[alt-photo] Re: UV Point Light Source

Peter Friedrichsen pfriedrichsen at sympatico.ca
Mon May 23 02:04:49 GMT 2011

All of these posts to my question have been very informative, and I 
am taking it all on and pondering my options but now I want to 
respond to etienne's post, with the results from a fairly basic experiment:

Etienne, I tested your "IME" using a visual check with an LED light 
source. I used a Philips 2w white dragon LED and projected the light 
about 10 feet. From this distance, I held up my negative in front of 
a translucent screen and viewed it from the back with a magnifier. I 
then placed a 1/8 inch glass plate under part of the negative, with 
the other half pressed against the screen and I noticed that the 
image at 1/8 inch separation was softer; BTW this negative has a 
resolution of about 600 dpi. The angle  width of the light source is 
certainly much less than the sun and I am very roughly estimating 
that it may be 1/5 of the sun's 0.5 degree angle or 0.1 degrees at 
the distance mentioned. Moving further away (about 30 feet) there was 
no improvement in resolution.  Finally, I proceeded to use a thin 
glass slide of 1/16 inch thickness and at this separation distance, 
the sharpness was preserved

The limiting factor seems to be light diffraction, which keeps a lid 
on resolution no matter how small the light source.

Peter Friedrichsen

At 03:57 PM 05/22/2011, you wrote:
>Peter wrote:
>>I am imaging onto some lumpy surfaces and want to retain the image 
>>sharpness when exposed onto it. The sun is great for this but it is 
>>not reliable here especially in the winter months. I even went so 
>>far as calculating the shadow sharpness of the sun based on the 
>>exposed surfaces distance from the negative, and that told me that 
>>I can image up to 900 dpi even if the negative is 1/8 inch away. 
>>The only thing I came up with so far is a naked xenon arc which is 
>>probably risky to use since I know those bulbs can sometimes 
>>explode violently without warning, and can also blow a big hole in 
>>the bank account. Any ideas?
>Carbon arc, xenon arc, or ultra-high-pressure mercury arc appear to 
>be your choices for artificial point-ish UV sources.  Of course, as 
>you stated, the lamp would need to be used without a reflector to 
>preserve the small source size.  They all have operating arcs on the 
>order of 1/2 inch, so they would need to be 60 inches or more from 
>the negative to match the angular size of the sun.  Mercury arc 
>bulbs used for home tanning lamps are similar, though usually with 
>an arc an inch or more long -- but they do not have the power to 
>expose most alt processes from such a distance.
>Plate burners greatly reduce the risk from bulb explosion, although 
>they limit the lamp distance to around 30", so the effective angular 
>size is about double that of the sun.  I used to use an old 
>carbon-arc plate burner with the reflector assembly removed for 
>exposing Pt and carbon materials.  It provided great resolution, but 
>the fumes, noise, and residue finally persuaded me to switch to a 
>xenon arc plate burner, which I again stripped of its 
>reflector.  Both machines were obtained free from printing houses -- 
>the carbon arc when they switched to xenon, and the xenon when they 
>switched to laser typesetting.
>Note that you can clearly see the difference between a carbon print 
>exposed emulsion-to-emulsion and one exposed through the base of the 
>negative (only .005-.010" gap), so I would expect a 1/8" gap to show 
>a substantial reduction in sharpness.  IME, this is true of prints 
>made using the sun as the light source, as well.
>Best regards,
>Alt-photo-process-list | http://altphotolist.org/listinfo

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