UV camera project build question

ProfHankD wrote:

petrochemist wrote:

You will need to be careful selecting your UV pass filter, all the old film era ones allow NIR through as well (which wasn't seen by film) Digital sensors are sensitive to NIR so the NIR will often swamp the detector. Only a few very expensive filters like the Baader U2, or carefully selected stacks of multiple filters will work acceptably.

The filter stacks may well prove to be too thick for placing on the sensor -the option I'm considering for UV needs at least 3mm thickness of BG38 glass together with a U330. That's a 5mm total filter thickness probably significantly more than the existing hot mirror (usually ~1.5mm thick).

If I'm understanding this correctly, the OP is talking about a mod to a Canon IXUS 105, which is a little old PowerShot supported by CHDK .

That means the lens is extremely hard to replace. I'd also be more than a little concerned that the sensor stack itself might block enough UV to make this problematic -- MaxMax claims the CFA and microlenses actually block UV . Incidentally, it is a CCD sensor in most PowerShots, not a CMOS one -- does that help or hurt?

Anyway, it really all depends on the wavelengths of interest, doesn't it? My understanding is that to about 365nm is doable with conventional optics, but shorter wavelengths require quartz optics and other extravagant things. It depends on what you want....

It's not really intended for UV nor for use with cameras, but you can get a theatrical gel filter color swatch book quite cheaply and each filter comes with its spectral profile. Take a look at Rosco's web site to see some profiles -- for example:

Combining two or three filters from a Rosco pack might get you close enough to what you want in UV sensitivity. For example, the above + #375 Cerulean Blue might not be a bad starting point, although you'll be down more than 2 stops around 400nm (and lower elsewhere). Of course, you'll probably need longish exposures and a bright UV light -- and bright UV lights are seriously dangerous.

In any case, good luck.

IIRC there's no inherent preference for CCD or CMOS, but the thickness of silicone in the sensor can have a significant effect on it's UV sensitivity. On average shorter wavelengths (such as UV) penetrate further than visual light before being detected - this principle is the way Foveon sensors get their colour information.

Typical camera sensors being designed to capture visible light are not particularly sensitive to UV, but are sensitive to NIR - the reason a hot mirror is installed. The rise at the right of the spectra You've supplied shows considerable NIR is transmitted by this gel - FWIW I've not found any gels (other than diffusion ones) that don't transmit NIR well though I've only run about 50 of them on the spectrometer at work. Nearly all transmit more throughout the NIR (700-1100nm) than they do at any visible wavelength. They're definitely good for IR photography!

Schott U360 transmits more (4x?) UV than NIR (still enough IR to cause issues). Small circular examples are available for about £30 - 25mm ones should be big enough for a compact camera.

The wavelength of interest certainly affects things, If you're after anything below 190nm just the air itself will block UV pretty rapidly. Smaller lenses will typically transmit more UV so IF it's not coated the lens in a P&S has a good chance of being usable down to nearly 300nm. Most UV photography is done in this range by the time you've got down to 250nm you're into germicidal UV considerably more dangerous. General UV light sources for Labs. are usually around 365nm, with 254nm sources used for sterilization.