LED versus flash bulb in photography - Page 1
Forum section: electronics / general

author: Unknown
author_rank: Unknown
timestamp: May 08, 2014, 12:51:00 pm
content: *REMOVED BY MODERATOR AT USERS REQUEST*

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 08, 2014, 03:54:58 pm
content: LEDs actually light up faster than flash tubes do: less than a microsecond. For photography purposes, you can call both instantaneous. Unless you're doing high speed photography, you probably don't even have a shutter anywhere near that fast!By efficiency, flash tubes and LEDs are comparable; I think LEDs are a little better. This isn't like comparing to incandescent, both are in a whole other class. And the average consumption is miniscule if you're doing flash photography.Flash does extremely high peak power, LED does not. This means, for a given illumination level, the flash exposure can be teeny (< 1/1000), but you would need a massive number of LEDs to achieve nearly the same effect (several work lights worth of illumination might get you closer to 1/200 second?). The difference is, the LEDs can be on for a long time, so if you want your set as bright as daylight, just use enough and you'll get there.Think of the LED on a camera phone. When taking a picture in dim conditions, it lights up like a flash -- well, it tries to, but to do any good, it has to be on for a pronounced length of time, 100s of ms, so you get just as bad a picture from a shaky hand as without a flash.Regarding existing hardware, an LED lamp may take anywhere from ms to 200ms to turn on, and may have undesirable ripple in the output (anywhere from line frequency modulation to 100s of kHz). The former would be the most relevant to photography, as the illumination will be random depending on exactly when you hit the shutter. Most lights, for illumination purposes, I think will have fairly stable output.If there are LED fixtures on the market specifically for photography, they'll probably be better with regards to stability.Tim

author: georges80
author_rank: Frequent Contributor
timestamp: May 08, 2014, 05:10:09 pm
content: ^ agreed. A flash can generate a very bright very short light pulse.LEDs (with decent CRI) would be great for fill lights/flood lights etc. No so good for flash use. LEDs can typically be overdriven a little in pulsed mode, but still no where near enough (before bond wires etc vaporise) to match the instantaneous light output of a flash.LEDs can definitely be turned on very quickly (and back off again) and a decent driver will provide very clean transitions. Here's a picture of one of my LED drivers. The top trace is the PWM input control, the bottom trace is a 50MHz bandwidth Tek current clamp over one of the LED wires. 3A nominal drive to the LED.cheers,george.

author: georges80
author_rank: Frequent Contributor
timestamp: May 08, 2014, 06:07:10 pm
content: "Can you please help me by telling what is "The top trace is the PWM input control, the bottom trace is a 50MHz bandwidth Tek current clamp over one of the LED wires. 3A nominal drive to the LED." I mean the Tek current clamp. You apply a PWM to some circuit that is the input for a circuitry that provides PWM to the LED?This is another thing that intrigues me. The PWM here is for "constant current" that is PWM in current with peak value 3A, or? Constant current 3A is not enough?"The top trace is the external PWM control to the driver - i.e. each time the top trace goes high the driver starts to drive current (3A in this case) to the LED load. As can be seen it takes about 50uS from the control signal going high (e.g. a flash trigger) and the driver producing for 3A through the LED load (the bottom trace is showing 1A per division). The LED obviously turns on VERY quickly. The turn off is even faster.The driver (in this case) is a constant current driver (fully current regulated). It can output up to 6.7A (if so adjusted), though for this test it was adjusted for nominal 3A output.PWM can mean the input CONTROL signal to a driver (to turn the driver output on/off at whatever rate the user requires) or could mean the PWM control that the driver internally uses to drive the inductor/FET combo. In this case I'm talking about the input control signal.If you want to learn more about various LED drivers (I also have a FAQ) then visit my website - the link is below.Given your questions, you have some reading/learning to do - there is PLENTY of info out on the web to learn the basic nomenclature.cheers,george.

author: georges80
author_rank: Frequent Contributor
timestamp: May 08, 2014, 06:54:07 pm
content: No where did I say buy from me? I'm providing you information that you can use to read and learn. I have NEVER pushed any of my stuff on this website.The technical sections of my various drivers and my FAQ have a lot of info that I've put together over many years to answer typical questions from various customers. Yes the drivers cost money, feel free to go on ebay/alibaba and buy chinese drivers (I'm sure they are very well documented...)I put most of that info on my website together so I wouldn't have to answer the same questions over and over again from my customers. You can benefit from all that information for free. At least read through the information and then you can better determine what is possible and what is needed to put together a LED system that you can turn on/off at some specific rate. You'll need to read up on the various LEDs to determine their drive current and what pulse current they can handle.I doubt anyone can give you a simple answer of 'yes, buy 10 leds, connect xyz driver and you'll have a great setup for photography'... Even the choice of optics and how you plan to blend all the various LEDs together to get a flood or spot and at what angle is a project in of its own.So, my suggestion (take it as you will), is to at least read up on how drivers work, the type, the current output, how they can be controlled externally, how fast they respond to a control signal, buck, boost, efficiency, LED specs, optics, heatsinks, etc etc...Control input PWM can be used for dimming, it can be used in single shot... All depends on specific needs. PWM dimming can be a major issue with video since it can 'beat' with the video. Constant current dimming can have tint issues since the white phosphor on LEDs causes a colour shift at different drive levels. PWM dimming (since it is at constant current) has less tint shift issues - but then you have the video issues... Lots to learn...cheers,george.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 08, 2014, 07:51:49 pm
content: Quote from: pereczes on May 08, 2014, 05:42:05 pmSuppose you have enough static lights to take a good picture at 1/100. That's usually a very well lit room, though not like broad daylight. While the shutter is closed, the lamps do absolutely nothing, photographically speaking. If you could synchronize the lights to the shutter, so they are only on while the shutter is open and exposing the "film" (whether actual film, or a CCD or other sensor), you could take exactly the same photos, with a fraction of the overall power used (because, unless you're shooting video, your shutter is closed most of the time).Give or take things like automatic focus / exposure / F-stop / ISO, which you may or may not have set, which are calculated based on the static scene. Obviously, you have to have it set correctly for the scene with lighting.Now, suppose you wanted to take photos at 1/1000 instead. You need ten times as many lamps, to provide ten times the illumination, for 1/10th the duration of the previous example. The energy is the same, but the cost just went up, because LEDs can't handle too much peak power.Whereas, since the flash tube is already in the 1/1000 to 1/100,000 range (depending on design), with high peak power, so you don't have to do this sort of scaling with flashes when the exposure is longer.Now... if you had a super-high speed shutter (in the micro to nanosecond range), the flash pulse would be many times longer than the shutter opening, and you would again have to use a great many lamps to achieve the same exposure. (On the plus side, this sort of photography was often used forextremely luminous subjects, so lighting wasn't a big deal.)QuoteAs mentioned, LEDs unfortunately don't do pulses very well. There are a few reasons:- Thermal -- chip and connections (bond wires)- Saturation: at high current densities, the LED itself becomes less efficient, and you suffer diminishing returns. Effects from charge carriers, recombination and reabsorption (the LED acting as a photocell to its own internal light!), among other sundry physics, play a role here.- Phosphor: the LED itself is blue; some of this is converted to a broad yellow, giving an overall white. But it can only do so much, for similar physics as the LED itself. (I don't actually remember offhand what all mechanisms limit a phosphor, but they should be similar, at least.) It's also slower, so whereas the blue light might be controllable in the 10MHz range, the yellow part won't -- something to keep in mind if you're transmitting data over ambient light.This is easy to imagine if you remember those "nite glo" phosphors, that can be "charged" with a bright light, then keep on glowing for hours. Every phosphor has a different decay curve, some fast (~femtoseconds), some slow. What happens is, a given "center" in the crystal (which might be a rare earth dopant) gets excited by some energy, then after a while, decays to the ground state, releasing light of some color (with less energy than the original stimulus). While an atom is stuck in the excited state, it's *not* giving off light, so the power transfer of a phosphor has to be inversely proportional to the time constant. LEDs are pretty bright, so their phosphors have to be fairly fast to keep up (without needing a massive chunk of phosphor, that is). Trying to pump more energy into the phosphor will either accomplish nothing (it goes right through, not being absorbed), or pushes atoms to even higher energy levels, causing damage (ionization, permanent chemical change, dislocations..). Which I think is a major player in the life of a phosphor as well.- CRI: the efficiency of the phosphor varies at different intensities, too. Unless the device is characterized at different currents, it's probably only correct at the rated current. (This could be one good reason to PWM LEDs to control brightness, rather than regulating DC current.)QuoteAnd certainly not a hundred at once!QuoteYes.. well, unless the application requires it, the driver can be pretty much whatever speed the designer wishes. Typical figures might be:- < 2us: linear current regulators, PWM (unregulated), very fast switchers, special purpose (optical data, etc.)- 10-100us: medium to fast switchers, regulated PWM (e.g., backlight driver chips, which can be on-off PWM'd, or analog throttled); the scope shot above is a perfect example of this sort of design and performance- 10-50ms: soft start circuits; line powered switchers; passive LED strings (e.g., Xmas lights -- usually pulsing at line frequency)- 100ms+: Active PFC switchers (the long time constant is necessary to filter out line ripple), intentionally long time constants (to 'soften' transitions?)So if you're looking to explore this practically, be mindful that probably no one really cares about how fast their light is, so if you want pulsed drivers, you'll need one that's documented as being fast enough. Nothing that needs, like, custom design and certification, but you may have to test a few products (like those eBay drivers) and see which ones work to be sure.Tim

author: dannyf
author_rank: Super Contributor
timestamp: May 08, 2014, 08:26:10 pm
content: QuoteWow!

author: cyr
author_rank: Frequent Contributor
timestamp: May 08, 2014, 10:54:56 pm
content: Quote from: pereczes on May 08, 2014, 05:42:05 pmStrobes are often rated in watt-seconds, the total energy of a full-power discharge. A battery-powered strobe may be something like 50-100Ws. Most of the energy will be output in about a millisecond or two (seehttp://www.gock.net/2012/01/flash-durations-small-strobes/). The peak power is then on the order of 50kW (!). A quick google search says that the efficiency of a flash tube is typically 40 lumens/watt or so, so that would give about 2 million lumens peak.That would roughly be equivalent to 10kW or so of LED lights. Or 1kW if you can go with 1/50 s shutter speed, 100W at 1/5s etc...

author: Someone
author_rank: Super Contributor
timestamp: May 08, 2014, 11:50:32 pm
content: Quote from: pereczes on May 08, 2014, 05:51:15 pmGreat answer here:Quote from: cyr on May 08, 2014, 10:54:56 pmA good photo flash tube could be driven down in sub millisecond duration, and specialised scientific flashes take it into microseconds.CREE provide some excellent information on pulse driving of their packaged devices (not just the bare die), which show it would need an awful lot of LEDs to match a typical professional photo flash. Professional equipment tends to be at the bleeding edge, if it were effective to used LEDs as flashes you'd see products popping up.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2014, 01:03:29 pm
content: Also consider "light engines" which are metal assemblies with up to 100 LED dies integrated. Typically 100W for $30. Ten of these would get you to 1kW. I guess if you can get CREE XM-L T6 that cheap, you're paying about the same per watt.A pulse of extra-high power, up to 0.2 seconds long, would probably be safe from damage without a heatsink. You would have to wait until the array cools down before repeating the pulse, so if you want to take a lot of photos in rapid succession, you may need a heatsink. The CREE XM-L T6 is rated for chromaticity up to 3A (nominal at only 0.7A), and if the trend continues, maybe it wouldn't be much worse at 6 or 10A. This much peak power would reduce your array from 10x10 to 6x6 or so, which is certainly a cost savings. It is impossible to provide any assurance, however, because a thermal pulse response graph is not provided. The LEDs will certainly be beyond manufacturer's ratings, and will certainly age much faster (hundreds or thousands of hours, versus 10k-100k hours at ratings). As a flash, this still may not matter, as the on-time is brief.For best CRI, you might be able to improve the spectrum by adding blue, cyan or red LEDs to the array, just a few to adjust the overall color. I think you would want to quantify the spectrum under pulsed conditions first, and then augment it until it meets requirements. If you don't have a spectrophotometer or CRI meter or whatever... just do what looks right.Tim

author: dannyf
author_rank: Super Contributor
timestamp: May 09, 2014, 01:08:03 pm
content: QuoteOr you could just get a flash light,Cheaper, easier to use and simpler. No need to worry about power supplies too.This is a case of a wonderful high-tech applied to the wrong place.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2014, 12:27:47 am
content: Quote from: pereczes on May 09, 2014, 01:32:05 pmYes. Thermal stress in the die and bonding; atomic/chemical/molecular damage to the die and phosphor. As far as I know, aging accelerates with higher currents, so it's not just proportional to average current flow.Tim

author: mzzj
author_rank: Super Contributor
timestamp: May 10, 2014, 10:39:47 am
content: Quote from: pereczes on May 09, 2014, 01:24:35 pmBuy 5 or 10 cheap chinese flashes instead of SB-910's and diy/modify them to your satisfaction?I Think it should be pretty obvious at this point that LED's are less than optimal for high power flashes.5x SB910 is around 500Ws. Assuming that you overdrive the hell out of your led's they have similar efficiency to flash tube. (for leds 3x pulsed nominal current drops efficiency to roughly 40% of the rated)If your maximum flash duration is 1/50 seconds that would mean 25kW peak power for leds.1/50 second flash duration is totally useless for anything that moves like kids or animals, so you want probably something like 1/500 second flash duration.500Ws with a 1/500 exposure is whopping 250kW of peak power. You can overdrive your leds with 3-4 times nominal current before effiency drops too much, so for a 250kW pulse you need leds worth of 83kW.83kW set of leds is "somewhat" large and expensive, 100W bridgelux leds are something like 20 euros per piece, so the total for the LEDS ONLY would be 17000 euros.1750 Euros vs. 17000 euros sound like you put your money for the wrong horse.

author: mzzj
author_rank: Super Contributor
timestamp: May 10, 2014, 06:23:31 pm
content: Quote from: pereczes on May 10, 2014, 04:58:54 pmI can tune my calculation to make leds look even worse.If you dont need the power SB910 can offer why dont you just buy smaller, cheaper flashes?Or buy cheap chinese flashes with 20 euros per piece and add your own controls to them?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2014, 11:54:56 pm
content: Quote from: pereczes on May 10, 2014, 04:40:13 pmIf the dimmer can be set to 100% and the LEDs are still within limits, you're fine.If the pulsing occurs at line frequency, it's probably not the greatest, but the die temp won't vary all that much. Reason being, the magnitude is lower so the heating is lower (it's at rated power, not several times), and the duration is still relatively short (thermal time constants are often in the 100s of ms -- though I'd like to see that graph in a datasheet to be sure!).If the pulsing is at high frequency, it doesn't even begin to matter; thermal effects just don't show up in the microseconds.Tim

author: Someone
author_rank: Super Contributor
timestamp: May 11, 2014, 10:21:30 am
content: Quote from: pereczes on May 09, 2014, 11:14:45 amFrom a rough calculation you'd still get more light into an exposure from a studio flash through a diffuser than such a hypothetical LED panel so there is still lacking a particular application where a LED flash would have some interesting advantage. Studio flashes have duty cycle limits (even some incandescent instruments have duty cycle limits) so thats not a barrier to entering the market.You can run flashtubes in a quasi-continuous situation that flashes them at dozens or hundreds of Hz, but lighting instruments instead use lamps designed for continuous light if that is what is desired. LEDs provide a similar efficacy to gas discharge lamps when run continuously, but they still don't scale well up into the higher powers (we're just starting to see LED based projectors). The only way LEDs scale up is by adding more sources, where as incandescent and continuous gas discharge scale into the 10's of kW in a single source easily. So far the only niche where LEDs have found good use is space constrained situations where a diffuse light is acceptable.Your combo flood/fill/flash would have the power equivalent of a tiny diffused flash lamp and a 3-4 fluorescent tubes, but at a greatly inflated cost, where as those tools separately could do many other things in combination and things the LED panel could not.Lighting is as much, if not more, of the art as any other aspect of photography. You pick (and often rent) what tool is needed to suit the end result.