Alley in Pisa, Italy

by Lars Kotthoff

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I've been following recent developments in light-emitting capacitor technology with interest. This new technology produces a complete-spectrum light (like sunlight or incandescents), instead of the spiky spectrum of even the best LEDs and fluorescent lights. And it comes in durable, flexible panels up to 3' x 6' (1m x 2m), in pretty much any color temperature. And, looks like products will be coming to market at the end of the year, for not-unreasonable prices. Looks like they're targeting advertising displays and commercial lighting initially, but given the properties it seems like this is even better for photography than the LED-array panels which are catching on these days.

The only catch is that the current models aren't very bright. They're planning to get more so, but right now engineering samples have a luminance of 200 cd/m², with 600-1000 cd/m² samples planned for the end of the year. That's not particularly bright in absolute terms, but here the whole surface is the light, so it's different from an LED or a single bulb. It seems like the most apt comparison would be a softbox (which, theoretically, such panels might replace). But I'm getting kind of twisted up trying to figure out the equivalent with formulas. Rather than trying to compare candelas and lumens and watts, how can I put this in straightforward camera exposure terms for a given situation?

Let's say I had a 1000cd/m² LEC panel which was half a square meter (roughly like a 28"×28" "traditional" hotshoe-flash softbox). With my subject a meter from the panel, what aperture and shutter speed would give me a correct exposure at ISO 100?

And, how would that compare with a, say, GN 36 flash through that 28"×28" softbox? What about incandescent light of 100W, again, with whatever is needed to diffuse nicely so the light source is effectively that area?

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Soft sources have to be closer than hard ones because they emit less light. But if you go closer you have a stronger light falloff (square law). This can be an issue if you have a moving subject but only a small (1x1m) softbox. –  sbaechler Feb 2 '13 at 22:09
    
@sbaechler: Sorry, I'm having trouble seeing how that's relevant to the question. Can you explain further, in the context of the question? –  mattdm Feb 2 '13 at 22:51
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'And, how would that compare with a, say, GN 36 flash through that 28"×28" softbox?' Not well at all, as far as I can determine. That 500cd panel would be approximately equivalent to a single high-output 65W-ish CFL photo-type bulb optimally placed in an Apollo 28 (allowing for significant losses in reflection and diffusion). That'll barely give you a hand-holdable speed with a typical fast "portrait" lens wide open at an ISO of 200. You'd have to turn the flash power down to open up all the way with a GN36 flash at ISO 100 at 1m. (Not confident enough in my math to make this an answer.) –  user2719 Feb 3 '13 at 4:03
    
One thing to remember if you're going continuous is that the light is now impacted by shutter speed. Continuous light is like ambient, it hangs around for the full range of the shutter and that's not true of a flash. Not necessarily a problem, but something to bear in mind because some techniques are now out as a result (e.g. dragging the shutter). –  John Cavan Feb 3 '13 at 4:37
    
Let's say you shoot a portrait of two people standing next to each other and the source is coming from 45% to the right, 1m away from the person on the right side. The distance to the person on the left is 2m. The person on the left now only gets a quater of the amount of light as the person on the right, which means two stops darker. If the source was 3m away (4m to the person on the right) then the person on the left is less than one stop darker. –  sbaechler Feb 4 '13 at 9:38

1 Answer 1

You can translate cd/m^2 + Area directly into lumen.

lumen = cd/m^2 x m^2.

That is, 1 lumen of light energy will illuminate a square metre of area with a brightness of one candela.

So your 1000 cd/m^2 source over 0.5 m^2 = 1000 x 0.5 = 500 lumen.

Modern available LEDs are achieving 200 l/W (just) (Cree XM-L2 top flux bin, lowest Vf), with LEDs with typical values of over 150 l/W being commercially available off the shelf (I have some). But, even allowing 100 l/W the 500 lumen source is equivalent to 5 Watts of input.

That's minimal compared to alternatives - it's well under a 100 Watt incandescent bulb.

You can get much more output from some of the new 60W equivalent LED bulbs with CRI's of over 90 and people starting to focus on providing CRIs of essentially 100. At the 5 Watt level you could achieve a very high CRI by simply mixing a range of low power LEDs of different wavelengths with your main "white" emitters.

Note that the lifetimes claimed as possible only approach those of good quality power LEDs (50,000 hours +) when expensive encapsulation is used. Quoting:

  • "For our device to reach 20,000 hours, they still have to be encapsulated, but they are not so sensitive as OLEDs," said Carroll. "If you use expensive encapsulation, you will get 40,000-50,000 hours."

The article you cite is from late 2012. Given the typical time from lab to value for money buyable product I see nothing written there that suggests that the new devices will be available within a year with reasonable outputs and/or at reasonable prices. I'd be immensely pleased if they were.

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The conversion isn't quite that simple, since you have to go from an approximate point source to a truly diffuse light. Current LED array panels cast multiple shadows and require either a short throw and two stops of diffusion or a long throw to a 1-stop diffuser to smooth out sufficiently. That's why I estimated a 65-watt CFL in a softbox rather than a 23-watter. The principle is still correct, but obviating the need for extra diffusion gains some (but not nearly enough) efficiency for photographic application as soft light. It'd probably be beautiful home area lighting, though. –  user2719 Feb 3 '13 at 6:57
    
(Still gets an upvote. Just nit-picking.) –  user2719 Feb 3 '13 at 6:58
    
How well under a 100 watt incandescent bulb? What if the incandescent bulb were diffused to that same half square meter? –  mattdm Feb 5 '13 at 3:28
    
I'm not convinced by the CRI numbers for LEDs, by the way, since the CRI standard has so few samples that it's easy to game. The mix of LEDs you suggest is probably the way to go, but there's still that diffusion issue Stan mentions. Anyway, that's incidental to the question, for which purpose let's temporarily pretend I don't care about any alternatives. :) –  mattdm Feb 5 '13 at 3:36

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