The main application regarding Questioned documents and the use of alternate light sources revolve around “ink differentiation”. Many inks have different formulations, even within the same apparent color type. A tunable forensic light source can be used to identify slight variations in ink type by viewing ink responses as the color of the light is tuned through the visible and infrared regions. Regardless of the skill of the forger, this examination would reveal that 2 different pens were used on the document.
There are 3 ways inks may differ visually; fluorescent, absorptions, and transmission. The problem with all 3 is they typically occur in the Infrared beyond the visual range of the human eye. While the illumination from the alternate light source can occur from the visible through the infrared, almost always the detection takes place in the Infrared above 700nm. Accordingly, detection or documentation will require the use of an infrared sensitive digital camera or the use of infrared sensitive film.
Below are a series of images used to test the authenticity of a French Passport. In all cases the “reference” or real passport is the image on the left. The images on the right represent the passport with a questioned authenticity. In all cases, note that the camera is visualizing the passports in the Infrared, beyond 715nm and our ability to see with the unaided eye. In the series of images, the only thing that changed was the wavelength of the illumination (forensic light source). In this case the wavelength changed by selecting longer wavelengths. Under visible light these passports looked identical.
As you will note, visualizing in the Infrared, the questioned passport always displayed a difference from the reference passport.
Reference left, Questioned right; note on the questioned image the bright pattern under the “RF” that does not exist in the reference.
Reference left, Questioned right; note on the questioned image the stronger pattern on the left half and the lack of bright “RF” imprint.
Reference left, Questioned right; note on the questioned image the lack of “RF” on the left half of image.
All inks are designed to absorb in the visible, to be visible to the human eye. The color of the ink can vary greatly but in the visible spectrum they will absorb light. Those same inks when exposed to differing illumination wavelengths of light from the visible through the infrared and visualized with an infrared sensitive camera the ink may appear different. If it fluoresces, it will appear white in relation to other inks. If it transmits, it will disappear or be invisible.
Below are a series of images to test the authenticity of a bank check. In this instance to see whether or not it has been altered in some way.
In the above image, the ink as written appears normal.
At this wavelength, the “Fifty” and the “5” become partially transparent and begins to fluoresce.
At this wavelength the “Fifty” and the “5” are clearly fluorescent, while the rest of the text is becoming transparent. This image clearly allows for the differentiation of the inks; inks from two different pens.
The inks are generally not engineered deliberately this way, it just happens because the companies making the ink formulations know that humans cannot see the infrared so they do not care what the ink looks like in the infrared. Happily, this allows for the detection of altered documents as it is usually very easy to differentiate between 2 inks made by different manufacturers even when they look very similar in the visible spectrum.
It is even possible to separate the inks from 2 pens from the same manufacturer if they differ in the batch from which they were created.