When shooting in alternative light spectra such as UV, it is important to get the white balance right particularly if different equipment or light sources are used at different times. This is so that the results (false colours) are reproducible and comparable to each other.
Then at least when you acquire a gallery of images, these can be interpreted against subsequent images you take. This is particularly important for UV-video, as getting the white balance right in the video clip will mean no need to adjust this in post. Overlook this important step, and it would undermine all the efforts you have made into mastering UV-imaging.
Ideally this custom white balancing should be done for each camera, lens and lighting combination using the same standard reference. The Labsphere is the current recognised reference standard for UV white balancing, but is exceedingly costly £££. Hence many people use white PTFE sheets which can be bought rather cheaply on eBay £. But is it up to the task?
To find out if your white balance reference is suitable (unless you have a Labsphere), it would be worth taking a picture of it with your UV-imaging equipment in the light you will be using. If you see significant variance in colour or brightness within your white balance reference, then clearly it needs some work on it to make it useable.
This is particularly so with Xenon flash, as the white reference will be overexposed when you adjust the exposure for the subject. Hence when not using my standard setup, I will take two images, with one exposed for the reference (typically -1 to -2 EV) – so this can be used for white balancing in post.
I have chosen to use sintered white PTFE discs (low ££) because it is much cheaper than the Labsphere, is fairly matt with less variance over the entire surface in UV compared to PTFE sheets available on eBay. I have incorporated it into a lens cap for ease of access.
How to white balance in UV?
Each camera system has a different way of doing custom white balance, so do look into your manual for the recommended way for doing this. I only know of the methods for Sony and Olympus cameras.
For Sony cameras, they use the central circle to do white balancing. So all you have to do is place that central circle over your white balance reference and shoot. Just be aware that with many different photo processing programs, the white balance from Sony RAW files will not be retained in the image – so needs to be adjusted again in post-processing.
For Olympus cameras (and also for Panasonic cameras I have heard), the whole sensor must be filled with the white balance reference to set it. Hence the only way I can do this would be to place my reference at an angle just in front of the lens to reflect the UV into the lens. I have found this method to be more accurate than shooting through the reference. Just be aware that with my Olympus cameras, I do have to add about +1 EV for it to register the custom WB (rather than give an error message).
I have also managed to shoot in a variety of conditions and create white balance profiles on Capture One Pro for future use (e.g. direct sunlight, in the shade, with Quantum X2D, with Olympus FL-300R etc). That way I have a better chance of choosing the most appropriate profile for each set of images taken.