Light Emitting Diodes

smirkbyfire · July 13, 2004

I think I might have posted this question here before, but it's still on my mind. I heard a radio story on npr about people in developing countries using LED's (big versions of the lights in watches, calculators, clocks, etc.) to light their homes. They use signifigantly less electricity. My thought was is there a way LED's could be used for cinematography? I am envisioning a large board LED with four or five seprate color temperatures (made up of tiny LED's next to each other in a grid pattern, white, then red, then blue, then green over and over again). This way, you could simply choose the Kelvin degree you wanted, and the corresponding pattern of lights would activate. It could even be controlled by some hand set device that also acts as a dimmer. If anyone has any thoughts about this, or has used something like this, speak up. I don't really know if LED's would be bright enough to match current lighting techniques, but they might be useful for some special effect, or simulating a wierd future world atmosphere. Well. Thanks for any replies.

Nate Downes · July 13, 2004

Actually I use LED's for some in-camera effects. The LED's are small enough that I built a light ring for chroma-keying effects that fit up to my camera for super-cheap.

For now they're not bright enough for a lot of cinema stuff, but for some limited-scope work, they're great.

Tony Brown · July 13, 2004

They get used quite a lot. There are a couple of quite punchy (for they're size and weight) ring lights and some flat panel screens which I'm using next week.

Anyone know how bright 300 nits is?

John Pytlak RIP · July 13, 2004

Kodak and labs uses LEDs for darkroom lighting and "path lighting":

http://www.kodak.com/US/en/motion/support/...on/page01.blind

http://www.fpchollywood.com/supplies-lab-s...b-lighting.html

LEDs (even the "white" ones) have fairly discontinous spectral characteristics, so you need to check color reproduction.

Anyone know how bright 300 nits is?

http://www.schorsch.com/kbase/glossary/luminance.html

Standard unit of luminance is candela per square meter (cd/m2).
(also called Nits in the USA, from latin "nitere" = "to shine").

There are several older units of luminance:

Apostilb (deprecated) 1 asb = 1/pi cd/m2

Blondel (deprecated) 1 blondel = 1/pi cd/m2

Candela per square foot 1 cd/ft2 = 10.764 cd/m2

Candela per square inch 1 cd/in2 = 1550 cd/m2

Footlambert (deprecated) 1 fL = 3.426 cd/m2

Lambert (deprecated) 1 L = 104/pi cd/m2

Nit 1 nit = 1 cd/m2

Skot (deprecated) 1 skot = 10-3/pi cd/m2

Stilb (deprecated) 1 sb = 10'000 cd/m2

Typical luminance values are:

1.6 * 109 cd/m2 Solar disk at noon (don't look!)

600'000 cd/m2 Solar disk at horizon

120'000 cd/m2 Frosted bulb 60 W

11'000 cd/m2 T8 cool white fluorescent

8'000 cd/m2 Average clear sky

2'500 cd/m2 Moon surface

2'000 cd/m2 Average cloudy sky

30 cd/m2 Green electroluminescent source

0.0004 cd/m2 Darkest sky

Phil Rhodes · July 13, 2004

Hi,

A nit is a candela per square meter. 300 wouldn't be much for a typical LCD image display, which is where the term is most often found.

Phil

Tony Brown · July 13, 2004

Hi,

A nit is a candela per square meter. 300 wouldn't be much for a typical LCD image display, which is where the term is most often found.

Phil

Makes sense, the unit I looked at was set at 10% output which they told me was 300 nits (I just nodded like I knew what they were on about B) )

Phil Rhodes · July 13, 2004

Hi,

The other thing about LEDs is that they don't use "significantly less electricity" in the kind of application you'd be thinking of for film.

They use a lot less in situations like indicator lighting, that's traffic signals or the "on" light for your computer, the red rear light for your bike (but probably not the white front one) and things like darkroom safelights. The reason they're good here is that they actually produce coloured light; you're not producing white light then chucking two thirds of it away in a coloured filter. This makes the lamps used in traffic signals and car signals seem very inefficient when you take the light out of the complete device against the power input. Another reason that LEDs are often thought to be efficient is the excellent luminous efficiency of LED video displays when compared to LCDs; remember here that an LCD throws away half the energy in the polariser and another two-thirds in the colour filters, and the power consumption of an LCD does not fall (more than minutely) as you change the picture content.

In fact, most LEDs (40 lumens per watt, or so) are much less efficient than a moderately good fluorescent tube (eighty plus) in terms of absolute radiance versus power consumption. The white ones (30) are about as good as a high-efficiency tungsten halogen lamp like an MR-16, but have a far poorer spectrum and CRI. It's also worth mentioning that most LEDs can only achieve their maximum possible efficiency at very low current. To keep the size, weight and cost of the device to a reasonable level, they're normally driven for best output, not best efficiency. White LEDs are far from the best and once you start to filter them for better CT, you're removing the blue spike which comprises most of their output.

LEDs are not a low power option; look instead to battery-powered fluorescent or cold-cathode solutions.

Phil

Alvin Pingol · July 14, 2004

White LEDs are far from the best and once you start to filter them for better CT, you're removing the blue spike which comprises most of their output.

And aren't "white" LED's nothing more than blue LED's with phosphorus coating over the diode?

That in itself probably lowers light output/efficiency significantly.

Phil Rhodes · July 15, 2004

Hi,

> And aren't "white" LED's nothing more than blue LED's with phosphorus coating

> over the diode?

Yeah.

> That in itself probably lowers light output/efficiency significantly.

Yeah.

As UV LEDs rise in efficiency and decrease in price, we can expect to see better white ones, or at least white ones with less visible blue overspill. But it'll be some time before they can compete with even decent halogen.

Phil