High-power LED flash: too good to be true?

Started Aug 17, 2013 | Discussions thread
nelsonal
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Re: High-power LED flash: too good to be true?
In reply to cyrano, Aug 27, 2013

Their product description describes it as a 350W peak LED flash source, (at 150W constant source it's great for video). For very small video work, it's got many of the advantages of a Joker light at a price that's probably close to the rental! LEDs that I've seen can hit about 100 Lumens per watt, so lets give them the benefit of the doubt. That would make this a 35,000 Lumen source.

However, it's nowhere near a speedlight. For photography Lux seconds are the operative measure. If we imagine a 75J strobe with a 70 degree reflector (ie an alien bee B400 at half power) that's close to the energy of most speedlights (the speedlight would need to be at a fairly wide setting). That's about 3500 Lumen Seconds. At 3 meters that's 300 Lux seconds of luminous exposure.

The LED cube is 35000 lumens, but in a photo exposure, it needs to deliver light in say 1/100th of a second. Which means luminous energy is only 350 ls in the relevant exposure. Which means that with a similar 70 degree spread (I didn't see much documentation about a reflector/fresnel) this will only generate 34 Lux seconds of luminous exposure. So our speedlight is producing about 10x the useful light for a photo (it gets worse at higher flash sync speeds). So great for video, where $500 bought a glorified grow lights while this is battery powered, smaller, dimmable, and could have a higher CRI.

I'd love to hear my math is wrong, but this doesn't seem like it's quite up to speedlight power.

It appears the equivalence is a fairly tight reflector (30 degree) on the cube guide number either 16 or 22, both in meters, vs a wide reflector on the strobe (14mm looks close guide number 15 also in meters.  At the closest reflector setting the 580 EX II has a guide number of 42!

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