How are"cold lights" built

[Kevin J. Kolosky]

I went to Aristo's web site and all I found there was a catolog for their light sources. Does anybody out there know whether I can walk into a store (hardware, lighting, etc.) and buy one of these cold light tubes so that I can put it in a housing. I know of the Horowitz stabilizer as it was developed in conjunction with Zone VI and I suppose could possibly be purchased from Calumet, although I thought I read somewhere that Calumet might not be carrying Zone VI stuff anymore. Anyway, what I am looking for is more specifics. part numbers, etc. Thank you everyone for your contributions. Kevin

[dileep prakash]

Kevin: Have you thought of using a Mercury Argon grid like Ansel Adam's. I too am planning to build a 4x5 enlarger with cold-light (and using my 4x5 field camera in front of the light box). Mercury argon grid with a diffusing glass in front should work fine (you may need to add a focussing photo-flood lamp behind the grid for better focussing since the grid is pretty low in intensity).

If you have any more data on the Mercury Argon grid please let me know.

Dileep

[Robert A. Zeichner]

There are two issues that I'd like to address. 1. The design of the grid and container in which they are housed is a bit more critical than initially thought. I had a great deal of trouble with an old cold light housed in a can. Apparently the eveness of illumination was ever so slightly skewed. This created problems for me at the extreme edges of my prints. I didn't realize this until I bought a new head that had a perfectly even design. Even a piece of opal glass (mine was actually plastic) isn't enough diffusion at close proximity to the tube. 2. I had to compensate the output of this original design with a light yellow filter to allow me the use of a full range of filters with VC paper. When I got my new head (Aristo VCL-4500), a dual grid type, I not only was able to ditch the filters which made exposure estimation a lot easier, but I discovered I now had at my disposal, a far wider range of useable contrast grades. The phosphors in the tubes, I'm told make the difference. Maybe the construction of the actual enlarger is less critical when it comes to materials and configuration, but I would be mindful of having an even light source of the right color temp. and pay close attention to the alignment of neg. carrier, lensboard and easel.

[Charlie Stracl]

A cold light is a specialized flourescent light. It differs from the ones you buy at the hardware store in these areas.

1. Geometry: The diamater of the tube is small, about 1/2 inch. They get a lot of convolusions (bends) in a small area to give even illumination.

2. The phosphors they use are selected for the appropriate color of light: blue for graded papers, green & blue for variable contrast. These provide high intensity for photographic paper. The standard flourescent tube provides a wider spectrum to match what we need for visual uses.

3. They are thermostatically controlled. A simple fluorescent tube varies a huge amount based on a lot of things. Temperature, how long it was off before it was turned on, how long it's been on, etc. Temperature seems to be the characteristic that best controls these variations, so the cold light heads have a thermostat and a warmer to mimimize the variations. For perfect control, however, you have to monitor the light output and compensate for the variations. Zone VI used to build a stabilizer to do this. You added a photocell to the head and the stabilizer "watched" the light and adjusted the voltage to make the intensity even. Later, they and others made timers that adjusted the time according to the light intensity.

4. Standard fluorescents glow long after you've shut them off. I suspect the cold head manufacturers do something so the cold heads don't do this. I just don't know what. Probably has to do with the phosphors.

Can you make your own? Sure. But it probably won't work well because of all these issues, and it would cost you a lot to address each of the problems. If you want to make your own, try incandescent lights, which is what Ansel Adams did. Then you only have to worry about heat, and the current, and ventilation because of the heat. What am I saying? Just buy a cold light head from Aristo or Calumet (Zone VI).