Re: UV was Re: IR film & X-Rays ?

[tom@xxxxxxxxxxxxxxxxxxxxxx (Tom Benedict)]

I'm afraid I blew away the name of the person who mailed these
questions, but I'll go ahead and answer them sans the "so 'n so wrote"
line:

>Geoff says:
>1. I've heard that the wratten 18a is darn near opaque and that only 
>about 1% of a flash is UV, so that the exposures have to be really long, 
>like 2 to 15 minutes.  Is this right?

Not sure, but I'd believe it.  It'll depend on the total power output
of your flash and the sensitivity of your film.  At a guess I'd say a
flash has more UV output than a normal incandescent, simply because of
the color-temperature of the light.

>2. What film would you recommend for taking UV photography?  I have been 
>thinking of Tmax P3200, pushed to something like 25,000.  Does this sound 
>reasonable?  What about a color film?

TMax 100, at least, works ok for UV (note I said "ok").  Exposures are
long, the wavelength range is quite short, and focus shift can get to
be a problem (same problem as IR, but in the opposite direction).

Another alternative might be to pick an emulsion designed to work in
the UV.  Kodak makes a number of emulsions for taking UV spectra.
I've never used them, but I'd expect they'd work better for what
you're after.  I'm not sure what formats you can get them in, but you
can always ask.

>3  I have heard that the sun is the best UV source available so I have 
>been planning to take the pictures outside in the sun.  Does this sound 
>okay, or would another UV source be better?

I'd start with the sun.  It's plenty bright, and by the time you get
into higher-powered UV sources, you start getting into the potential for
serious problems like nasty sunburn or blinding your subject.

>4. I figure keeping a bird still for 2 to 15 minutes won't work, so I am 
>planning on starting on feathers.  Can anyone think of a way of 
>photographing a bird without blinding me and the bird in the process?

Not yet.  I'd say getting a fast emulsion would be a high priority for
solving this one.

>5. I have heard the the glass in lenses and the metallic coats fluoresce 
>in UV.  Is this true?

Yes.  Glass is especially good at blocking UV.  [A side note:  Silicon
wafers are patterned using UV lamps and a UV-photosensitive etch
resist.  As a result, photoresist masks must be manufactured on
soda-lime glass or quartz substrates.  It was only after a few weeks
of trials I discovered that ordinary float glass just don't cut it.]

>A professor I have worked with at the university 
>thinks a special lens costing about $2,500 is needed.  Is this true?

Likely, if you need full-range UV photography.  You can start with
what you've got, though, and see if you get what you need.

>To get around this, I am think of using an old rollei TLR because there are 
>fewer lens elements and the coating seems thinner.  I have also thought 
>of using a pinhole camera (this is where the xray film would come in) 
>and exposing the stuff for about 8 hours.

The pinhole camera will answer some questions since you can get the
full UV spectral range without any attenuation.  You can also use a
much smaller pinhole (shorter wavelengths), and get better resolution
as a result.

>I have also heard that 
>the fluoresence only happens in the far UV and I am interested in the 
>near UV because that is where birds see.  But I guess the film wouldn't 
>know that and would still get the UV in the far UV range, right?

The UV that excites fluorescence typically happens in the mid to far
UV, but the fluorescing emission happens in the visible.  Depends on
what you're after.

>6. Anybody got feathers they want to send me?

Only what I find in the park.

>7. Anybody got a spare 18a to loan in the name of science <grin>?

Love to help you, but...

>8. I will also take pictures of the same feathers in IR and visible 
>light, also.

Good idea.

>9. I have also thought about using a video camera since one like my 
>granpere's records down to .8 lumens.  Do camcorders record UV?  Do 
>CCD's?  If they do, how is it displayed (if it is recorded wouldn't it 
>help much if it only played it back as UV)?

CCDs can record in the UV, but it typically takes special coatings to
get them really responsive down there.  The idea is to coat them with
a material that will fluoresce and emit at a wavelength the CCD is
already sensitive at, but that is transparent to longer wavelengths.
These are typically B&W chips, and it shows up that way.

>10. And now a question for my sister:  How deep does IR penetrate in 
>freshwater?  in saltwater?  How about UV in freshwater?  in saltwater?
>What about different colors of visible light?  (she is interested in fish 
>vision and when she asked her physics professor he said he didn't know 
>and wasn't sure where to look it up).

?!@  You got me.

>I appreciate any help or suggestions, including "go look it up yourself", 
>if it includes some idea of a good place to start looking.

I'd say the best bet is to go out and experiment.  See what you get. 

>Thankyou,
>Geoffrey Johnson

Hope this helps.


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Topic No. 13