View Full Version : B/W Light temperature?

23-Jan-2014, 23:20
Do black and white pano films favor one color temp more than another? Specifically a standard warmish soft white CFL vs a daylight CFL. I saw a post Alex Timmerman's made testing different types of lights for wet plate. I'm testing an idea shooting inside with long exposure using CFL blubs. I was reading his post and it got me to wondering yes, there are X lums hitting the meter, but are they as strong or is the film as sensitive to that color, would I need to an extra stop or drop one? I assumed daylight bulbs would be best but now im wondering if something warmer like a normal bulb would be stronger, or does it just add more contast? Or does it matter either way?

Alex Timmerman's Post: http://collodion-art.blogspot.nl/2014/01/i-have-seen-light_15.html

Mark Woods
23-Jan-2014, 23:49
Silver emulsion is only sensitive to daylight. That said, it's the tricks of the chemists that allow Panchromatic and Color films. If you look at the the B&W film ASAs, you'll see that in a tungsten environment the film responds 1/3 stop slower. This is pretty accurate with Kodak films. I do the same compensation for Ilford film stocks. Test. Test. Test.

24-Jan-2014, 11:43
Interesting, guess I will have to tinker a little.

Mark Woods
24-Jan-2014, 11:51
Enjoy the journey!

Kirk Gittings
24-Jan-2014, 12:09
"Silver emulsion is only sensitive to daylight."

What does that mean?

24-Jan-2014, 12:11
I assume he means its most sensitive around 5600k, unless someone has other info?

24-Jan-2014, 12:14
Color Temperature, as that term is applied to photographic film, is the ratio of red to blue light.
It does not know or care anything about green.

This makes sense since the emissive spectrum of a thermal light source is highly predictable.
If you know the red and blue components you can accurately predict the green level.

It gets very confused by any source that does not follow the nice even spectral curve of a thermal emitter.
This is why good color temperature meters have a separate green/magenta channel to check that value.

The problem tends to be acute with fluorescent emitters, which have a huge spike in the green range.

- Leigh

Mark Woods
24-Jan-2014, 12:27
All of the film emulsions based on a silver halide only respond to daylight, approximately 5600* to 6000* or higher (hence the used of UV filters at high elevations). Only through dye masking is color achieved or panchromatic response achieved. This is also the reason why B&W films have a lower ASA in Tungston light. Regarding the green emulsion in color film, it can be problematic since part of the imaging in HS films has been shifted to it and reduced from the blue emulsion. This can be very tricky to time out, and if one knows the amount of increased density of the green emulsion, one can use a CC Magenta to correct it to the "ideal" density. BTW, there is no stop compensation when doing this since the CC Magenta only affects the green emulsion.

I hope this helps. It's probably TMI. :-)


Drew Wiley
24-Jan-2014, 12:43
At this point in the evolution of the light bulb (or in this case, a technological de-evolution backwards), CFLs tend to have a pretty wacko spectrum. So you have
to test specific films in relation to specific bulbs. This will affect both the speed (ASA) of the film as well as how various colors translate into relative gray values.

Drew Wiley
24-Jan-2014, 13:43
Given the intended application, I should have clarified something else. CFL's being designated as "soft white" or "daylight" are just marketing labels based on alleged
ability to substitute for ordinary lightbulbs. The spectra is otherwise discontinuous, unpublished, and unreliable. In other words, those terms mean very little from
brand to brand, or even within the offerings of a specific manufacturer, when attempting to relate junky consumer CFL's to tungsten filament counterparts. There is
very little standardization of this kind of thing. So again, you just gotta test, test, test.

Mark Woods
24-Jan-2014, 16:21
Right on Drew. There are color corrected bulbs and fixtures you can buy. The ones in the stores like Home Depot work well, but the fixtures from companies like Kino-Flo are very accurate.

24-Jan-2014, 19:16
Most of film manufacturers publish the spectral response of each film and although basic color meters, typically used outdoors just compare red and blue, the real ones measures each channel individually, including the green. After all, pan emulsions gets energy from all the wavelengths in the visible spectrum so all energy should be considered.
Again, check the specs of the emulsion in the manufacturer's web site and it should show if any exposure compensation is needed under different light conditions.

Bill Burk
24-Jan-2014, 21:07
Alex Timmermans, the collodion artist, has to be concerned about the amount of ultraviolet in his light sources. As Mark Woods points out, without special dyes to make the emulsion panchromatic, collodion is "blue-sensitive". Since the alternative processes get most of the exposure they need from light that your eyes (and your usual meter) can't see, you can't trust an ordinary meter reading to calculate the correct exposure for collodion. So light sources that give a lot of ultraviolet are really important for that process, and exposure is still experimentally determined.

I just picked up a small ultraviolet meter (you know the type, designed for sunburn prevention). It reads fine by sunshine, but just inside the car windshield it goes down to zero. So maybe my car windshield is designed to block UV light, or maybe the UV is blocked by ordinary windowglass. Anyway you can always block the UV with a UV filter and then you won't have to wonder about exposure compensation in the mountains. I found the meter got a reading from a couple inches away from a 500w photoflood, a big 40watt CFL and a specialized UV light source that I have. But I couldn't get it to read above zero by enlarger light, "60 watt equivalent" LED, CFL and halogen bulbs. I don't think these lightbulbs give significant ultraviolet - of the type that would affect your exposure --- to a panchromatic film.

Paraphrasing what I read in an old LP Clerc's Photography book (I can pull out the reference if you are interested), when films were given different Tungsten and Daylight ratings, it wasn't that film has different speeds in different lights... it was to compensate for a quirk in earlier (Selenium Solar Cell) light meters - they are very responsive to the warm light of Tungsten... The old meters were more sensitive to red light than film. So to ensure that you don't underexpose (because the meter needle is too high for the actual light)... Manufacturers suggested different (lower) film speed ratings for Tungsten (the "so-called" Tungsten speed was never an ASA speed).

I confirmed that explanation for myself using an old Weston Master II exposure meter. By Tungsten it tells me there is more light than my more modern Sekonic thinks - while in daylight the two meters agree fairly well.

My experiments tell me modern meters and modern panchromatic films do not need different Exposure Index settings for Tungsten vs. Daylight. (Though in the mountains I might add a UV filter to help tame excess ultraviolet). But when you experiment with old processes, old ortho or x-ray films, or old meters... Then you really have to think about Tungsten vs. Daylight speeds.

Mark Woods, I'd be interested to know if the 1/3 stop difference you quoted is still relevant for modern film and equipment. You have the experience and may be quoting carefully-conducted experiments. I work for Kodak, but I do these experiments for my own enjoyment at home, not using EKC equipment. I could easily be off by 2/3 stop, and I believe you may have done similar tests to a higher standard.

24-Jan-2014, 21:14
Here is an example of which DW writes above:

Mark Woods
25-Jan-2014, 10:08
Hello Bill,

I wrote a paper/article for the Camera Guild (Local 600) 15 or so years ago. I made exposures in both daylight and tungsten light and read the results on a densitometer, then plotted the RG&B emulsions. That's where I learned that the imaging characteristics of the blue emulsion had been partially shifted to the green emulsion. I won't get into the reason why, but the HS emulsions were redesigned. I discussed this with the Kodak engineer, Alan Masson (now retired), in Hollywood. He confirmed my results. I also did a CT test with my Minolta Spot Meter 5 from about 10,000*K to about 1700*K (the approximate CT of a candle flame). The meter was reasonably accurate in daylight and to 3200*K, but began to give low readings for exposure, i.e., indicated there was more light then there actually was, as the CT went lower. At the 1700*K range it was a stop off -- the film would be underexposed if the meter reading were used. The article is title, Candle Light to Daylight. Here's a link to it: https://cameraguild.com/member-resources/techtips/from-candlelight-to-daylight.aspx The Wikipedia article on Color Temperature is also quite good. The reason for the shift, according to my friend Alan, is the meter sensor is based on a narrow band green sensitivity and is "color blind" to the warmer tones. I spoke to my friend Cinematographer Alan Daviau, he told me that his mentors told him to open up as the sun set, and the closer to the horizon it was the more he opened up the stop. He said he didn't know why, but it worked.

I hope this helps.

Bill Burk
25-Jan-2014, 11:18
Mark Woods,

Thanks! I really enjoyed the article. You explained the point the meter responds to the "near-green red" more than the film does... And this confirms that the need for a "Tungsten" rating is to compensate for light meters which overestimate light as you approach the color temperature of candlelight.

The problem could be solved if a light meter were designed to agree more closely to film's response. In the conclusion of your article you give a rule of thumb to give one additional stop of exposure as the sun hits the horizon.

I agree, that's our answer here.

Mark Woods
25-Jan-2014, 11:57
Thanks Bill. I think I got out of being a cinematographer at the right time now that most things are electronically acquired. I was thinking about this the other day that the amount of knowledge I have is obsolete in what was my profession. Fortunately I can use it with my 8x10 Deardorff. :-)

Drew Wiley
27-Jan-2014, 10:32
Somewhere I have on hand the published spectrogram of the sensitivity of the kind of light meter I premdominantly use (Pentax digital spotmeter). This was very important information because of the frequent reliance of the cine industry on this particular meter. But it also happens to be a meter with a peak spectral sensitivity analogous to human vision, in the green. Minolta light meters seem to have been very similar. Of course, you had Fred Picker's ole tweak on the Pentax, alleging to
make it more suitable for "typical" pan black-and-white films. But that subject has been hashed over many times before.