Pimping my exposure unit part 2

[Rockers]

Let' s assume one real crafty person would want to pimp his exposure unit. 1200W metal halide. Distance to screen 60 cm. how about bringing the whole light source closer to the actual screen, let's say by 10-15 cm, that's including the reflector. That should bring the burn time down I guess. Of course we would probably not be able to expose really big screens anylonger, something we never did anyway. We generally deal with 23x31 frames only. Any other disadvantages I might have to take into account?
This would be only a temporary fix until we can splash some money on a bigger unit.

[Frog]

Moving the light too close could give you a hot spot in the center.

However, assuming that you did the math, to establish the ratio of the maximum screen size diagonal in relation to the stock distance, you should be able to figure the modified ratio.

[Chadwick]

No.
The distance between the light source and the screen is set so that
your positive, when being exposed, has the illusion, if you will,
of having a light source that is equal across the board.
This results in more accurate exposures.
Moving the light higher would have a negative effect on accuracy of exposure,
as well as increasing the temperature of the glass.
.02

Of course..if you're re-thinkin all the math, power to ya.
Most of this stuff has been hammered several times already though.

[Frog]

Chad, the distance is set, but can vary with the screen size, much like our units which have their  tilt frame vertically mounted to a wall, and the light unit on wheels with marks on the floor corresponding to common screen sizes.
So, rather than distance being fixed, it is the pattern established by the reflector.
Then, it's a matter of figuring exposure times as they increase or decrease exponentially with the distance.

Here is an example using completely made-up numbers as original size and distance

[blue moon]

let's not forget the issue of undercutting as the light gets closer. going away from a point light source will introduce more light coming from the side.

pierre

[Frog]

But I believe that is still all relative to the size being exposed at the same pattern, though there may be a point of diminishing returns with a given light/reflector/pattern.

When a flat stock shop exposes an eight foot screen at it's given distance, it generally doesn't see a problem when wheeling the light closer for a four foot screen, or vice-versa.

The estimated exposure times in many of our emulsion tech sheets are figured at 40" distance, and then folks like Ulano give the adjustment factors from 20" to 72". Though there may be a practical minimum and maximum distance, there is definitely some room to play.

[Evo]

let's not forget the issue of undercutting as the light gets closer. going away from a point light source will introduce more light coming from the side.

pierre

The undercutting would happen if you moved the light closer, but kept the same size glass/screen to expose. This would mean you'd have to widen the angle of the reflector to match the glass, and the angle of light hitting the ends of the screen would increase.

Moving the light closer but keeping the same angle of reflection effectively a) decreases the area that can be exposed and b) increases the light intensity on the exposure area.

So you need to calculate by taking the ratio of the current lamp-to-glass distance vs the diagonal of the current exposure area, and use that to calculate how far to raise the lamp so the diameter of the light pattern matches the diagonal of the exposure area for the maximum screen size - in this case a 23"x31", therefore 38.6".

You are effectively "miniaturizing" the exposure unit, but keeping the same intensity of lamp. Much like using a stand alone lamp and vacuum frame, and moving the lamp closer to the frame.

[Rockers]

let's not forget the issue of undercutting as the light gets closer. going away from a point light source will introduce more light coming from the side.

pierre

The undercutting would happen if you moved the light closer, but kept the same size glass/screen to expose. This would mean you'd have to widen the angle of the reflector to match the glass, and the angle of light hitting the ends of the screen would increase.

Moving the light closer but keeping the same angle of reflection effectively a) decreases the area that can be exposed and b) increases the light intensity on the exposure area.

So you need to calculate by taking the ratio of the current lamp-to-glass distance vs the diagonal of the current exposure area, and use that to calculate how far to raise the lamp so the diameter of the light pattern matches the diagonal of the exposure area for the maximum screen size - in this case a 23"x31", therefore 38.6".

You are effectively "miniaturizing" the exposure unit, but keeping the same intensity of lamp. Much like using a stand alone lamp and vacuum frame, and moving the lamp closer to the frame.

Came to the same result. I should be fine since 23"x31" is all we do day in day out. By bringing the light source up by 10 cm or 4 inches I can still use frames the size of almost 35 inches in width and hight. Ok next step, I will talk to my handy man and see what he thinks.

[RonH]

As has already been mentioned, moving the light source closer to the vacuum glass will affect the evenness of light across the usable area.  I can think of a couple of other things to consider.  I am not suggesting that you not try this because that will be up to you, but these are a couple of areas that you should be aware of. First is that the reflector in each of our units is designed for covering the stated area.  By moving the light source you could affect the collimation of the light hitting the screen.  The second concern would be that you could affect the lamp temperature, which could in turn affect the lamp output and life.  How much or how little affect you could see is hard to guess.

Ron Hopkins
NuArc Sales Manager
M&R Sales and Service Co.
O-770-254-1584
C-847-997-2487

 

[mk162]

also, I am really not sure how much exposure time would be shortened..if any.

I would worry about other things..or switch it over to a brighter bulb if you want to shorten times...of course this would involve a lot.

[Sbrem]

move up to a 5K unit... how about a faster emulsion? Lots of stuff out there.

Steve