LED, the convenient yet dangerous light source

Like any tool, you have to choose the right ones or you’re going to end up in trouble.

I love LEDs. Well, mostly. I have my tricks for dealing with them, and I’m going to pass some of them on to you. Read on…

This year we bought a new artificial Christmas tree, and my mandate was to find one “without those awful LED lights.” The tungsten-and-colored-glass bulbs that I remember from my childhood are long gone, and while I don’t completely hate the new colored LEDs, the colors are so pure that they make my eyes bleed just a little over time. They also strobe, which I dislike a fair bit.

When confronted with a roomful of candidate trees, I pulled out one of my favorite location scouting tricks. I rolled my eyes insanely, doing my best to generate light streaks on my retina from all the different trees. As low quality LEDs tend to flicker a lot, it was easy to separate the tungsten bulbs from the rest because they produced unbroken streaks of light. LEDs produce streaks with gaps in them, so they’re easy to pick out (and ignore when you’re buying a Christmas tree).

I’ve done this fairly regularly on location scouts over the years. By moving my eyes in circles or back and forth very quickly, I can determine which lights in a location might be problematic by looking for broken streaks. Sometimes I can assess how problematic they might be flicker-wise due to the size of the gaps. While this isn’t a foolproof method of determining anything more than where problems might lie, it has proven helpful. It also alerts me when a common type of location light source has been switched over to newer LED lights whose color might be less than stellar.

I have several LED tricks up my sleeve. Here are three of the better ones.

METAMERIC FAILURE

There are two common types of LED lights: warm/cool and RGB. Warm/cool lights are driven by phosphor LEDs, and their respective spectrums are fairly broad. The broader the spectrum, the better the resulting color, so while they are deficient in a couple of areas (saturated red and cyan) they generally look pretty good.

The Kino Flo Select Series are a good example of this style of light. I’ve been happy with them so far, although I’ve found that their color is a little bit off when it comes to matching other kinds of common lighting instruments (like Kino Flo tubes). Still, the quality of light they put out is very pretty.

RGB lights are another story. Saturated hues are the result of a dye LED emitting light along a very narrow spectral band. While they are pretty to look at, they are not particularly good at making other things look pretty. You can only reveal an object’s true color by illuminating it with light that contains its entire spectrum, and RGB lights don’t do that: they emit light along three very narrow wavelength ranges with huge gaps in between, so they can generate what appears to be white light but can’t reproduce accurate color in important things like… oh, I don’t know… FLESH TONE.

If you’ve seen the series House of Cards (the modern U.S. version) you’ll have seen this in action. Most of the car shots are executed on a green screen stage, and the background plate is projected across large video wall panels suspended overhead to create reflections and highlights in the car’s surface that match the moving background plate. This works great for night work, where there isn’t much white light in the environment, but the day/exterior shots tend to be another story. The light isn’t so bad when the car is passing underneath “trees,” but if it breaks out into broad “sunlight” the actors tend to look a little bit dead. There’s not enough spectrum present in those three red, green and blue spectral spikes to elicit a proper flesh tone response, so while the light looks neutral it makes skin look flat and pasty.

There’s one particular light that has caught on in the industry like a house on fire, and yet the quality of skin tone that it reproduces is not great. It’s an RGBW fixture (red, green and blue dye LEDs combined with a cool white phosphor LED) that is incredibly punchy and versatile, but because it only has a cool blue LED to round out the light’s spectrum it doesn’t make people look good at all. They look… okay, and I guess they look good enough for these lights to be used on a lot of very expensive features and TV shows, but there’s no warmth or life to the skin. People look like dull, moving alabaster.

If anything, their flesh tends to look a little blotchy because the narrow dye LEDs react to very narrow ranges of color in flesh tone, and those ranges have abrupt boundaries. For example, most skin blotches tend to be reddish, which means that the narrow band red LED will punch those red areas up while the narrow blue LED won’t react at all. The result emphasizes skin imperfections.

There are two ways that I can spot spectrum-compromised lights in the wild. The first is that skin tone looks roughly the proper color but doesn’t look warm or healthy, and often looks blotchy. The second is that the quality of light just looks wrong: I don’t know how to describe it accurately, but the word I use most often is “metallic.” It feels like the specular highlight off a shiny chrome object, but harsher. This makes them easy to spot at trade shows, as the white light emanating from many of these compromised fixtures feels “thin,” “metallic” or just wrong.

I shot in a studio the other day that was ringed with these lights. The lights were bright but somehow the color felt thin. Skin looked bright but washed out, and specular highlights felt like old school video highlights that rolled off or clipped too soon.

Some RGB lights are great effects lights. I’ve used a variety of RGB lights to create all sorts of saturated lighting effects, and they work really well. Best of all, they turn on and off instantly, which makes them great for light chases or emulating video screens. (It’s now possible to drive some of these units using video signals.) Not all of them do white light well, though, so it’s important to use the right tool for the right job.

COLOR GAMUT ERRORS

Once, while setting up a lighting effect for a scene with highly saturated colors, I noticed that one of them—blue—was clipping garishly on faces. I looked at the hue on my on-camera monitor’s vectorscope and saw that it was way outside of the Rec 709 gamut. “Do you have a way of desaturating that blue?” I asked my gaffer.

“Sure,” he said. “There’s a desaturation control on the lamp that adds white light.” He turned it up a bit and suddenly the blue wasn’t clipped anymore.

Clipping a saturated color on flesh tone is a pretty awful look. It emphasizes blemishes but also creates flat pools of color that look like areas of luminous grease on cheeks and foreheads. If you want an idea of what to look for, try lighting a face with super blue Kino Flo tubes and see how a typical camera responds.

This is a pretty common issue, and I’ve seen it on network TV shows. It’s possible to create highly saturated hues that video cameras, especially Rec 709 video cameras, can’t handle. Blue is a particularly vicious color because it has the lowest luma value of any hue. Many codecs, and all broadcasts, utilize Y’CbCr encoding, where luma (Y’) is stored separately to color but is also derived from colors in the image, similar to the way the human eye senses brightness. Creating the luma signal means taking of 21% of the red signal, 71% of the green signal, and 7% of the blue signal and creating a new channel that does nothing but describe how bright things are.

I tried an experiment a while back that shows the pitfalls of working with very saturated hues in this color scheme. I created a pure blue image (0,0,255 in eight-bit RGB) and brought it into DaVinci Resolve. In the RGB waveform I was able to clip the blue fairly easily by pushing the gain way up, but I couldn’t push it past 7% on the luma waveform. In Y’CbCr encoding, a pure, saturated blue can never be brighter than than 7% luma. It still clips, though, and quite easily.

The RGB waveform decodes the Y’CbCr signal into RGB and shows what each color channel is doing, but it also scales them to fit the full range of the waveform. Luma doesn’t do that: it’s only looking at the Y’ component. The RGB waveform will show you this clipping, as will a vectorscope. The luma waveform, and false color exposure assists (which look at luma only) won’t.

What this means is that you can light a set, or a person, with a very low level saturated blue and find that it is clipping everywhere.

There’s a bit more leeway with red, and a lot more leeway with green, but this is something to watch out for when working with very saturated hues. It’s worth it to check a vectorscope for out of gamut colors at 1x gain, or look at an RGB wavefrom for clipped color channels.

There are several solutions to this chroma clipping problem:

Reduce the saturation by mixing in white light. Some fixtures are set up to do this automatically. Arri SkyPanels have a desaturation control that works in this very way. I’ll watch my vectorscope during this process and make sure the blue falls within the Rec 709 boundary. (Make sure gain is set to 1x, or make sure there’s no added gain, as we often view charts on vectorscopes at 2x gain and that would shortchange your saturation levels.)

Mix in a different color of light. A really pure, saturated blue hovers on the very edge of violet. It’s a very dark hue. Many blues in the real world tend to be a bit brighter, and that’s because they have a little green mixed in. Many people have a hard time distinguishing between a pure blue and a greenish blue based on hue alone, so adding some green might pull that saturated blue back into range and make it more striking at the same time. Red is similar: adding green or blue to an out-of-gamut red will shift the hue a little bit but might bring it back into range. A lot of real world reds are on the yellow or blue side while still reading as red.

The best solution is to reduce saturation by adding white light if possible, as that will preserve the hue without changing it. If your LED light doesn’t offer this as an option, you can push it through some diffusion along with a small amount of light from a broad spectrum unit like a tungsten fresnel.

YOU THINK ITS YELLOW, BUT IT’S NOT

I walk into a lot of locations lit by LED lights, and I’ve gotten to the point where I can tell instantly if I’m in trouble. If the light looks to be a pleasing warm golden hue then I start to relax a little bit, but if it feels yellow then various muscles in my body clench. The feeling of yellow light from LEDs almost always means they will photograph green.

One of my old video engineers taught me this decades ago. He was painting a camera with a paintbox, and I mentioned that the flesh tone in the shot appeared too warm. “That’s not warmth,” he said, “that’s green.” He pulled out some green and the flesh tone snapped into place. Apparently too much green, in combination with red flesh tone, makes people look yellow, which implies too much of both red and green. Balancing those out by adding blue, though, just makes things worse. Pulling out a little green does the trick nearly every time.

I’ve noticed the same thing with LED lights. If they look yellow by eye, they’re going to be either bright straw (a greenish yellow) or luminous green on camera.

The under cabinet lighting in this shot, from a commercial for Oriental Trading Company, looked very yellow by eye and photographed very green. Adding some white fill to the background mitigated this effect, so they look more yellow than green. Still, I can feel the green struggling to get out. There wasn’t much I could do, though, as the lights brought the background to life and I didn’t have time to replace them. I’m quite pleased with this result. (Shot on a Sony FS7 in Cine-EI mode, 3200K white balance. The actors are lit by colored Kino Flo tubes, and the wall to the right is lit by flashing colored LEDs.)

There’s not a lot one can do in this situation. Sometimes choosing a different white balance helps. Some LEDs and fluorescents look less offensive at 4300K or 5500K than 3200K. Under controlled circumstances it might be possible to match your cine lights to the background lights using a technique I outline here. I’ve almost never had any luck gelling built-in LED fixtures because minus green gel isn’t meant to work with something as narrow in bandwidth as the green spike in an LED: the gel tends to have a gentle roll off whereas the LED’s green spike is quite narrow, so eliminating the green completely often makes the light look magenta because it’s now cutting too much of the rest of the light’s spectra. If I resort to gelling an LED light I find that slight under correction is better, as slightly yellowish light looks better than slightly magenta light. (Magenta is not a common color of light in the real world.)

I own a color meter, but I find that it is descriptive rather than prescriptive. It tells me that my eye isn’t lying when I see an LED light is green, but it never tells me how to correct it. That’s because color meters are meant to work with light that has a broad spectrum, and they are confused by sharp color spikes. I’ll bring my color meter on a scout to determine if a light is going to be a problem, but any gelling happens once the camera arrives. This isn’t an ideal way to do things as it takes time, but if you have the gels ready to go and a reference light on standby it can happen very quickly.


I find myself using LED lights quite a lot, but there’s been a bit of a learning curve as I figure out which lights can be used in the foreground versus the one that can only be used in the background. I’ve gotten to the point where I can mostly do this by eye, but the camera is the ultimate arbiter. Many LED movie lights are very good, but there are still some marginal ones out there and price doesn’t always determine quality. Still, you have a better chance of good color using an expensive LED light than a cheap one.

Art Adams
Director of Photography


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Art Adams - Director of Photography

Cinematographer Art Adams shoots spots, visual effects, web/interactive/mobile and high-end marketing projects. His website is at http://www.artadamsdp.com.

Art has been published in HD Video Pro, American Cinematographer, Australian Cinematographer and Camera Operator Magazine He is a current member of the International Cinematographers Guild, a past member of the Society of Camera Operators (SOC), and an industry consultant and educator. He lives in the San Francisco Bay Area.

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7 Comments on "LED, the convenient yet dangerous light source"

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Dithermaster
That’s funny, I used the same scanning technique to make sure we got an LED tree this year because I hate it when the incandescents burn out! I’m not a fan of the flickering, but I’m less of a fan of burned out strings. You should check out the LED fixtures from ETC; they use 7 different colors of LEDs so they don’t have the problems that RGB or RGBW fixtures do. These folks understand colorimetry and have a number of video cameras and other measuring equipment so they can understand how the LED sources are picked up by various… Read more »
Art Adams
Testing lights on camera is a big thing! I’m impressed that they do that. Long ago I worked on the Kelvin Tile, the first broad spectrum motion picture LED light, and we had five dye LEDs and a phosphor LED. It looked great on prism cameras of the time (this was back when the Sony F900 was a thing), but when single sensor cameras hit we found it looked awful, as it made people look pasty and pale, whereas on prism cameras it made them glow with warmth. I haven’t used the ETC LEDs yet. Thanks for putting them on… Read more »
James
Great article, I have encountered many of those quandaries with LEDs as well. Convenience vs true color representation.. One those tends to win out on my sets 🙂 I see companies like Falcon Eyes promising 750w LED fresnels (allegedly 6k tungsten equivalent) and can span 3000-8000k, as well as dimmable without color shift. They are rated at CRI 92 or 95, but I have to wonder as you state what that really means for color representation. But, as a caveat, they have on light fans, which of course is a can of worms in and of itself. I’ve been highly… Read more »
Michael Taylor
Excellent post… although I have to admit the complexity of this Brave New Digital World makes me glad to be retired after a long career in set lighting. When I came into the biz, the carbon arc was the state-of-the-art BFL for exterior work, 4K HMIs were still a tantalizing rumor, and tungsten ruled the filmmaking world for night work, interiors, and on sound stages. By the time I left early last year, LEDs were coming on very strong, but it will take a few years to fully sort them out. Have you used any of the new Mole Richardson… Read more »
Art Adams
I haven’t used the Mole LED 10Ks. I think I’ve heard good things about them, but not sure if they are in our area yet. I’ve learned that I need to look at a light source myself rather than trusting reviews, though. I think I’m more critical about this stuff, although maybe more critical than necessary sometimes. But… I have this thing about hating surprises and loving predictable results. 🙂 It is definitely nice to be able to do more with less power. The Arri M series HMIs are a dream, especially the M18 which has become my newest workhorse.… Read more »
Michael Taylor
There’s no substitute for actually getting your hands on the gear — I wouldn’t trust a review either. I had a chance to use a few of the newer Arri HMIs on one of my last shows in LA, and they were great. I suppose it’s the nature of modern life that just as the state of the art in any given technology reaches its apogee, a the Next Big Thing kicks down the doors — imperfect and troublesome, but with advantages that cannot be denied. Nearly fifty years ago, I toured the Smithsonian and saw a Rolls Royce piston-powered… Read more »