Cold Light optics question - anyone want to take this on?

[JW Dewdney]

Hey- so I have a fairly modified DeVere 504 I've been using for years. Originally it was a Dichro model but I gutted the head to put in a cold light (it's a nice aristo unit- though a BIT small for complete coverage - I think it's the V54 mk.2 or something - it's a later model).

SINCE the coverage is a tiny bit problematic (should be fixed by using a fixture for the light in the correct position) I've been trying to figure out a way to get even more coverage out of it. I have a diffusion panel sitting DIRECTLY over the negative inside the bottom of the diffusion chamber. BUT - I'm ALSO trying to start printing 6x17 negatives. I milled out a 6x9 carrier to approximately 6x13 and that seems to work great so far.... but I want to take the head apart and mill out other obstructions so I can hit at LEAST 6x16 or so (really pushing the limits but I think it can be done)

At any rate - here's the problem:

CAN I move the light source UPWARDS an inch or so and get BETTER coverage? It's a bit of a mind game trying to think things through. From the perspective of illumination ONTO the diffusion panel you would think that higher is better. But If you think of it from the negative stage - the further away the source IS, the smaller it appears. Perhaps the right quality of diffusion panel would obviate that - however I was hoping someone here might have futzed around with this issue before (?).

Thanks

[smithdoor]

Can you post photos I have done a lot of modifications to ever thing under the sun But I have not work on Cold Light. I will say you problem can do this modification

Dave

[JW Dewdney]

Hi - well I will have to make drawings - as the two possibilities for light spacing are inherently unphotographable - at least without cutting a cross section through the head. But I don't really want to do that...!

[ic-racer]

You don't say how big the lamp is. I think it will be futile to print a negative larger than the lamp. Best results would be if the lamp is bigger than the negative by about an inch. Even with the appropriate sized lamp farther away tends to darken the edges and make a hot spot in the middle.

I spend a lot of time re-working my Aristo to make it more even (14" lamp over 10" negative) while I was looking for a dichroic head. I was able to improve it a little, but it still needed a center filter.
http://www.largeformatphotography.in...l=1#post319386

[Harold_4074]

I recall a discussion similar to this one from years ago; the idea was to make a "custom" center filter by printing the image of the film gate onto sheet film, and tinkering with the exposure and development to make a mask which exactly compensated for the edge falloff.

At the time, I wondered how one would deal with the nonlinearity of the film curve, but I think it might work of you used the straight-line portion of the curve and very dilute developer; the total density needed would probably be very small. Doing it in two steps would be even better, because the second mask would need an even smaller range than the first, and over a narrow enough range every film curve is effectively linear.

[Nathan Potter]

This is just a geometrical optics problem and some existing dimensions and sizes of parts would be needed to give any specific advice, as ice-racer says above.

But as a general comment you could consider an arrangement that utilizes a Fresnel of an appropriate size to more than cover the largest format you need. Backing the distributed white light source further away from the film plane might aid in coverage, but the focal length of the Fresnel needs to be such that the exit rays from it are roughly infinity corrected for best results.

A key limitation with a Fresnel is to make sure it is not in a plane where the lines will be in focus at the image plane.

Regardless, any attempt to increase the coverage will result in less light at the projected image plane.

Nate Potter, Austin TX., Chatham MA.

[Bill Burk]

Just make a gray print without a negative you'll see how even the coverage is in any case.

[paulr]

I recall a discussion similar to this one from years ago; the idea was to make a "custom" center filter by printing the image of the film gate onto sheet film, and tinkering with the exposure and development to make a mask which exactly compensated for the edge falloff.

I did this, and it worked. I used lith film, and adjusted contrast through trial and error by increasing development time. I tested with a borrowed exposure meter, and then when it got close with prints of the enlarger image.

I also tried to build a diffusion box like what the OP describes, and this was a resounding flop. I wasn't even trying to increase coverage ... just to improve evenness (before I made the mask) and to make room for a filter drawer above the negative. Increasing the distance between the negative and the light source made the edge falloff much worse. More than I'd want to try to correct with a mask.

[Ginette]

It is my intention to replace all diffusion panels on my coldlights with this product and try to double it like Jerry explain here http://www.largeformatphotography.in...ead.php?106071

[mdarnton]

That masking idea is great--I will try to remember it for any future problems. It reminds me of my first enlarger, which was a Testrite 6x9 diffusion enlarger. The only thing between the bulb and the film was a round piece of glass that was frosted on one side, and then on the up side had an additional deposit of something white with diffuse edges sprayed on the glass in the middle, directly under the bulb.. When I started reading this thread I immediately figured I could do the same thing with an airbrush and drafting ink, but a self-made film mask that exactly replicates the specific probem in negative is really elegant!

I think part of the problem with making the box larger but not the light is that the light probably needs to be somewhat larger than the neg, so that lens/neg/light form a cone, ideally, so that at the corners the light is shining through the film pointed towards the lens--after all light is directional, even with diffusion.