Films for UV Colour Photography

Prerequisites for UV Colour Work
In theory, any film capable of recording long-wave UV; ie. the UV-A/B bands, should be eligible for UV Colour Photography. One simply has to emply a UV bandpass filter that blocks all visible light from being received onto the film surface. Such is not the case, however. Why is that?

Firstly, the UV bandpass filters available to us are not perfect. They transmit some additional spectral bands in the deep red and near infrared. These sidebands are strongly attenuated by the filter, but still with a transmitted energy level capable of exposing the film. Secondly, we have to remember that UV radiation is just a fraction of the visible light energy. So, even for a film equally sensitive to UV and ordinary light, the effective UV speed would be far lower than the film's rated speed. If energy outside the UV range reaches the film, this would result in an image where the UV contribution is literally swamped by other spectral bands. Most modern films additionally incorporate UV screening layers in their design. Thus, much of the UV radition impinging onto the film is lost even before the film layers can be impacted. Besides, most lenses pass very little UV because of their coatings that absorb UV. Add this to the fact that the colour coupling dyes rapidly loose sensitivity towards the deep violet and UV range, and it's clear that UV colour photography is not an easy task. However, "not easy" significantly differs from "impossible"!

I found early in my experimental UV photography days that tungsten-balanced films are preferable to normal films for UV colour work. There are several reasons for this. They have an enhanced sensitivity to blue light that also helps record UV better. Simultaneously, their lowered sensitivity to yellows and reds means the UV colour signature recorded on the film is less likely to be contaminated by the far red sideband of the filter.

Tests revealed that Kodak tungsten films (EPY, E160T, E320T) were less suitable for UV colour photography than Fuji RTP. The UV images obtained with the Ektachromes were soft and exhibited muted colours. A reddish tinge frequently overlaid the Kodak images. This colour pollution of the UV images was much less evident on Fuji RTP, which rendered UV scenes with vibrantly rich colours in blue, white and red. Fuji Provia (RDP-II) gave quite useful images too, but not up to the standard possible wth RTP. So, I have settled for Fuji RTP as my preferred UV film.

Shouldn't Everything Be Blue?
Many people have commented to me that the UV colour images on my web site were unexpectedly "colourful". Shouldn't everything turn out shades of blue?

The colour palette in UV Colour Photography results from the balance between transmitted and reflected radiation in a number of spectral bands. The interpretations of these colours are as follows;

Scene Reflectance				Resulting Colour

UV	Blue/Green	Yellow	Red + Near-IR
High	High	High	High	Pinkish White
High	Low	High	Low	Bluish White
High	N.A.	Low	Low	White or Light Blue
Low	High	Low	Low	Darkish Grey or Blue
Low	Low	Low	Low	Black or Dark Blue
Low	Low	High	High	Red
Low	Low	Low	High	Purplish Red

The table outlines the extreme cases. Contingent upon the reflectance properties of individual areas, mixtures of the basic colours are possible. Thus, a restricted range of magenta, light yellow, and dull green colours can result. However, these extraneous colours normally are quite muted in their visual impact and seldom extends over large parts of the image.

UV Photography Techniques
Working with UV is basically not much different from ordinary photography. The main difference is since UV photography is done through opaque "black" filters, one must compose and focus the image first, then add the filter. My UV-Nikkor 105 mm f/4.5 lens has a hinged filter to make this process extremely simple. For non-quartz lenses, visible and UV focus points are different. As a first approximation, focus normally and then align the setting against the IR mark on the lens. Stopping down will ensure adequate depth of field unless you are shooting close-ups. Most non-UV lenses will exhibit low contrast and sharpness in the UV range due to uncorrected optical aberrations. This adds to the diffused effects of the UV light field on its own to give soft and dream-like pictures.

A last point to remember is that many UV pictures will appear quite dark to the eye. The reflectance patterns we take for granted in visual light simply do not exist in UV. Much foliage and verdure will reflect little or no UV at all so landscapes typically take on a sombre darkish tone. The sky registers in light, washed-out colours unless a polariser is added. Some objects, such as water surfaces or rocks, may exhibit specular UV reflections and thus appear very bright. Practice with a known scenery and repeat the photography frequently to get more familiar with the UV rendition. For details and flower close-up photography, a dedicated UV flash unit simplifies UV photography immensely.

Using IR Colour Film for UV Images
The new version of Kodak's Infrared Ektachrome (EIR) can be E-6 processed; however, colour rendition and image contrast will be less than optimal, Kodak literature says. This film has, in common with other IR films, a high and potentially useful sensitivity within the UV range.

I have done some test shooting with this film using my UV-Nikkor 105 mm f/4.5 lens. The pictorial results are detailed elsewhere; here it suffices to say that UV colour work with EIR is slightly problematic. The reason for this is evident when it s recalled that even UV bandpass filters have a transmitting sidelobe in the near IR. Because energy levels in this side band surmount that of the UV band, this results in a highly "contaminated" UV image. The problem can be abated by bringing the main object up close to the lens, while ensuring that the background receives as little light as possible. However, using a dedicated UV flash is less successful because again a UV bandpass filter has to be employed over the flash head.

Using a digital camera for UV photography
When I got my D1 camera, I immediately began experimenting with UV (and IR) digital photography. I still haven't solved all teething problems, but evidently the digital approach has a very promising potential. I easily obtained black/white UV (and IR) images with the UV-Nikkor 105 mm f/4.5 mounted on D1. Colour images are possible, but I need to sort out the problems of colour-balancing these shots on the D1 first. Using D1, UV sensitivity appears to be quite high, at least the level of Fuji RTP film and possibly higher. I have to be a little vague here simply because the incident UV levels here in Norway are very low during winter, so getting the experimental images is easier said than done. I shall soldier on to refine my procedures before the onset of spring and the early spring flowers in a few month's time.

Added July, 2002: For an updated review of digital UV and IR photography, click here.