LED studio lights, Take Two...

[iceweasel]

LED studio project

Goal
Build a controlled LED lighting system that would allow me to illuminate a small, white enclosure in a range of colors and brightnesses.

Description
The enclosure will measure about 24” by 24” and stand about 24” tall. Think of a cube with two contiguous sides missing. In photography this enclosure is sometimes referred to as “cove box”. They’re frequently used to photograph small items.

Colors of the LEDs should be red, green, blue and white.

Narrative
The details of the rest don’t matter so much as the emitters themselves will be hidden under a small “stage”. The whole idea is the reflected light against the white backdrop. My thought was 3 or 4 LEDs of each color should be sufficient. Ideally it would run on AC and have four dimmer controls, one for each set of LEDs. Each color group should be separately controlled so that all may be run at full output of any portion thereof.

So my question to you, the much more knowledgeable photo wrangler is, is there any place where someone could find a schematic to build a rig such as this? Is it something that is impractical based on the concept alone? Any specific suggestions for types of LEDs or other components? Any good sources for any of the components?

 

[paulr]

I don't know if it makes sense. White leds have sort of a peculiar spectrum whose effect on photographs may be a little hard to predict. Leds also aren't all that bright, so you'd need a lot of them to get anywhere near the lighting levels that you'd get with regular studio lights and filters.

 

[idleprocess]

He's only trying to illuminate a volume of ~8 cubic feet. With good RGB balance, it could be done with some experimentation.

White LEDs would likely mess up the color balance, with their excess of blue and defeceit of red emission - although you might experiment with warm white LEDs.

In terms of lumens per watt, red LEDs are the brightest, green are median, and blue are dimmest. You might be able to get by with a R:G:B ratio of 1:1:2 with some sort of cap on the red to keep it from overpowering the other two colors.

EDIT - ratios backwards

 

[iceweasel]

Sorry, I obviously wasn't very clear above. The LEDs are not meant to light the subject here at all. They merely cast a colored light on a white backdrop. What I'm looking for is a way to "mix" the colors giving a numerous different colored backdrops using only LEDs. The white ones are there so that if I need plain white (well, bluish white), I can get it.

Thanks.

 

[Doug Owen]

This sounds interesting.

While I don't know of a specific schematic to suit your needs, it's easy enough to make one up. I'd suggest four constant current channels, each made of a sense resistor, FET, potientiometer to set the level and half a LM358 to control it all. The four channels can share a common reference.

Doug Owen

 

[iceweasel]

YES!

Doug,

While I lack the vocabulary from what I do know I think you're correct. If constant current means that I'll be able to adjust any of the color "channels" at any time, up or down, independently of the other, then yes. If by common reference you mean one power supply (some kind of wall wart I suppose), yes, that would be ideal. How the rest would be connected I don't know either but it seems as though, once I had the proper materials, it wouldn't be overly onerous to wire up (given my extremely rudimentary soldering skills).

the whole thing, in fact, could be wired point to point (if that's a proper term use) because the assembly will be hidden from direct view by the "stage".

The only other concern would be heat sinking. Would bare LEDs need to be heat sinked in some manner?

Thanks Doug.

 

[gadget_lover]

Re: YES!

You asked about heat sinking;

Yes, the LEDs have to be heat sinked if run at more than 100ma. The heat sinking can be quite simple (a lid from a tin can has a lot of surface area) or rather complex (the little fins on a CPU heat sink are to increase surface area but the fan pushes more air over the those fins).

Watch out for polarity when mounting two or more LEDs to the same metal heatsink or other conductive mount. Red and Blue (IIRC) use oposite polarity for the slug that is glued to the heat sink.

Keep in mind that you will be concetrating the heat from several sources if you put the power supply in the same area as the LEDs. Simple vents or a small fan may be necessary to bring in cool air.

It sounds like a great idea.



Daniel

 

[Doug Owen]

Re: YES!

Sorry to mislead you (it seems), I do know how to wire such a gizzie up. No problem, really.

Heatsinking depends on how much light we're talking....how much are we talking? Perhaps this is not a fair question to ask, but do you have a feel for how much light you'll need?

Doug Owen

 

[iceweasel]

background lights

Doug,

Sorry I really don't have the knowledge to communicate in this any real terms. I really can't tell you how much light I'll need. It's my thought that, given the white backdrop is designed specifically to reflect light, evenly, the needs will be relatively minimal. I'm guessing that four average LEDs would be enough to illuminate this area. But I do need to take into consideration that there will be a fair amount of light (from other sources) directed at the stage, which would tend to wash out the LED colors. Maybe I need to revise up my thinking here. Perhaps it would take more, let's say 6 or 10 LEDs of each color to work. So we're talking about an array of, at the most, 40 LEDs. Does that only complicate things in choosing potentiometers?

Then, suddenly, it seems my budget for materials has doubled.

 

[Doug Owen]

Re: background lights

No, the potentionmeters don't change, but the total current requirements for the drivers and supply go up as we need to drive more. Somewhere along the line we need to decide how to dirve them (all in parallel, series/parallel, etc.).

Doug Owen

 

[markus_i]

Re: background lights

A few more questions/inputs:
- what type of LED are you thinking about? There are the ordinary 5 mm LEDs, which also come prepackaged in an RGB version (i.e. R, G and B LED in the same 5 mm case). You would need more of those, but then you can string them out along one (or several) edges of your box to achieve a more distributed light source (also less trouble with heat in that way).
- on the other hand, you can use high power LEDs like the luxeons, for which also specific RGB reflectors exist (see here how that might look like). This will give you a wide angle colour-variable spotlight, and I don't know if that would be suitable for your setup.

Bye
Markus