Ultraviolet and multispectral photography

UV Photography - What and Why

Ultraviolet (or UV) photography means taking photos using only ultraviolet light, rather than using visible light like a normal photo. Just like Infrared photography, the world can look quite different in ultraviolet to how we see it normally.

The main use for ultraviolet photography (for my purposes at least) is photographing flowers. Some flowers have hidden patterns or markings that show up only under ultraviolet light. Bees and other insects can see in ultraviolet, and it is thought that these hidden markings are used to help insects identify where the nectar is.

So far I haven't had much luck at finding any flowers that have a good UV marking on the petals. So I think the UV nectar guides are probably not on the majority of flowers. However, all flowers do have a very dark centre (where the nectar is) when viewed in UV compared to visible light. In visible light the centre is normally bright yellow or white, in UV it is almost black.

The above photos (top visible light, bottom ultraviolet) are of a crustose lichen. You can see that (what I think are) the fruiting bodies (called apothecia) are very dark under UV light compared to visible light.

What equipment do you need?

UV photography requires quite a bit of specialist equipment. First you need a camera that is sensitive to UV light. Next you need a lens that transmits UV light (most modern lenses have coatings applied that block UV). You also need a filter that passes UV light while blocking visible light and infrared - these are very expensive. And lastly you need a UV light source, the sun is only suitable if your subject will not move at all.

For the camera, I have the Fuji IS Pro. There are various companies such as LifePixel that can convert standard cameras to full spectrum or ultraviolet cameras as well.

For the lens, I currently have an EL-Nikkor 80mm/5.6 lens. The EL-Nikkors seem to be generally well regarded for their UV transmission, e.g. see EL-Nikkor lenses in UV photograph and [UV] EL-Nikkor 75mm - 80mm - 105mm for UV?. (Both those websites are very good references for UV photography). Because the lens has no focusing mechanism, it requires using a focusing helicoid. My set up with the lens is as follows:

• Extension tube
• Nikon F to M42 adapter
• M42 17-31mm focusing helicoid
• M42 to M39 adapter
• EL-Nikkor 80mm/5.6 (pre-N) lens
• 34-52mm step-up ring

The EL-Nikkor 80mm/5.6 lens I bought is the older version with a chrome mount, there is also a newer version that is all black. The newer version is denoted with an 'N', i.e. EL-Nikkor 80mm/5.6 N. The version of the lens I have has a 34.5mm front filter thread. There doesn't seem to be any step up rings in this size (I spent a loooong time searching for them). So instead I bought a 34-52mm step-up ring and epoxied it to the front of the lens.

For the UV pass filter, I am using the Baader Planetarium U 2" filter (also known as the Venus filter). There are a range of UV pass filters available, but most have problems in letting through infrared. The Baader filter has very good cut off of visible and infrared light, while still passing a good amount of UV light.

The only problem with the Baader U filter is that it is very expensive. I bought mine direct from Baader in Germany, but it would actually have been cheaper to buy from a UK reseller (the postage was about £40).

When you get the filter, it is recommended to flip the filter glass for using it in front of a lens (instead of behind a telescope). This is a bit tricky, but not too problematic.

The filter consists of the metal filter ring, inside this is the filter glass. Screwed down on top of the glass is a retaining ring. The retaining ring has little notches at either side, and these can be (very carefully) gripped using the point of some scissors or tweezers etc. The retaining ring can then be rotated and unscrewed.

Notch in the retaining ring on the Baader U filter

Filter after unscrewing

Carefully tip out the filter glass, then put it back in the filter the opposite way round. Then screw the retaining ring back on top.

The Baader U filter has a 48mm filter thread, so I have a 52-48mm step down ring to enable attaching it to the lens. I could have used a 34-48mm step up ring on the lens, which would have meant no step down ring was needed, but I think I couldn't find any 34mm step up rings in that size.

For lighting I have several modified Vivitar 283 flashes. Standard flashes do produce UV light, but have coatings on the flash tube or the front lens that block the UV light produced. So the flash needs to be modified to remove the UV blocking elements and allow UV light output.

I must point out this website which gives quite a bit of info on modifying the flash for UV photography: BeyondVisible Ultraviolet & Infrared Modified Vivitar 285HV/285/283 Flash - Nikon SB-140 Alternative. The modification is pretty simple. Undo the flash, then remove the yellow fresnel lens that covers the flash part. This is clipped on, though it is clipped on very tightly. (The fresnel lens blocks UV, which is why you must remove it).

After removing the fresnel lens it is a good idea to replace it with some clear plastic that does not block UV light. This is just to protect the flash tube. I cut some plastic from a CD case for this purpose. Then do the flash up again, and you now have a full spectrum flash.

I found that with one flash, the light output was not enough for a good picture. So I bought some more Vivitar 283 flashes and converted them as well. I now have four full spectrum Vivitar 283s.

The Vivitar 283 may not be voltage safe for connecting to a digital camera. You can check the trigger voltage if you have a multimeter, but I don't. So I just use a radio trigger and hot-shoe to PC sync adapters on the flashes to trigger them.

Multispectral photography

Multispectral imaging involves photographing the same subject under different spectrums, and then combining these into a single image. I have tried this by photographing the same subject in UV, IR, and visible light. I then create a new document (in Photoshop), and use the visible light photo for the green channel, infrared for the red channel, and UV for the blue channel.

Getting the images to align for creating a multispectral image is very difficult though. As well as focus shift issues, the camera will move slightly as you add and remove the different filters needed for each image.

For the multispectral images below, I didn't correct for the focus shift, so the UV and IR photos don't have the focus point at the same point as the visible light image. I've included the visible, UV, and IR images so you can see how they look separately as well as combined into a single multispectral photo.