UVB from a halogen bulb

[Stefano]

Halogen bulbs emit a low but detectable amount of UVB light. Here I recorded UVB at around 310 nm.

Full-spectrum Canon EOS M, SvBony lens (more detail here), double Chinese 310 nm bandpass filter + 2*2 mm thick ZWB1 filters. The bulb, which is supposed to run at 12 V, has been powered at 16 V.

 

I extracted the green channels only from the raw images.

 

Lens fully open (about f/2.2), ISO 100, 15 s exposure:

 

Lens stopped down (about f/4.5), ISO 400, 15 s exposure:

 

I can tell I recorded UVB only (without contaminaton) for two reasons: the raw color is green (as I have obtained from a 310 nm LED, for example) and a magnifying glass, which I know it's transparent down to at least 340 nm and it's opaque at 310 nm, completely blocked the light.

 

[Fandyus]

This is an experiment I would definitely have done have I had the gear for it. Thanks a lot for showing us!

[Stefano]

You are welcome!

[colinbm]

Is this a quartz halogen light ?

[Stefano]

It is a car headlight lamp, as far as I know they have a quartz bulb.

[colinbm]

Thanks

[Nate]

Super cool experiment. I had no clue those lamps went that low

[colinbm]

12 minutes ago, Nate said:

Super cool experiment. I had no clue those lamps went that low

There is not much there that is useful, even in UVA, they are dim.

[Stefano]

Thanks Nate. Just to prove that you can use these lamps for UVB illumination, I took this image of this lens, which passes most UVA but blocks UVB.

 

The setup is the same as before, with the SvBony lens wide open. To obtain the above image, I took 23 photos at ISO 25600 and 30 s of exposure, extracted the green channels only and then stacked them. On the left there's a rock used to hold the bulb above the lens (I needed a heat-resistant material, as those bulbs get extremely hot). This was more of a fun experiment than anything useful, as I hope it's clear that halogen bulbs are not the best UVB light sources, as Colin said.

 

[colinbm]

Thanks Stefano
"  I took 23 photos at ISO 25600 and 30 s of exposure ", with these figures, if you waited a bit longer you would have seen blood come out of the stone.....just saying...

But you did it.

[Nate]

42 minutes ago, Stefano said:

This was more of a fun experiment than anything useful, as I hope it's clear that halogen bulbs are not the best UVB light sources, as Colin said.

Oh yeah, I figured. Just pretty cool to see such experiments below my working range of UV

[Stefano]

The most surprising thing to me is that my camera can see at 310 nm. The sensor still has the CFA, microlenses and the original coverglass. Some cameras, even if modified for full-spectrum, can't see UVB because of the coverglass.

[Nate]

@StefanoIt would be interesting to see how low my 77D goes, but I'm ok if it goes to 340nm

[dabateman]

Wow cool you can push it to see some UVB out of a hallogen light. I was surprised to get a 350nm bump from my halogen flashlight when I placed the 8mm zwb1 filter over it. 

Using a hallogen light for UV imaging is one of the few light source were you will push mostly IR through the Baader venus u filter and will see very little UV. This is a great test for your IR blocking control setup.  I wouldn't recommend it for UV imaging.  But you seem to be very patient Stefano.

 

The problem is a flower might just burst into flames before you would be able to finish the image. Andy even had that issue at the other end with hot hallogen for SWIR imaging. 

[Stefano]

Those Chinese 310 nm bandpass filters are rated at OD 5. One of them wasn't enough as it leaked at around 340 nm (raw orange), and also some red/IR. Two of them should have an OD 10 blocking, which is huge and it's enough to push UVB with a halogen bulb. Even if the actual blocking is OD 4, that's still OD 8, which is twice (squared) the needed blocking for UVA in sunlight.

 

Edmund Optics sells OD4 310 nm bandpass filters with a bandwidth of 10 nm and a peak transmission of ≥70% (here). Two of them would have OD 8 and ~50% peak transmission. They would cost $1000+ in a 50 mm size, so by far not something I can afford now. You would need two to eliminate leaks. Remember how Jonathan had leaks with his Invisible Vision 308 nm bandpass filter: https://www.ultravioletphotography.com/content/index.php?/topic/2375-baader-u-and-invisible-vision-308nm-filter-comparison-for-skin-imaging

 

 

[JMC]

Stefano, all filters will leak under certain circumstances, especially when used in applications they are not designed for. The Invisible Vision showed that behaviour when being used for something it wasn't designed for - imaging in sunlight with a normal camera - so it is not surprising that it behaved like that.

 

As with all things, context is important to remember.

[Stefano]

Yes, that's correct. Your filter wasn't faulty, it probably matched the rated OD, you just need more blocking to take UVB photos under sunlight.

[ulf]

The OD was likely also defined for a normal incidence (90° to the filter surface) and not as good when used with camera lenses, for any light beam ≠90°.