How is blacklighting or transmitted lighting used in document examination?

The use of blacklighting, or transmitted lighting, in document examination is primarily achieved by placing a light source behind the document to enhance transparency. This technique is effective for revealing features within the document that may not be visible under standard lighting conditions. By shining light from behind, it allows examiners to see through to the details of the paper itself, which can include watermarks, the structure of the paper, and any alterations made to the document.

This method is particularly useful for examining security features in banknotes or IDs, as well as revealing previous writings or marks that may not be noticeable otherwise. Utilizing transmitted lighting helps to detect forgeries or modifications that would go undetected with surface illumination methods or other non-invasive techniques. It effectively highlights the differences in opacity and the interactions between light and the various inks or markings on the paper.

In contrast, illuminating the document from above or reflecting light off the surface do not provide the same depth of understanding as transmitted lighting, especially when it comes to evaluating transparency and identifying hidden features. Applying heat may uncover changes in the document but is not part of the blacklighting technique.