VC Paper and enlarger bulb color temp.

How much does an aging enlarger bulb really affect contrast? I have a chromega enlarger head that uses a halogen lamp. The lamp has been in there for ages and its color temp (measured at the easel) is now below 3000K instead of the 3400K that the bulb is spec'd at. I use the magenta and yellow filters for contrast control on Ilford MG IV and I adjust for what looks best. The fact that the bulb is aging makes little difference, except that I'm now doing personal film speed and development tests where I'd like to be reasonably sure that I'm simulating a filtration close to 2.5 when I'm doing the printing part of the test (I don't have a densitometer). So, I'm using an actual Ilford 2.5 filter and leaving the dichro filters out of the picture. But... How much difference is the reduced color temperature of the bulb going to have on that contrast value?

There's not much on the web about it. Some articles say it matters and some say that it doesn't matter much. I don't see any reference in manufacturer literature to suggest that VC filters are made for a specific tungsten color temp and not all enlarger bulbs are 3400K. I could put a new lamp in there or I suppose I could dial in some compensation using the magenta and cyan filters. Is it worth it?

I could just do some tests to find out how important the filters are, but I'd rather not do it uncessarily if someone already knows the answer. Advice would be greatly appreciated.

-Dave

It seems to me that since the entire basis variable contrast paper is that the existence of two emulsions, one low contrast and one high contrast, that are designed to be sensitive to different colors of light, then a change in color temperature in the light source due to aging will have an affect.

Whether the affect is significant is another matter. It seems to me that a change from 3400K to 3000K is not that great, and the resulting change in contrast may not be noticeable.

But more importantly, I make decisions about contrast by visually inspecting each print. It is entirely possible that I would never notice that the color temperature of the bulb in my enlarge has changed resulting in a change in the effect of the the VC filters that I am using. The reason is that I make judgements about the required filtration on a print by print basis along the way, the process is self-correcting and adapts to any change in the effect of filters caused by bulb aging.

The only way that I might notice such changes is that if I try to make a duplicate print several years later using old printing notes, I might find that the newer print is more or less contrasty than a vintage print.

Dave, you might try this website for an illustration of color temperature. This is the blackbody curve for different temps.

http://www.mi.infm.it/manini/dida/BlackBody.html

There is a significant shift in the curve for those different temperatures. I suspect that you are more interested in the ratio of blues and greens. Doesn't look like that ratio changes much. Your paper and processes could easily change more. Follow Monophoto's advice. Forget the science and go with the art.

Well, dammit...

I answered this earlier but must have forgotten to hit the post reply button. Old age, I guess.

In response to Louie, I too adjust the dichro filtering by visual inspection for each print, so never gave the actual equivalent filter grades much thought. But, the Barry Thorton article specified that the test prints be done using a "normal" contrast filter of 2.5. This is, in fact, the first time I've used actual Ilford filters since I got the enlarger about 10 years ago.

Anyway, I just happen to have two other lamps, at least one of which has never been used. I measured the color temperature of all three lamps and they were all within 100 degrees Kelvin of each other, so even the one I've been using since I got the enlarger a decade ago hasn't changed that much. I'm not a prolific printer, but even I have printed one hell of a lot of pictures over that time.

Now, here is the clincher. The color temp read read about 2600 to 2700K at the easel for all three lamps even though they're rated for 3400K. The meter is a Gossen Color Pro 3F, by the way, and all other lights were off in the room.

LG, I checked out the link. Pretty cool. I'm and engineer, so that's probably why I sometimes tend to get wrapped up in the technical details. Thankfully, I'm also pretty lazy, so I don't often go through the work of evaluating my processes like I've been doing lately. I feel like I should have learned a lot of this stuff a long time ago, though...

Thanks for the help, guys.

-Dave

How could one control the difference anyway..buy a new bulb everytime you print?

Claire Senft said:

How could one control the difference anyway..buy a new bulb everytime you print?

Click to expand...

Well, the color temp of halogen bulbs diminishes with time, but much more slowly than ordinary (non-halogen) bulbs. Most of the time it wouldn't matter because you correct for it when you pick the contrast filter based on visual inspection. One of the books I have really does suggest changing it periodically. Of course, one could keep it constant by using color correction filters. I have a whole shit-load of those since I used to shoot transparency film under lots of different kinds of lighting. That also explains why I have a color temp meter that rarely gets used now that I shoot B&W exclusively. Maybe I should trade it for a densitometer...

BTW, I was born and raised in Milwaukee. Whereas the rest of the world has gas stations on every corner, Milwaukee has taverns. Makes me homesick just thinkin' about it.

-Dave

Dave

As others have already said, there is no need to worry about the bulb if you print by 'eye'. I do change the bulb on a regular basis, but I need to do this to keep things consistent for my tests. This is not required fro normal printing.

BTW, don't confuse filter numbers with paper grades. My Ilford 2.5 filter produces an ISO grade of 2.9 on MGVI in Dektol 1+2.

I imagine one could control the color temperature by changing the voltage to the lamp. More volts is more current is more power is more heat is higher Kelvins. At some point, the lamp goes poof. The hard part would be getting calibration. I don't know how accurate the color temp meters are. Very spectrally dependent. Your paper is aging and ripening and that will throw everything off. And again, what difference does it make if the prints look good?

Loose Gravel said:

I imagine one could control the color temperature by changing the voltage to the lamp. More volts is more current is more power is more heat is higher Kelvins. At some point, the lamp goes poof. The hard part would be getting calibration. I don't know how accurate the color temp meters are. Very spectrally dependent. Your paper is aging and ripening and that will throw everything off. And again, what difference does it make if the prints look good?

Click to expand...

Ah-ha! Maybe the color temp is low because the volage is low. I don't have a stabilizer. Just the Omega transformer. A lower voltage would explain why the lamp, rated at 50 hours, has lasted over a decade. LOL!

And, one more time, I was doing a test published by Barry Thornton designed to determine personal film speed and development that will allow a "normal" negative exposed over a range of zone I to IX to print with normal grade paper which he says is 2.5 when using variable contrast paper. I assume the point is to center the negative in the printable contrast range so there is room to increase and decrease the contrast according to the needs of the image. Seems to make sense to me.

-Dave