Testing enlarger lamp temperature

[Marco Buonocore]

Can anyone see anything wrong with testing the colour temperature of an enlarger lamp using a colour meter? Specifically, a unit like the Kenco Color Meter KCM-3100.

I've been using a point light source run through a variac, and am curious to know how much the colour temperature shifts as I change voltage.

In the future, I would like to compare LED options with the tungsten bulbs I have used.

I'd like to know that my method is sound before spending too much time fooling around.

 

[ic-racer]

B&W or color? If B&W you can use an uncalibrated stepwedge and multigrade paper. See if there is any difference. If none then no need to be concerned.

 

[Gerald C Koch]

The color temperature is determined by the energy of the photons produced by the bulb which in turn is controlled by the voltage. The energy distribution follows the Maxwell-Boltmann curve. As can be seen in the article total photon energy falls very rapidly from the peak on both sides of this curve. Therefore using a Variac to change the light output has a very significant effect on the color temperature. Lowering he voltage will shift the color temperature toward the red. This will effect the response of both variable contrast B&W and color papers. Probably not a good idea unless you are using only fixed grade papers.

https://en.wikipedia.org/wiki/Maxwell%E2%80%93Boltzmann_distribution

LED's have their own problems as the their spectra may not be continuous. So a particular blue or green LED may be a poor match for a particular variable contrast or color paper resulting in a different contrast or color balance in the case of color papers than expected.

 

[Sirius Glass]

It the enlarger lamp is one that is made for the enlarger, then I trust that the enlarger bulb is in the temperature range and the dichroic filters will adjust the light for black & white and color printing.

 

[Doremus Scudder]

Marco,

I'm not sure what you are trying to do. If you've been using your Variac and want to see how much it affects color temperature, then fine. As Gerald points out, you are likely changing it significantly, which makes a given amount of attenuation to the eye even more so for the paper, since you are losing the blue and green portions of the spectrum faster than the red.

If you are printing color, this will have a real effect on the corresponding colors in the print and result in significan filtration requirements, which will reduce exposure times even more...

For black-and-white, this will have the effect of giving a greater green component to the attenuated light in relation to the light at full power, thereby altering the response of VC papers (e.g., difficult to get higher contrast with attenuated light). If you're only using graded papers, then it will simply lengthen exposure times, all else being equal.

The question arises: why use a Variac in the first place? If you need less light for longer exposure times, there's always the aperture on your lens or ND filtration, both of which will not affect the spectrum of your light source.

To answer your question: Since the meter reads in degrees Kelvin, you should be able to get a good comparison. And, since it reads in CCs too, you might even get an idea how to correct your color filtration to a certain extent (the meter is designed for filtering film, however) How exactly that translates into spectral components compared to the response of any black-and-white paper(s) you are using is a different issue and may take some testing. The step-wedge suggestion by ic-racer above is a good place to start.

Best,

Doremus

 

[Marco Buonocore]

B&W or color? If B&W you can use an uncalibrated stepwedge and multigrade paper. See if there is any difference. If none then no need to be concerned.

I should have stated, I'm printing B&W. That's a great method of testing, ic-racer - really simple. I love it. I'll try it when I'm down in the darkroom today.

 

[Marco Buonocore]

With regards to the variac, I also should have noted that I am only using it with the point light source. The reason for that is that you cannot use lens aperture as a means of controlling light, as the aperture blades cast a shadow on the easel with a point light. It's brilliant for old fixed grade papers, giving at least a 1.5 grades increase in contrast. It's less useful with VC paper, as the color temperature is around 2300k on the setting I'll be using it at. This is well shy of the optimum range that Ilford suggests, between 2856k and 3000k. I might introduce a cooling filter and see how that works.

Lee has a calculator that seemed useful:

http://www.leefilters.com/lighting/mired-shift-calculator.html

I've got a Stouffer 31 step 6x6 neg that I use to compare paper contrast, so the results of adding a filter should be pretty apparent.

 

[Marco Buonocore]

I'm just following up on this if anyone is curious.

I used a Kenko Color Meter KCM-3100 to take readings.

After measuring the colour temperature of a variety of light sources that I've got down in the darkroom, I was surprised to find out that most of them were warmer than the Ilford ideal range, which is between 2856k and 3000k. I got that number from Ilford Technical service, who were (as usual!) most helpful. If a lamp has a warmer colour temperature, it will produce less contrast with variable contrast paper. Primarily I use VC paper, so this was a concern.

Here are some of the readings:

Durst Atlas 300W bulb - 2350k
PH211 75w - 2640k
PH212 150w - 2590k
PH213 250w - 2870k
Saunders colour head - 2750k
Omega colour head - 2920k
Point light source on mid setting - 2280k

This is by no means a proper exhaustive test - I just checked what I had on hand. I work in a group darkroom, and we've got 12 Omega D5s with PH211 bulbs, and the figure above is an average of those. They were consistently below Ilford's recommended values. Not as warm as some of the other sources though. I was mainly interested in addressing the Durst Thorn 300w bulb and the point light source, as that's what I am personally using. I figured I would add a cooling filter to the system and see how it behaved. I used the following mired shift calculators:

http://www.pointsinfocus.com/tools/mired-calculator/
http://www.leefilters.com/lighting/mired-shift-calculator.html

I bought some Lee gels sheets - 202 and 218 blues - and put them in my filter drawer. I measured the lamps again, and was able to get both lamps to around 2900k, which was just where I wanted.

I made a series of step wedge tests, with and without the cooling filters, and with both the Thorn bulb and the point light. I made them at grade 2, grade 5, and with no filter at all. In all cases I got a significant boost in contrast with the cooling filters. Less than a grade, but more than half. This was right across the board, which was great news. I'm not knocking IC-Racer's approach for testing in post # 2, but in the end, that method didn't tell the whole story.

Why fuss around, you ask? Well, it was a bit of a rabbit hole. I was initially curious about the colour temperature of the point light source, but as often the case in the darkroom, one thing leads to another. I mostly do split grade printing, and I want my grade 5 filter to deliver the most contrast possible. It suits my style of printing. When I'm burning locally with a grade 5, I'd like it to be as hard a contrast as possible, if you follow me. The point light source was a logical conclusion to that train of thought.

I suspect that the situation I was in was not the norm. That Thorn bulb of mine has got to be 20+ years old, maybe more, and I gather that bulbs tend to get warmer in colour temp as they age. The point light is quite warm on low to mid settings due to being run through a variac. Both these sources were well off the ideal. If I was using one of the PH211s that was a touch warm, I don't think I'd be concerned at all.

Hopefully I won't have to do this again!

 

[ic-racer]

If you already had the meter, then that is a great use for it. Thanks for sharing. I use a color analyzer just because you can frequently get them for free. Those generally only can show differences between light sources, and can't give the temp values you got.

 

[Marco Buonocore]

Yes, the meter belongs to a colleague of mine. I don't think I would buy one just for the task, but it seemed the right tool for the job. Using a colour analyser is a great idea; I think I've got one buried in the darkroom somewhere!

One last note on the topic: I was very surprised to find that, in terms of actual light output, the PH211 75w bulbs produced ever so slightly more light than my Thorn 300w bulb. This was obvious in looking at the step wedges, and by using my Darkroom Automations enlarging meter.

That came as a real shock to me, for two reasons. One, that 300w bulb runs HOT! I usually know a print is done when I can smell the dust cooking on the top on the enlarger head. To think that the relatively dim 75w bulb kicks out the same amount of light, with much less heat. I wish I were aware of that fact, especially when making bigger prints, or when lith printing. The Thorn bulbs are so physically large, and apparently the frosting is quite substantial - it amounts to being a very diffuse light source. I had never considered it as such. Comparing it to the point light source is quite amazing.

 

[alanrockwood]

...The energy distribution follows the Maxwell-Boltmann curve.
https://en.wikipedia.org/wiki/Maxwell%E2%80%93Boltzmann_distribution...

A slight correction. Photons from a Tungsten light approximately follow a Planck distribution.