How do I compare a continuous light panel's brightness vs. flash through a softbox?

Question

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?

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.