LED array

[Tim Hole]

I am seriously considering (what I have been meaning to do for a while) building an LED array (lightpanel).

For the shoot in Poland in August I want to punch the available light a bit. As it will mostly be shot in available light.

I'm thinking about 4 x 100 LED arrays. That could be used separately or together. If I use 10mm Super bright white LEDs I have a couple options in power. 12x AA battery 14.4v (2500 mAh) - in 100xLED array draws current of 500 mA (Fw Voltage 3.2 [current 20 mA]). LEDs in 4 series with 82 Ohms resistors. This would last for good few hours. In 400 LED array draws 2000 mA - batteries wouldn't last very long may an hour or so.

Alternative to the AA batteries is to use camera batteries. Small cam batteries are around 7.4v so two batteries (tend to be same as AA and hold around 2500 mAh). Problem with this that I would probably have to buy a battery charger and destroy it to get the mounting plate.

The last alternative is to use a V-Lock battery. One battery is 14.8v so everything stays the same except I would have to borrow a battery and buy V-Lock plate for £30-50. This would probably power all four arrays for most of the day.

My biggest worry as always is the light the LED are throwing out. Not the amount of light but the balance of colour. I don't want my blues or reds to look black. I am going to experiment by buildin one 100x LED array for £30 and then test it. £30 is nothing at the end of the day is it. Getting the right LEDs is the problem. There is plenty of them out there...

EDIT: I noticed that you can buy warm white LED's that are rated 2500 - 3500K. They might work for indoor and then gel them for exterior.

Thoughts anyone?

[Peter Mosiman]

Please let me know how this works out Tim. I am very curious.

Good luck!

[Andrew]

12x AA battery 14.4v (2500 mAh) - in 100xLED array draws current of 500 mA (Fw Voltage 3.2 [current 20 mA]). LEDs in 4 series with 82 Ohms resistors. This would last for good few hours. In 400 LED array draws 2000 mA - batteries wouldn't last very long may an hour or so.

You've assumed your AA batteries are 1.2 V, have you? They're rated at 1.5 usually which would give you 12 * 1.5 = 18 V, quite a difference. It's probably a safe assumption that you'll get 1.2 out of them but you will have to safeguard in case you actually get 1.5.

LEDs in 4 series with 82 Ohms resistors.

So if you had a 14.4 V supply you'd have a 1.6 V drop (14.4 - (3.2*4)) over your current limiting resistor. Then you'd lose 32 mW in each one * 25 banks of four is 800 mW, quite a lot. You might find that it's more efficient to have fewer LEDs in your banks but you'd have to do the calculations.

If the batteries were fresh and giving you 1.5 V then your supply would be 18V, the drop over the resistor would be 5.2 V, the current through your LEDs would be 63 mA, enough to at least shorten their lives considerably, maybe even blow them. You'd also be burning 326 mW * 25 = 8.2 W! Check the datasheet to see what the maximum current of the resistors are. You might want to use a 14 V low drop out voltage regulator to make sure that you don't overload it.

But it sounds great, those Litepanels are way too expensive! Their trick is to get the LEDs to proper colour temperatures. Do you have any idea what the colour temperature will be like? LEDs tend to cover a very narrow spectrum and the colour temperature may vary with voltage, another good reason to use a voltage regulator. I'm very interested to see how you go with this!

-Andrew

[Peter Majtan]

Tim, I will send You e-mail later on. I might have a product ready for You to test in August. I have single LED capable of delivering 400W light output at daylight color rating. I can built any light equivalent up to 10K ARRI-SUN...
PM me Your e-mail.

:D Peter

[Peter Mosiman]

Tim, I will send You e-mail later on. I might have a product ready for You to test in August. I have single LED capable of delivering 400W light output at daylight color rating. I can built any light equivalent up to 10K ARRI-SUN...
PM me Your e-mail.

:D Peter

How does one become a tester of this kind of product?

[Peter Majtan]

How does one become a tester of this kind of product?

By having good relationship with the manufacturer... :devil:

:D Peter

[johnvid]

I been waiting for the Led lighting to become affordable, would be nice even if it could provide a nice subtle fill, or reporter type top light at night, but my mate got a PAG one whch turned out to be crap so I have held off....

But that one Peter has sounds interesting, let me know more info too...

Also a dimmer function would be good, does it keep colour temperature?

[Tim Hole]

If I take the plunge which might be the only way. Will keep y'all informed.

Andrew. Yeah I am literally going by the ratings on a selection of 2500 mAh batteries that I have been looking at. The majority are 1.2v but having gone back and looked now I see there are quite a lot of 1.5 out there also.

I will make my discisions about the array when I find the diodes I will end up using. The difference in forward current varies dramatically, which obviously then affects the mA that is being drawn. I am inclined to agree with you on the amount of diodes in the banks. I'll work out the most efficient array when I find my diodes.

As for colour temperature...who knows!! ;) Its the kind of thing that you only know once its built (to be sure anyway). The warm white for example are just white LED's with a film over them. I'm not sure what kind of information is available for most. the best way is to buy some and experiment.

Peter's ominous (as always) pledge to the LED world sounds very interesting. Hmmmm!!!!

Its not just having a good relationship with the manufacturer...it involves some animal sacrifice as well. So far its only been a rabbit and two voles - Can't afford a goat.:devil:

[Andrew]

Also a dimmer function would be good, does it keep colour temperature?

There are two ways of dimming LEDs, reducing the current driving them and Pulse Width Modulation (http://en.wikipedia.org/wiki/Pulse-width_modulation). The latter is the easiest to implement and the most predicable and it involves turning an LED on and off to vary the brightness. The master frequency is always the same but the time during the period when the LED is switched off varies. You can see examples of PWM waveforms at different duty cycles below. This works really well for dimming since you get precise control over the brightness and you'd keep the colour temperature the same. As long as you keep the frequency fairly high (> 6 MHz ?) I think that you would avoid the potential CMOS flickering problems you might get at lower frequencies.

http://www.eleinmec.com/figures/028_01.gif

[KurtF]

This isn't exactly a huge light panel, but I found it interesting. Also, some of the calculations and sources might be useful:

L.E.D. Lamp Build (http://www.fieldlines.com/story/2007/1/28/215243/977)

[Andrew]

This isn't exactly a huge light panel, but I found it interesting. Also, some of the calculations and sources might be useful:

L.E.D. Lamp Build (http://www.fieldlines.com/story/2007/1/28/215243/977)

Hey, wow, that LED wizard (http://led.linear1.org/led.wiz) thing looks handy!

[Dan Hudgins]

Because you want to match the color of the location it might be good to include maybe 50% white LED, and 25% RED, 15% Green, 10% BLUE, and have four pots to adjust emitter followers for their power (rather than PWM which might cause noise in the image on a rolling shutter). 2N3055 are low cost and can do maybe 5-10amps each with a heat sink.

That way you can dial in the right color and brightness with the 4 knobs.

[fde101]

I wonder if Ryobi's new TEK 4 system batteries would fit in well with a project like this...

[Dan Hudgins]

I was thinking it might be good to have some Orange LEDs along with the RED and GREEN LEDs to get better flesh colors.

If you add Yellow that will desaturate the colors like Sodium light, but a little wide band to the RED end might help, so maybe Orange LED could make the results look better from the camera's sensor's point of view, maybe 60% RED and 40% ORANGE for the red supplement LEDs.

Cyan LEDs can also de-saturate the colors, so if you add extra POTs to control the Orange, Yellow, and Cyan LEDs you can control the color balance as well as the saturation to some extent. I don't know if violet LEDs would add anything useful. You could have IR LEDs for night shooting, but maybe better on its own panel.

Also using LEDs with different peak wavelengths can help get more natural colors, and reduce the "speckle" that the laser like single color LEDs can make with some surfaces and diffusers.

[Stefan Christou]

I've been working on a similar thing myself. I'm aiming to eventually make a ring-light to go around the camera lens but I'm starting with just a basic camera light/hair light.

This instructable has some good circuit designs for powering LEDs: http://www.instructables.com/id/Circuits-for-using-High-Power-LED_s/

The long and short of it is that you need a constant current circuit and just using a resistor is a waste of power. LEDs are very sensitive to changes in throughput, which can change depending on their operating temperature.

You should note that AA batteries are almost never 1.5 volts and vary depending on the chemicals used to make them. For example Nickel-metal-hydride AA cells are in-fact 1.2 volts each.

The other thing to do is test your LEDs before you buy them in bulk. Although LEDs hold their colour far better than incandescent lights when they are dimmed, a bunch of high power bulbs I bought had a green tinge to the outside of their beam. :violin:

And also take into account you will probably need to soften them with some kind of diffuser. Banked LEDs can create multiple shadows and LEDs can also be mega offensive to the sensibilities of your "talent". Some people just HATE them being shined at them. (:furious3:)

[Tim Hole]

Some very nice input!

Very interesting points Dan. I wish I had more time than I have right now to work on this. Way too much effects work to do. But in the current climate we can't afford to complain now can we. With the time issue I doubt I can get to build for the shoot in August. Gonna have to improvise elsewhere. I will have to put it on the back burner until September. Buying two nice tactical torches for the night shoot in the woods. The ones I played with kicked out some serious light. One of them is apparently rated at 900 lumens but that is an exaggeration slightly.

Yeah I saw the instructables light. Its a good jumping off point. The thing is once the tech is worked out they aren't complicated. Its all about experimentation.

[Stefan Christou]

:thumbsup:

I'll keep you posted how I'm getting along too.

[Tim Hole]

:thumbsup:

I'll keep you posted how I'm getting along too.

Yeah man please do!