Collodion is only sensitive to visible light, not UV

[Tim Meisburger]

That sounds reasonable. I don't have the equipment (or brains) to do this sort of thing, but I think that this kind of objective testing is clearly what is lacking. I'm not sure I know enough about chemistry, optics or physics to pose intelligent questions, but this is what comes to mind:

- spectral sensitivity of fresh collodion
- change in spectral sensitivity of collodion over time (older collodion has less speed, but does its sensitivity to various spectra change as well?)
- spectral transmission of lens designs (long/wide, triplets, etc) and coated vs. uncoated.

Garrett and others have pointed out that its easiest to just shoot a plate and get on with it, and I agree. But I've been made curious by the contradictory assertions, and limited data that seems to suggest that the common belief that UV is important in wet plate may be unfounded. Or maybe not. Just a bit o' fun, like a crossword, with limited practical implications.

[A_Tabor]

The road to science is all about asking dumb questions and finding the answers to them till you learn enough to start asking better questions and figuring out the harder answers.

I will admit that I am highly biased because I'm a geek and I enjoy optics, but I would encourage you to spend time reading up about some of the early works and experiments done by the pioneers of the field. Science is fun and interesting, and the more you learn about something then the more questions you can ask and things you can poke at or play with. And photography is all about light, so the more you understand it then the more you can control it and compose images exactly to what you want them to be.

If you do want to get into experimenting with this kind of thing and doing your own tests, then getting a fairly simple and cheap triangular prism is probably a good place to start. Cutting edge labs might have billions of dollars worth of gear and gadgets, but much of the early work that we base all our current knowledge on was done with shockingly simple tools like what a $5 prism is today.

Get yourself a few dispersive prisms, and play around with them. Shouldn't be too terribly hard to rig up something to throw a rainbow pattern into a dark box and onto a white sheet you could observe, then angle the pattern so that you have the violet and where you expect the UV to be. Carefully mark the visible parts and where it ends, and then without moving the prism or box replace the target paper with the light sensitive media you want to test. Develop, and compare to your marked target and see how far into the UV things go.

Testing through an actual lens would require a little more setup. But one thing to remember when testing this kind of thing is that you really only need narrow strips rather than using full plates at a time.


Hopefully others can chime in with ideas and suggestions. I might sketch something up this weekend if others feel it would be useful to the discussion.


Remember: Science is fun. (At least till things catch fire, then they're exciting.)

[Tim Meisburger]

What a great idea! I'm not sure where I can buy a prism here, but I'm sure they would be available somewhere.

[Struan Gray]

Transmission gratings are reasonably cheap on eBay, some with highly UV-friendly substrates such as fused silica. A dispersing prism would also work, but their transmission can often drop off fast in the UV or IR (that's why they disperse :-)

Personally though, I would make a bar of LEDs of varying wavelength and photograph that. Make it small enough to contact print, and you can avoid lens transmission issues if you're interested in the emulsion only. With some UV LEDs you get fluorescence from the plastic packaging, so if you're a purist you could add a UV-only filter for the LED part.

[Fr. Mark]

Science is fun...til something catches on fire...then it is EXCITING!

How true, how true. Been there done that, thankfully, lived to tell about it. Perhaps the most dramatic incident:

Saw a professor flash burn off his eyebrows once in a lab "oops" that thankfully did not ignite the 30 gallons of ethanol/KOH cleaning bath next to the sink or we might have been front page news...there was a tiny amount of sodium metal a smidgen of water for ignition and maybe a 1/4 cup of acetone involved as fuel and maybe some residual isopropanol. I had my back to it, heard a whoomp sound, felt a flash of heat and just managed to see the flames licking the 14 foot high ceiling before it was over. Total elapsed time, maybe three seconds.

I think I mentioned that some of our favorite chemicals are highly energetic. Wear safety glasses and gloves, be careful and have fun.

[barnacle]

Just a thought: might the 1872 spectragraphs have been taken with a reflecting telescope, with no glass in the way to eat any UV? Seems a reasonable assumption; most astronomy uses reflective optics.

Neil

[Struan Gray]

Draper used a reflector for the Vega spectrum, but switched to a refractor later.

http://www.jstor.org/stable/983130?s...n_tab_contents

[defacto]

Yesterday we shot a lot of plates under constantly changing skies, from direct sun to heavily obscured by clouds. It was a pain in the butt, so this morning I have been looking at UV meters to try to determine if it would be useful to get one for wet plate. My (hour of) internet research has enabled to be definitively state that UV is not relevant for wet plate, as wetplate is only sensitive to visible light. Here is why I can can confidently state this with a level of certainty only the internet can provide:

• There are three types of UV: A, B, and C.
• UV C cannot penetrate the atmosphere.
• UV B is 290 – 320 nanometer, but does not penetrate glass so cannot get through your lens to the collodion.
• UV A is 320 – 400 nanometer, and can get through glass (unless it has a UV coating). 95% of the UV that reaches earth is UV A
• According to Lund Photographic the spectral sensitivity of collodion is 403 to 521 nanometers. According to the internet violet (color) begins at 380 nanometers and extends to 450. This means that there is a disagreement between the description of UV A extending to 400 nanometers and the description of violet beginning at 380 nanometers, but both would agree that by the time you get to 403 nanometers you are in the visible spectrum.
• So, collodion is sensitive to almost all of violet, all of blue, and half of green, so a lightmeter that measured only this section of the visible light spectrum could be accurately used to predict wet plate exposure.

I rest my case!

see here:
http://www.alternativephotography.co...uv-meter-ppm2/

if collodion were not susceptible to UV why does the face tanning light speed up exposure. QED

[Vaughn]

...One last thought, a couple summers ago I took a photo of my brother with an Ortho X-ray film and his skin in the photo was blotchy in a way that was not obvious to just look at him. I'm suspicious it was because of sunscreen and uv absorption....

Photographing my boys with X-ray film, I saw freckles on Calder I never notices before. He was also wearing red.