[alt-photo] Re: UV Point Light Source

[etienne garbaux]

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,

etienne