# Exposure height from light source??!!



## shaneT (May 11, 2010)

Hey Forums,

My partner and I are building a custom exposure unit and I was hoping you might be able to help us out with some info.

We're looking at a build out with the following specs:

Light Source:a single point metal halide 1000w bulb/ballast with shutter & cooling fan (we just can't really afford a 3000w right now) We'll also be adding a yellow safelight inside the light chamber as well.

Vaccum: 3mil Closed cell neoprene blanket (cloth on one side) sealed to a 48"x60" aluminum screen frame with weather stripped bottom side to create vacuum seal on glass. Suction will come from a 5cfm vacuum pump (we haven't decided yet whether to tube in to the edge of the frame and drill holes around the inner edge of frame or to go straight into the neoprene. 

Glass: 48"x60", 3/8" thick, low-iron, annealed, flat polished edge.

The main design element we're stuck with is how far to mount the light from the glass. We've read that rule is that the light source should be 1.5x the max diagonal distance of the artwork. If we are going to be burning a large variety of screen/artwork sizes, then how do we determine the distance between the glass and the light bulb?

If you have any insight on this or can think of any tips based on the other specs it would be much appreciated!

Thanks!!


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## miktoxic (Feb 21, 2008)

it would behoove you to find a test pattern, print it out to film and then expose it. 

a lot of variables.....what kind of emulsion. how do you coat your emulsion? (once on each side, twice?)

i use a 500 watt halogen work light placed above my screen at 16 inches for 13 and a half minutes. so with my bad math using a 3000 watt bulb would bring you down to somewhere over 2 minutes at that same height. 

i've never heard the rule that you described. i burn all size screens at that same height.


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## binkspot (Jun 12, 2013)

Yes the 1.5 rule applies. What ever the largest image you intend to burn would be your standard. Once figured out the bulb is set there and stays. It does not change if your burning a 2"x2" left chest or a 15"x19" back.
Example if your largest image is 13"x16" the diagonal would be 21" 
21x1.5=31.5" 
So the bulb would be roughly 32" from the bottom of the screen. It's not an exact measurement, more of an educated guess. 
This is calculated so the UV light will hit all areas of the image about the same, mostly so it hits the outer edges.


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## Dickerson259 (Apr 2, 2020)

I'm a newbie so this might seem kind of an off question. Why on led and other units do people make the height so much lower? Haven't seen that one explained very well.


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