LED arrays

[Mfranquelo]

Dear all,

Does anybody have any experience with high power LEDS ? in arrays if possible... im planning to shift from flash to LEDS for space purposes!

I was having a look at those ones : http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Arrays-NonDirectional/XLamp-CXA2530

[Tyler J]

I tried to use some with my face scanner, however they are way too bright.
You'll need to use a pwn controller to reduce their luminosity, because COB led behave weirdly when you reduce their current.
Plus you'll need a softbox.

Instead I use a regular white led strip, it's perfect.

[Mfranquelo]

Sorry i didnt specify my shooting conditions:

- Im shooting at f16 plus polarizing filter reducing almost 1.8 stops
- Lights should be at 60cm from subjects face

** Bear in mind  that im not using LEDs as constant illumination, but just with a pulse width of 20 ms or less... (like the flash one).

From what i've read flash output at full power equals to almost a crazy million lm's ... since i dont need full power at that distance.. (i need about 1/8) that is about 320k lms. If each LED has about 15k at 1500ma ... i'd need about 20 of them. Is this correct ?   

http://parts.arrow.com/item/detail/cree/cxa3590-0000-000r00cd0e3#yRQQ

Thank you for your response Eildosa 
 

[Magnus]

Hello Mfranquelo!

I have done some experimenting with LEDs but mostly for use with continuous lighting (brightness is a problem in lots of different ways, hehe). One can ofcourse use PS with smaller chip cameras.

Recently I have used these lights http://www.leds.de/en/LED-strips-modules-oxid-oxid-oxid-oxid-oxid/LED-matrix/Power-BacklightMatrix-290x203-70-Nichia-LEDs-white.html.
A benefit of those (also true for led strips as Eildosa mentioned) is that they usually do not require any additional cooling. Since you will pulse them it might not be an issue at all though.

Those LED panels (in a row so not optimal, have it setup like that now, sorry) gives me:
 
f2.84 with one
f4.01 with two
f4.07 with three
f5.6 with four

All 1/60th, ISO100 and 60 cm away (Sekonic lightmeter).

The user mala here might be someone to ask since he has experience in LEDs.

It will be interesting to follow this.

Eildosa, can you share some more info about your setup?

Best, Magnus.

[Mfranquelo]

Thank you a lot for that information Magnus! 
Cooling as you said is not an issue since the temperature generated in ~1ms pulse is close to nothing.

Thing is that your exposure time is 1/60, my "exposure time" should be 1/1000 at f.16 (i need it to be that fast because my cameras shoot in movement), i need a huge amount of light (flash light).
I would use a flash, but i cannot since all of them are too big for my needs.
As a record i tried to build my own mini-flash from flashlightbulbs webpage, but its too dangerous for someone that doesnt perfectly know it. (You can even die from a flash capacitor discharge on you)

It would be wonderful if mala could share its experience!

Regards,
Manuel.

[Tyler J]

I'm an electronic hobbist, one of my current project is a face scanner, however I just had 1500$ to spend so I build one compose a 21*Coolpix L27.
I cannot set parameters surch as ISO or aperture, etc but I'm getting EXCELLENT result.
I opened them and put some wire inside in order to be able to remote control them.

They are on a semi dodecagon at about 25cm of your face set to macro mode.
It is still a WIP, I'm having trouve with their alimentation, I made some kind of non-orthodox alimentation by using the power dissipated by the cable to lower the tension, however since some of them have slightly different electrical caracteristic they need specific length of cable.



For illumination I'm using led strip that are always powered on. their luminosity can be set using a PWM controller.
I made my own but if you buy some led strip on ebay, they, most of the time, give one free.

For your leds http://parts.arrow.com/item/detail/cree/cxa3590-0000-000r00cd0e3#yRQQ
you'll probably need something like this :
http://www.cap-xx.com/resources/docs/Using%20Supercapacitors%20to%20Solve%20LED%20Flash%20Power%20Issues%20for%20High%20Res%20CamPhones.pdf
(using 100V capacitor of course)

Also, know that you can buy for a much smaller price high power led from ebay? they'll come from china and will need 30 days to get to you.
But from my experience those ebay LED are just as good as the one sold on regular website.

[Mfranquelo]

Thats amazing Eildosa!  I'd love to see scanning outputs of your system if possible.
Lots of thanks for the capacitor white paper i?ll have a look into it with my colleague.
(I got electroshocked with a capacitor from a flash, so im quite respectful to them) 

You could alimentate your cameras with wired fake batteries to a power source. (just make some moulds of the original ones) or maybe they may even sell them.

- About the LEDs, i was thinking about the CREE LEDS because their luminous flux is about 18k at 1800mA, the ones i saw in ebay were about 5k, so i'd need a ton more if i use those ones. But maybe someone knows another cheaper brand that delivers that luminous flux ?

This is getting interesting 

[Tyler J]

I need their battery compartment to be empty (because I'm using it to get the wire to go outside).
the problem is that those camera are powered with 3V and consume up to 3.3A.
with this current and this low tension, each centimeter of cable matter (0.12Ohms induce a tension drop of 0.4V)

The solution I found was to use 0.75mm copper wire of 2.4 meter and a 3.3V tension source, cable of this lenght produce a drop of 0.3-0.35V

the ones i saw in ebay were about 5k, so i'd need a ton more if i use those ones

yeah but a 10K led cost 7$
With 2 of them you get 20k for 14$, your led is 14k for 50$

[Mfranquelo]

I understand,

- About those LED's, the problem is that it doesnt specify the CRI, for LEDS its usually +80 but it may be less... color rendering a thus texture in agisoft might be very color innacurate. (It is not easily fix-able in post prod as well)

http://en.wikipedia.org/wiki/Color_rendering_index for those who dont know what it is.
- On the other - more expensive - LEDs model CRI is certified to be "Available in 70-, 80-, 90- and
93-minimum CRI options"

However as you say, the price for the cree LED's is too high to buy more than 20. Which is what i need. 

[Tyler J]

I did not know that, up until now I always though that white light was white light.
Well you could buy a prism, buy 1 7$ led and look at the spectrum.

I ordered a 5" prism, I'm going to see if my spectrum is continuous or not.

[mala]

Hi there,

I'm not sure how much I can help here...
I will agree that in order to compete with a standard flash unit (but it is notoriously difficult to actually get an accurate lumen reading for flashes due to their speed) that you would require COB LED.
In my experience it is not difficult to PWM or current dim a COB array but special attention has to be paid to how the individual diodes in the array are wired, often they can be more than one parallel string of series diodes.
Therefore there will be a minimum current usable across the array that may not be exactly logical ..
In terms of CRI, if you buy quality LEDs and choose something in the region of 5000-5600k I don't think you will have a problem.
Nearly all ( maybe 95%) of high CRI LEDs are so called "warm white" 3000-3700K  these are so numerous because they are used as a replacement for filament/tungsten lights....these actually cause more problems for accurate texture colours in photographs of skin than using lights of a higher colour temp.
Xenon flash by the way IS a much higher color temp ..... Basically use 5000-5600k or maybe even high as D65.

I do think that if you actually manage to get the lumen output you talk of,which is very rare to get stated output ....then this will be way too bright for your subject even at the very short times you talk of.
For example I have recently done a test shoot with 320 LEDs around a subject,each Led is 700 lumen, we use polariser film on lights (transmission 40%)
Our cameras have another polariser film(transmission 40%) camera exposure time is 0.5ms.
The resulting images are sufficiently lit.
Cheers,
Mala

[Mfranquelo]

My lumen output was just a very rough approximation, i'll implement the LEDS from less to more until i have the exposure i need.

If you have 320 x 700 that is around ~220k for 0.5ms. For my exposure time which is 1ms it would be  440k lumens, which is just 1 stop more, not much for the camera.

Im using polarizing filters too, not on the lights though. I have around 1/2 stop less light because of them. Since my subject is very near to the light source, you may be right that 440k lumens is too bright, i'll need to test it since this is very complicated to calculate on theory basis. 

What i need to find is a high power LED strip, with decent CRI for a low price, this seems to be impossible  , because wiring each LED could be a nightmare.
Have you wired those 320 LEDs ? could you suggest any good company for high power LED strips ?

Thanks for your response mala!
Cheers,
Manuel.

[mala]

I don't think you will find any high output high CRI LED strips for a low price i'm afraid.... these things are made as cheap decoration light and you may have problems trying to turn them on/off at the speeds you talk about.

If you were to use the orginal Cree COB LED you mention...  again controlling these at high speeds may in fact be quite expensive given the voltages required.

Personally I don't really understand what you are doing with the "moving arm design".... it does look cool but I think you will end up spending more money in the longterm trying to compensate for the movement of the cameras and additional light needed to shoot at the high speeds.... when you could just buy more cameras and have a fixed rig.

One interesting and cheap LED light source are LED panel lights like this kind of thing
http://www.ledhut.co.uk/45-watt-high-powered-panel-light-1800-lumens-600-x-600mm.html
they come in various sizes are bright and produce a very diffuse light..... but even the small 300x300mm ones will be too big to put on your rig of course.

mala

[Mfranquelo]

Yeah i agree that i wont be able to find it for a cheap price too. However turning them of/off is not a problem at all.

The "moving arm design" is definitely cheaper in my opinion, buying the equivalent in fixed cameras is : about 25k euro... plus cables filters... etc. I havent spent more than 10k right now in development, it will be even cheaper in later stages. Also for the future, it is cheaper to buy 8 cameras than to sell 48+ cameras and buy another 48. But this is another topic!

I will probably use Cree COB LED's or another cheaper solution from ebay... taking account that each LED takes 75v and 1800mA, finding a power supply that gives +40a and 75v is the hard thing.

- Using more LED's to achieve the same lumens is better in one way: the light is more diffuse since the area of emitting light is more evenly distributed, the downside is that the CRI of the cheaper and less bright LEDs could degrade the texture.

[Mfranquelo]

Mala,

May i ask which power supply did you used to power up your LED's at 0.5ms? I've looked a bunch of power supplies from meanwell, but im afraid that they wont be able to "charge up" in much less than 1ms.
(Power supply needs time to "charge up"). Have you used capacitors for this job ?

This one : http://www.mouser.com/ds/2/260/RSP-3000-spec-54071.pdf
It looks that it needs around 80ms to rise up.  Thats way too much.

Thanks for your input! 
Cheers,
Manuel.