Working outside the visible spectrum (Ultra-violet / infra-red) is made more easy, as filters are not needed to block visible light, if the document is lit only by the light of interest. Care should always be taken and appropriate eye protection worn when using UV light.

Professional systems are much more refined but the modern digital camera and (often free) photographic software make basic aspects of these techniques available to everyone.

Although the differences between the above images is minimal, with some documents, they can be quite pronounced, especially if the document is multi-coloured.  There are other very effective software based techniques. Such approaches can minimise the need for potentially harmful methodologies, using say, ultra-violet light , as pointed out by others.

However, there are few problems nowadays with recording UV images, especially if work is limited to UV-A (320-400 nm). Cameras can be modified to increase their sensitivity to UV light, but many are sufficiently sensitive to work without modification.

Fuji S5600 camera converted for full spectrum use plus B+W 403 filter. Illumination: Cheap, hand-held, 41 UV LED flashlight in normal room lighting (400 ISO, f3.2, 1/4sec.)

Fuji F30 compact camera (unmodified), 41 UV LED flashlight in darkened room, no filter. (400 ISO, f2.8, 1/15th sec.)

As an experiment, we took apart a £20 Premier fixed focus camera, removed the lens (held by two screws) and crudely prised out the hot mirror filter (primarily used to reduce IR transmission but also reduces UV) and then re-assembled the camera. In a darkened room, we illuminated the manuscript with the same UV LED flashlight as above and took the photo below. No filters were used. This illustrates how effective UV images can be produced with equipment (camera plus lighting) costing less than £30. The modified camera can only focus on nearby objects.

We find that UV LEDs produce crisper images than 365nm UV fluorescent tubes. The light from the LEDs also has a much more prominent visible violet component than from the tubes.