April 27th, 2008 @ 21:45 by NormMonkey
WHAT’S ALL THIS, THEN?
Jim Talkington is a photographer in Cincinnati, Ohio who makes a living making pictures. In today’s world, that’s worth a few nods of respect. Further, he shares his wisdom at his blog, prophotolife.com.
How awesome that is, or how Jim honed his amazing time management skills, is not what I want to explore today. Jim posted an article comparing the quality of various bulbs as photographic lighting. As one of the bulbs was a compact fluorescent (CF) and as I recalled how CF bulbs emit light differently than natural light, I thought I’d post a comment.
That lead to a follow-up article in which Jim compared light from a CF vs. an incandescent bulb. That (only three paragraphs later) is the topic of my mental meandering today. I’ve taken the bulb-ball and I’ll share where I’ve run with it so far:
I thought I’d start at my comment: my recollection that CF bulbs emit light differently than “natural” sources of light – i.e. things that make light by being very hot.
Some quick googling*extensive research* led me to a site full of spectrographic analyses of various sources of light:
Analysis of Sunlight
© The LED Museum
Analysis of Flame
© The LED Museum
Analysis of a 60W Incandescent
© The LED Museum
Analysis of a CF Bulb (5000K, 13W)
© The LED Museum
SOURCES OF LIGHT
Sunlight, flame and incandescent light are all governed by Planck’s Law of black-body radiation. They emit light at all wavelengths, with the peak changing according to temperature. Fluorescent bulbs work differently.
Wikipedia tells us that the gas within a fluorescent bulb is excited by electricity, causing light. Sadly, it’s mostly UV light, so the tube is coated on the inside with phosphors that absorb the UV radiation and emit visible radiation (some is converted to heat, but as we can touch fluorescent bulbs it’s not so bad). The mix of phosphors determines the visible light:
Fluorescent Lighting Spectrum from Wikipedia
Click for Explanation
Our Wikipedia page tells us “A typical “cool white” fluorescent lamp utiliz[es] two rare earth doped phosphors, Tb3+, Ce3+:LaPO4 for green and blue emission and Eu:Y2O3 for red.”
So unlike black-body radiation which throws light at all wavelengths, a fluorescent bulb throws most of its light at specific wavelengths. That’s not so bad, though, as those wavelengths are our favourites: the ones that the cone cells in our eyes enjoy; the ones that our camera’s Bayer filtered photosensors detect. The ones our TVs and monitors fling at us daily, namely: red, green and blue.
A PHOTOGRAPHER’S HOOT
In what way does all this matter to us as photographers, then? I suppose what matters most is how the bulb manufacturer chooses to mix their phosphors to make the light appear natural. It’s important to note that not all fluorescents are alike.
Look back at that Wikipedia page again. Note the spectrograph for the halophosphate bulb. “The color rendering ability of these older style lamps is quite poor” says the article. One of the graphs on the LED Museum page shows a “Dollar Store” CFL of this type, implying that they might be living among us today.
So, as photographers interested in using CF bulbs for lighting, we want to make sure we choose a bulb that uses the right phosphors in a mix that closely matches natural light. Looking at the results Jim posted in his comparison, I’d say that bulb he used does a pretty good job.
Jim did an excellent job of doing a well-controlled comparison. However, I can’t very well go writing an article about photography without taking any pictures myself, so I tried to replicate his results, to see what I could learn.
Unfortunately I don’t have a Gretag Macbeth colour checker chart. I did, however, find a “Stride Rite” shoebox at a friend’s house with a very colourful design (what can I say, this is a budget production). I set my camera on tripod in the basement with the shoebox 3-4 feet away. The lamp, scavenged along with 3 bulbs from around the house, I placed at camera right, about 1 foot away.
I shot RAW to avoid worrying about setting the white balance perfectly in-camera. I set the exposure at 1.3″, f/4.5, ISO 100 and took three frames: Frame 1 uses a Philips MarathonTM 15W CF bulb whose light has a warm glow (sort of) matching an incandescent’s (3000K-ish); Frame 2 uses a Pur-Lite 15W CF bulb whose CCT seems to be around 5000K, producing a very white light (perhaps I should say, Pur Wite Lite?); Frame 3 uses a 60W incandescent.
In processing the results, I set the white balance from the white “S” in the lower right corner using UFRaw. In The Gimp, I used Colour : Auto : Normalize to stretch the brightness of the image to fit the maximum range. (I did this after I noticed from the histograms that the brightness didn’t turn out the same across all bulbs. 15W CF is *supposed* to be 60W equivalent) Finally, I used a “Difference” overlay to make the differences clear. Anyways, here are the results:
Col. 1: CF3k, CF bulb, low temp.
Col. 2: CF5k, CF bulb, “pur wite lite”
Col. 3: INC, Incandescent
Row 1: histograms for the raw pictures (before I normalized)
Row 2: the pictures before normalization
Row 3: the pictures after normalization
Row 4: Differences (CF3k vs. CF5k / CF5k vs. INC / INC vs. CF3k)
(*) There is a small but noticeable difference, which the “difference” overlay makes apparent.
(*) The difference between CF3k and CF5k is similar to the difference between INC and CF3k. This suggests to me a couple of possibilities:
(o) The differences between phosphors is just as significant (or insignificant) as between CF emissions and natural light emissions;
(o) My own image processing (either WB adjusts or normalizing) might be confusing matters;
Instead of a final result, what I’ve done so far is a learning experience. In doing a better study of this I would change a few things:
(*) Reset the exposure between each type of bulb by using a voice activated grey card. Take a frame as long as the card’s there anyway, to be used for white balancing.
(*) Include more of each bulb type to act as a control. Perhaps the difference between a 60W and 100W incandescent is just as noticeable as between incandescents and fluorescents.
(*) Get a Gretag Macbeth colour checker chart, or at least have handy greyscales to check exposure and whitebalance against.
(*) Take better care in post-processing to be avoid introducing artifacts.
In the meantime, however, I believe that CF bulbs ought to be fine except for critical colour management scenarios I can’t even imagine. I suppose those involved in such scenarios know more about this than I ever will.
UPDATE #1: If you’re visiting from ProPhotoLife.com I’d just like to point out (before you waste your time) that the vast majority of this blog is not photo-related. Here’s a link to the few posts that are tagged ‘photography’, be ye inclined to explore further.
UPDATE #2: I did a bit more digging and posted another article which includes info on strobe light spectra, in case you’re curious.
 I tried to find out more about who did these but The LED Museum site scared me away with pop-up attempts and general Websitesthatsuck.com behaviour.
 OK, so they’re called X-Rite now. Who’s great idea was that? “Gretag Macbeth” has personality and uniqueness aplenty! “X-Rite” sounds like somebody pushed the button on the Random Marketroid Naming Machine. Very e-Web-2.0. I can feel the synergy.
 I’m *sure* these guys used that same machine. Pur-Lite is apparently by Reusable Technologies, Inc.
 It’s an unmarked, completely clear bulb (no frosting), but another bulb from the same fixture in the kitchen where I scavenged this one still has its markings: 60W.
 You say “OK, put the card down” and your voice activated system places it in front of the colour chart – that is, if you can tear her away from World of Warcraft.