, a monochromator, an imaging device (video camera and monitor), and two recording devices (video tape recorder and paper printer). Light from the quartz CS‑16 is infinitely tunable throughout the ultraviolet, visible, and infrared spectra, allowing the operator to select specific frequencies for viewing. Even though UV and IR light spectra are invisible to human sight, the camera's receptor can see them, and we can watch what the camera sees on the video monitor.

Light from the CS‑16 is directed from one of three (UV, IR, and visible) output ports through liquid or fiber‑optic cables onto the subject being examined. As the selected portion of the spectrum illuminates the subject, light is manipulated to test first for luminescence, and afterward for reflection, absorption, and transmission.

Manipulation is achieved in three ways:

1. While passing light through tunable dichroic filters (beam splitters that transmit some colors while reflecting others, varying according to the angle of incidence), controls that rotate those filters are calibrated to denote the frequencies that are being passed at any given setting. (Intensity is regulated with a diaphragm.) Dichroic filters are built into the Crimescope CS‑16 case that houses the quartz bulb and power supply. They direct the light to the three ports, into which the liquid or fiber light pipes are plugged to direct the output.

2. Passing light through blocking filters of two types is another way to vary the spectra.

Long‑pass filters transmit all frequencies above or below a selected level. Band‑pass filters transmit only a narrow segment of the spectrum. Sets of these filters are mounted on turrets, both inside the CS‑16 case and behind the camera lens. The camera's iris diaphragm also can be used to adjust the intensity of the screen image.

3. A diffraction grating monochromator separates light being passed through it into a rainbow. Using a calibrated dial to rotate the grating so that a narrow band of the rainbow passes though a slit while the rest is blocked, a pure frequency of light can be selected. The degree of purity is determined by the aperture of the slit. Light from the CS‑16 is piped through a fiber‑optic cable into one side of the monochromator, and out the other side onto the subject by a second fiber cable.

The first test is for luminescence. This occurs when an object illuminated at one wavelength radiates light at a longer wavelength. Stamp collectors employ this technique when they use UV lamps to see tagging ‑‑ invisible UV light directed onto invisible tagging phosphors causes them to glow, radiating visible colors that people can see. With the CS‑16, this principle can be used throughout all three spectra. Any band of UV, visible, or IR illumination can be cast onto the subject, and (through filters) the resulting luminescence can be viewed at whatever longer wavelength it occurs.

After testing for luminescence, the same object is examined to see which frequencies are reflected, transmitted, or absorbed. In some instances the system's sensitivity to absorption is sufficiently acute to read back printing (such as on U.S. federal duck stamps) from the front, or to detect the watermarks of stamps on cover. The equipment excels in revealing cleaned cancellations, altered or enhanced postmarks, and counterfeit overprints.