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Fingerprints || Part 1 || Introduction

Applications of Infra-red (IR) Radiations in Questioned Document Examination


Dr. Bhoopesh Kumar Sharma
Forensic Expert & Associate Professor

Dear Students,

In the last section, we have discussed the uses of UV rays in Questioned Document analysis, and now in this section, we will summarize the applications of Infra-red for questioned document examination.  

Infrared radiation (IR), is electromagnetic radiation (EMR) with longer wavelengths than visible light. It is therefore generally invisible to the human eye, although it can be felt as heat by humans. IR wavelengths extend at 700 nanometers (frequency 430 THz) from the nominal red edge of the visible spectrum to 1 millimeter (300 GHz). Most of the thermal radiation emitted by objects near room temperature is infrared.

UV-IR Spectrum

One of the most characteristic feature of IR rays is that, they are readily absorbed and converted in to the heat energy. Unlike, UV rays, it does not have any fluorescent effect and can not be viewed by the unaided eye. The basic source of IR radiation in forensic work is 1000 watt tungsten filament lamp. Various filters are needed to screen out all other light waves and to allow only IR rays to pass through.

Applications of Infrared in Questioned Document Examination:

1. Analysis of Inks: In any disputed or altered document, in which the use of two different types of inks is suspected, can easily be deciphered through infrared radiation. An example is given below in the image. This is due to the differences among the reflectivity of the ink used. One ink will be more reflective than the other under certain wavelengths of IR. Hence, one will be more darker and other will appear lighter. 


2. Analysis of Obliterated Writings: Writings can be obliterated in number of ways. For instance, by using correction fluid and then over-writing, or by damaging the previous text and then over-writing or in any other such manner. One ink will be opaque and other will be transparent at certain wavelength. Therefore, by changing the wavelengths of IR radiation, it is possible to read the obliterated matter easily. 


3. Examination of Erasures: Small particles of the ink or pencil often linger in the outline when writing is erased mechanically. With an infrared radiation or infrared imaging, it may be possible to make the writing clear.

4. Charred Document Examination: The burnt document examination, sometimes is only possible through infrared photography, however, it depends upon the type of the ink used and the degree of burning. 

5. Cancellation Marks: After their first cancellation, postal stamps are often reused. The cancelation mark residues may not be visible duets the stamp's dark background. An infrared photograph may reveal the remains of previous marks of cancellation.

6. Water Damaged Documents: By photographing the water damaged documents with the aid of light transmitted by the blue-green infrared blocking filter, the residual ink in the paper fluoresced in the infrared and this can be recorded further to decipher the written matter. 

Dr. Bhoopesh Kumar Sharma
Forensic Expert and Associate Professor


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