Making a VC Cold Light Head?

[Kevin J. Kolosky]

Has anyone here ever made one?

[jon.oman]

Interesting question. I would like to know this as well.

With a cold light head, do you need a set of condensors? Can you get along with just the head, a negative stage, and the lens?

[jeroldharter]

There was a guy selling beseler 8x10 coversion kits with a big spacer and an array of flourescent bulbs. Nowadays, I think LED's might be easier.

[Kevin J. Kolosky]

Yes, but will the LED's work for Variable contrast paper? And how do you control them and make them so that the light is even?

[jon.oman]

There is a thread on using LED's here:

http://www.largeformatphotography.in...l=1#post647152

[Graybeard]

Has anyone here ever made one?

A color head, such as a Chromega, provides both diffuse illumination and variable contrast (via the color filters). There is the considerable advantage that these use tungsten-halogen projector lamps which continue to be readily available.

If you're working with 4x5, there are plenty of these available. These fit directly onto an Omega (of course) and it should be a simple matter to fit a Chromega to most other enlargers (no condensors are required).

For larger formats, things are not so simple.

[Kevin J. Kolosky]

Here is a homemade one.

[pbryld]

Here is a homemade one.

Did you have the fluorescent tubes custom made?

[Sevo]

Yes, but will the LED's work for Variable contrast paper? And how do you control them and make them so that the light is even?

VC are probably the photographic application best suited for LEDs, as VC paper actually needs the monochromatic light natively delivered by (green and blue) LEDs. The commercial heads I've seen all had very even light, placing the LEDs in a mixed grid of blue and greens, and shining that grid into a secondary mixing box (or sphere) before it enters the enlarger mixing box - such a double mixing setup loses some light, but thanks to the low loss of the monochromatic LEDs, 50W worth of LEDs can be at least as bright as 600W in halogen through filters, so that even a 50% loss at the pre mixing stage leaves the LED head brighter than a conventional one. Controlling the contrast can be done with a dimmer (and dimmable driver) for each LED bank - sophisticated systems use one electronic controller that regulates both banks to a constant exposure at any contrast.

Focusing with blue/green is actually more accurate than with white light, as no chromatic aberration issues with exposure-irrelevant parts of the spectrum will irritate you. If you cannot adequately frame in a blue-green light, you'd need an extra set of white (or red) LEDs for a white (or RGB) framing light.

The most difficult issue actually is cooling the LEDs. 50% photon efficiency rather than less than 5% means that they produce more than a magnitude less heat per photon than halogen, so that heat emission into the path of light is smaller than from the best halogen + cold light filter rigs. But it still means that half the driving power ends in heat - and other than halogen bulbs (which have a working temperature of 800°C) LEDs work best/live longest below 50°C, and will immediately be destroyed somewhere above 120-150°C. So you need a fairly sophisticated cooling system to keep 50W worth of tightly packed LEDs in the safe temperature range.

[Kevin J. Kolosky]

"So you need a fairly sophisticated cooling system to keep 50W worth of tightly packed LEDs in the safe temperature range."

Which is why I think most photographers, if interested, would prefer to make a cold light VC head.

I wonder how much the Chromatic abberation has an effect on sharpness in printing. I know Peak sells a focuser that comes with a filter to help with that issue, but the price is quite steep. Still, lots of good photographers over the years have been able to make some pretty sharp prints without all of those fancy things.

I know one needs two tubes, one for Green (Low contrast) and the other Blue( high contrast). I know you place the B. tube above the G. tube in such a way that it looks like G.B.G.B.G.B..... when looked at from the bottom. I know you can use one transformer (60-80W) to power the both tubes, but the tubes must be switched on sequentially. You can buy a toggle switch at a Radio Shack. When it is standing up, it's OFF, when you flip it to right(BLUE) or left(GREEN), it's ON. I know you control contrast by exposing G.B. sequentially. And I know that a cold light does not generate heat so you could make the box with plywood, or you can have a sheet metal guy make one for you.

What I don't know is how you get the tubes bent, or which tubes to buy, and where to buy them.