While I have done quite a bit of ultraviolet photography in the past, I haven't done any ultraviolet induced fluorescence photography. This involves lighting the subject with UV light, but recording an image that only includes visible light. The idea being to capture any visible fluorescence caused by the UV light.
The difficulty in doing this is finding a light source that emits only ultraviolet light. If we have a source that emits UV and visible light, then the visible light output will likely overpower any fluorescence in our image. So that means a full spectrum flash is not suitable.
One possible solution would be to use a UV-pass filter in front of a full spectrum flash. This would require making some kind of filter holder to hold the filter in front of the flash. I also didn't really want to have my expensive Baader U filter blasted at close range by hot flash.
So that led me to look at UV torches. The torches most recommended for UV / UV induced fluorescence photography use UV LEDs produced by the Japanese company Nichia. However, these are very expensive. There are much cheaper alternatives available, but are they any good?
For this test I have three cheap torches that claim to be UV. They were sold on ebay as:
- Waterproof Outdoor 380-400nM UV Ultra Violet LED Flashlight Light Torch Lamp
- Aluminium UV Ultra Violet Blacklight 9 LED AAA Flashlight Torch Light Lamp
- Ultrafire G60 UV 3w Ultraviolet 365nm LED Flashlight Torch
The first two are both 9 LED and take 3 AA batteries. Both cost about 99p and strangely both had one LED not working (so they were actually 8 LED). The Ultrafire torch was probably a bit over £20 when you include the cost of the special batteries and charger it requires.
Before I go into my test, I should mention a couple of tests others have done, which test a different selection of UV torches: Extreme Macro: Lighting For UV Macro and BudgetLightForum.com - Review: UV light shootout, seven lights tested.
The first thing I noticed when I received the torches was that all three gave out a lot of visible violet light. The ultrafire torch gave quite reasonable results when shone through my Baader U filter. The other two torches gave very faint results when shone through the Baader filter, but it did make the fluorescence easier to see than with no filter.
So, I purchased from ebay
Optical Filter 330WB70 25mm Excite Fluorescence High Transmission 70%. This passes light from around 300 nm - 400 nm, cuts visible light, then starts transmitting some IR from around 810 nm. So by adding this to the torch, I can block any visible light it produces and turn it into a true UV torch.
Including postage, tax, and post office fees the filter came to around £40. So it puts the price of a cheap UV torch up considerably. But it still keeps the price a lot less than the torches using Nichia UV LEDs.
The test subject was the UV security feature on a Bank of England £10 note. This has a number 10 that cannot be seen under normal light, but fluoresces under ultraviolet light. According to the Bank of England
A UV lamp that emits light at around 365 nanometres is ideal for checking the fluorescent features.
I kept the aperture and ISO the same for each shot, but varied the shutter speed. I'll include the shutter speed below each image.
Ultrafire torch w/ 330WB70 filter black side facing out - 0.4s. The 330WB70 filter has a shiny red side and a dark black side. I decided to test with the filter in both directions in case it had any effect.
Ultrafire torch w/ Baader U filter pink side facing out - 0.4s. I tested to see if the direction the Baader filter was used in had any effect as well. You can see the Baader filter passes quite a bit more visible violet light than the 330WB70 filter.
In conclusion, I think we can say that the really cheap 'UV' torches are actually just violet torches that output a tiny amount of UV. The cheap but not quite as cheap Ultrafire torch does output UV light. However, I seriously doubt that its peak output is 365 nm. It looses a large amount of light when the 330WB70 filter is added, even though this filter passes around 70% of light at 365 nm.
I'd really like to try out the MTE U301 torch, which uses a Nichia UV LED, and see how that compares. However, at around £120 it's a bit much for me. Maybe if I get into UV fluorescence photography a bit more, then I'll consider it.