Background: UV radiation and UV photography

	How can we possibly envision the world around us seen through the UV-sensitive eyes of an insect? Some answers are obtained by using the advanced technique of Ultraviolet Colour Photography. Such images are presented here. However, in the end it is the human brain that must interpret the results to integrate these weird pictures into our ordinary world.

	The UV range starts below 400 nm, i.e. deep violet. Usually UV radiation is divided into three main ranges, these are UV-A (320-400 nm), UV-B (280-320 nm) and UV-C (below 280 nm). We humans are effectively "blind" to this radiation and it is felt mainly indirectly through sun-burns, irritated eyes and an elevated risk of skin cancer. The shorter wavelengths are the most dangerous in this respect.

	Modern optics pass very little radiation below 400 nm and this is exacerbated by current films being fairly insensitive to UV. Spectral sensitivity curves of some films are given here. So, getting a UV image onto film is easier said than done. Specialised, very expensive quartz optics are your best bet. For those not willing to spend a fortune on quartz lenses, your best alternatives are old, uncoated camera lenses (preferably pre-WW II), copying lenses or pinhole cameras. Fuji RTP is the currently best film choice for recording UV images. It must be used in conjunction with a "black", UV-transparent filter over the lens. Wratten 18A or equivalents provide the necessary filtration. The light source can be a blazing sun (the best), UV flash units, xenon or mercury lamps, or direct daylight. UV light is very diffuse to give soft shadows even under strong sunlight.

	More details on the spectral properties of daylight and of the filters used in UV photography can be found here.