LED based lights are a brilliant opportunity for improving many aspects of film making.

They are solid state and can be designed to be very robust, last for well over 20,000 hours, use as little as a tenth of the power of the lights they replace, can be tuned to deliver accurate sunlight balanced light, and do not generate the heat typical of other lights.

The Problems:

As with any new technology, there is a learning curve to properly spec and design LED lights for cinema applications. The major pitfalls include;

1 - high speed cameras can see flickering from the PWM (pulse width modulation) control
2 - beam patterns with artifacts that detract from the scene being lit
3 - poor color rendering, causing skin tones to look greenish

1- PWM - that flickering you sometimes see with LEDs

Pulse Width Modulation (PWM) is the most common method for controlling LED light output. By pulsing the power and adjusting the pulse manufacturers can easily and cost effectively increase or decrease the light output. But PWM control is not ideal for film. It can conflict with the sampling rate of the cameras causing visible flickering. The solution is analog control of LEDs, which is less common and more expensive, but well understood as a design approach.

The challenge with analog control is, once the power drops below about 50%, LEDs can ex-perience a color shift that does not occur with PWM control. To manage this color shift, ad-vanced control strategies marry PWM with analog control below 50% power levels to main-tain correct color rendering as the power drops.

2 - Beam patterns:

LEDs, unlike HID lamps which suspend the arc, or light source, in space, generate light di-rectly off the chip. This creates a challenge for light modifiers, which must be completely re-thought to work effectively with LEDs. Optics that work great for managing a suspended point of light, fail when tried on LEDs. Fortunately there are new optics being developed for LED lights that are appearing every year. A common example is the application of Fresnel lenes. In testing fresnel lenses with LED sources, one sees color shifts as well as rings of light variations as the lens is moved in or out. Leds are a distributed light source and not a point light source which requires a different optic approach. The application of fresnel lenses, which need to be specifically designed for the particular LED or LED array, only work well at a precise distance from the source if artifacts and light rings are to be avoided. A moveable fresnel is not a good concept when married with LED lights. Fortunately there are a range of neat light modifiers that are designed to work with specific LEDs and LED arrays to narrow the typically wide beam (120 degree native), to specified beam angles.

3- Color rendering and film

There is a great deal of discussion in the industry about accurate color rendering. As camera sensors have advanced, the importance of balanced color from LEDs becomes critical. Cheap LEDs can do a very poor job of delivering good color and the old ways of measuring color rendering, such as CRI (color rendering index), can be cheated by LEDs. So a high CRI does not necessarily mean good color from your light source. There are a number of emerging methods such as TLCI (TV light color index) that do a better job of ensuring the light source will perform well on camera.

But before we get into the science of good color from LEDs, lets review some basics. First; a lumen is defined as all the light visible to the human eye. This is a small fraction of the electromagnetic spectrum (shown below). Conventional lights emit infrared as well as ultra-violet light which heats the subject it is pointed at, but adds no value to subject illumination. LEDs are designed to emit primarily only visible light, which is part of their efficiency advantage.

So the first and most basic measurement that manufacturers should report is their lumen output. This is the useable light that our eyes can see. It has nothing to do with watts.

Second we need to make sure the light source is balanced. The basic measure is CRI, which for cinema, should be 90 or better. In addition to CRI, the TLCI measure (TV Light Color In-dex) should be 85 or better to ensure that no color correction is needed. Beyond that, the color temperature required is important well understand and thankfully easy to specify with LEDs.

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A bit of the science behind LEDs:

Below is a chromaticity diagram. This is easily generated for any light source by placing the light in a “lumen sphere” and measuring the light. The CRI as well as the TLCI indexes can easily be generated from the data captured when chromaticity is measured for a light source. The diagram shows that a white light is some combination of all the visibly spectrum ofLED02 light. White light is shown in the center, while pure or saturated colors are around the perimeter.

The Black Body Line (BBL) is plotted on a chromaticity diagram and relates to the color emitted by metal that is heated. Think of a blacksmith shop; as he heats the metal it glows red, then yellow and then white. This is shown as the black line starting on the right side of the chromaticity diagram, moving from the perimeter of the plot to the center. This color measurement is captured as degrees Kelvin, with “white hot” being in the center at 10,000º Kelvin. The sun is classified as 5600º to 5700º Kelvin and Tungsten light is 2700º Kelvin.

This Black Body Line relates to cinema lighting in that LED sources “binned” above the black body line start to generate the greenish hues film makers complain about. It is clear to see why. As you move above the black body line, you are moving into the yellow/green area of the chromaticity plot. The TLCI attempts to predict the light performance of any measured light using a formula calculated off the chromaticity data used to generate the plot for a given light source. Manufacturers have an easy way to ensure a high TLCI. They just need to specify a tightly binned LED that is sorted just below the black body line. For cinema ap-plications, tighter bins that live just below the black body line will generate the best color rendering for CMOS sensors and will rate the best on TLCI index.

In summary, switching to LEDs as a light source allows smaller, more powerful and more robust lights that will last. The first way to compare lights is by raw lumens, or visible light generated from a fixture. Second, buyers need to understand the color temperature they are looking for, which can range from sunlight balanced (5600ºK) to (Tungsten 2700º K). Next is the quality of the light. Does it render colors well? Measures of CRI and TLCI over 90 are best in class. And finally pay attention to make sure the light is pared with appropriately designed light modifiers, is driven by electronic circuitry that manages the light to minimize color shift without flicker.


Learn more at LM.TL/broadcastlights

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Light & Motion Industries
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