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

Started Aug 17, 2013 | Discussions thread
Phil Veteran Member • Posts: 3,141
Re: High-power LED flash: too good to be true?

nelsonal wrote:

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!

I ran into an LED vendor over the weekend at a harbor street fair last Saturday in Oregon. Among all the household and light rope goodies he had some marine spot lights. They drew 30 x 50 watts at 12V but of course were designed for constant use (in flash terms) as docking or fishing trawler work light.

I was looking at giant single surface LEDs an inch or more across with a very shallow reflector (aluminium by appearance) that was most likely the heat sink not an actual reflector. I could see the 50 being a 400-500 watt equivalence in constant use but who knows as a strobe, 1000 watt plus?

High brightness LEDs are often pulsed because our eyes fill in the blanks like movie frames and it saves heat at the LED. You can't do that in still photography unless you time it so that could be the basis of a flash. I'd think you'd have to preheat the LED to cut down the intensity of the thermal cycle, which I believe would be the limiting factor. Instead of re-charge recycle time you'd be limited by heat dissipation rates and thermal shock related lifetime reduction results.

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Phil Agur
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