Replacing Enlarger Bulbs

I recently changed the primary light bulb in the Chromega and I started to think of questions for this group. This activity was part of a general refurbishment of my darkroom set up.

1. I changed from an ELC (800 Lumens) to an ELC-HL (950 Lumens). There is an significant difference in brightness on the easel. My measurements show it is a tad over 1 stop brighter. I think that the difference is not only the change in lumens. The old bulb was not burnt out but, I just wanted some extra light.

After changing bulbs, I noticed one curious effect. It seemed that magenta exposures in the split printing technique are relatively much shorter than the reduced yellow exposures. The yellow exposures are cut in half but the magenta exposures are about one-fifth. I also changed developers but I don’t think that it accounts for the entire magenta reduction.

In addition to the bulb losing brightness, I suppose that it had become more yellow in its output over time. I actually noticed this effect on the easel with the back of a discarded print for focusing. The white-light image seemed much cleaner.

I had been using the bulb for about 7 years and I think that the brightness had diminished over the years. Do other people find this to be true; that the bulbs loose intensity long before they burn out? Does anyone monitor for this change?

2. Dichronic Filters. I looked at the filters and other questions arose. I have heard that they fade and that they don’t fade. The Omega spec sheet say they don’t. Any comments on this issue? Also, if I were to get a new magenta filter, could I replace the cyan filter with a new magenta and use both to get a higher magenta value? The maximum for a Chromega is 170 units. This is rather low for some papers and a full grade 5 cannot be achieved. Would this extra filter allow a higher magenta value or would it only be like the addition of neutral density?

Thanks,
RM

Bulbs will loose brightness over time, because the tungsten plates onto the inner surface of the glass. The halogen gas slows this down, which is why quartz halogen bulbs can burn at higher temperatures than a "standard" bulb, but it doesn't stop it. Often, the inside of the glass in a very old bulb will be completely black.

I think the difference you notice with the new bulb must be from a color temperature difference. I couldn't find any color temp specs for those two bulbs, but since they are the same wattage, something else has to change to get more light output.

My undestanding is that dichroic filters do not fade. They produce the color from a thin film on glass, and produce the color by interference rather by a dye as with colored filters. It's the same phenomena at work as with the anti-reflection coating on lenses.

A couple of things may be happening. First, and old bulb coats the glass with a bit of tungsten, despite the halogen scavenging. This will dim it a bit and may lower the color temperature. Second, a brighter bulb of about the same power consumption will have a higher color temperature (be bluer). Third, bulbs from different manufacturers may be built differently and may show different color temperatures. Generally, when you change enlarging bulbs, there will be a pretty marked change in filtration. This will also shift slowly as the bulb ages, but you adjust to the slow change and don't notice it.