JOHN LUFF /
03.01.2012
Originally featured on BroadcastEngineering.com
Portable lighting
Improved technology has made it easier to capture images.

The broadcast business is about capturing images and using them to create content that informs, enlightens and entertains. All of that starts with replicating the human sense of sight. Nothing could be simpler than lighting a scene and shooting it with a camera (which then samples the light and codes it digitally before passing it down a coax/triax/fiber — well, you get the idea).

The first part of sight, or image capture, is finding a scene with adequate illumination to make rendering the image possible. The human eye can adapt to an incredible range of illumination, from starlight to full sun, a range of about 90dB, or 24 stops, a ratio of around 108:1. It is that dynamic range that makes it possible for humans to operate as both diurnal and nocturnal animals.

Cameras are not as flexible. At any illumination level, the useful range of a camera is about 20dB to 30dB less than what the human eye can accommodate. Eight-bit sampling limits the dynamic range of a camera to about eight stops, while 14-bit processing can allow as much as 14 stops of dynamic range. Even at that, the camera can reproduce a contrast ratio of only about 16,000:1.

In outdoor and studio setups, lighting plays a critical role in setting the dynamic range of the scene. This makes it possible to capture a rendition of the content that is pleasing to the eye at the time of image viewing. It is important to note that this is the point at which “machine vision” kicks in; our eye might not think the scene was optimally lit, but the camera “sees” it as perfect. In the case of news footage capture, the ultimate challenge is to achieve the same end without bringing a grip truck and a staff of lighting technicians. Here the battery enters the story.

Battery-powered light

Film cameras used for news often had battery belts that plugged into a DC-powered light that was switched on only when film was rolling. Any adequate amount of light simply sucked so much power out of a battery that it was not practical to run it more than absolutely necessary. The same was true for early ENG and EFP cameras recording on 3/4in, 1/2in or 1in tape.

The sensitivity of the cameras was good; but they still required a lot of light, which equated to a lot of weight in batteries. Cameramen often wore two belts for powering the camera and the lights, or carried the battery on the camera and a second battery in the VTR slung over their back (or that of an assistant). Or they did all of the above with a second belt held for powering the light when the first belt ran low. In some cases, the sheer weight and volume of batteries meant a second person on the crew, who acted as both lighting and sound technician, was absolutely necessary. The issue is not weight, however; it is energy consumption and energy density, and the efficiency of the light in converting electricity to illumination power.

Progress in lighting

In today's production world, things have gotten much simpler. First, the camera requires less power (i.e. smaller batteries), is much more sensitive (requiring less light for acceptable performance) and weighs less. The weight is critical. Less weight in the camera means that a photographer can bring along more light. This, of course, replaces the weight lost in the camera and its battery, but it also means more energy. But, at the same time that the camera has made options for lights more appealing, and less critical due to the camera's own sensitivity, the science that drives portable lighting has allowed improvements in luminaries for portable use that have opened up a set of options that would have been just as stunning (if not more so) 30 years ago when ENG cameras started to evolve into useful tools.

The effect of research and good engineering has been to make the lights more efficient in terms of conversion of electricity into illumination, lower in weight and less costly to run due to longer life times. At the same time, battery technology has allowed marked improvement in the amount of energy stored in a unit of volume or weight. This results in the same amount of run time in lower weight, less bulky batteries or longer run time in the equivalent size/weight battery.

High-power LED and highly efficient halogen lights are the drivers from the illumination side of the equation. LEDs can provide nearly as much light as halogen for close-in work. Also, due to the extended size of many of the luminaries, the light is less harsh (softer) than what used to be seen on the top of ENG and film cameras. The power efficiency of LED lights for ENG use is about 7X to 10X better than incandescent sources, and they emit much less heat as well.

There is no free lunch, of course. LED lights have struggled to reach the other performance expectations of incandescent sources. The issues most often brought up are the ability to focus the instrument, and the color temperature of LED luminaries. Early LEDs looked OK to the eye, but produced strange effects on some scene content due to the nature of the light. LEDs produce light at specific wavelengths. By grouping LEDs of different colors, or by using phosphor coatings on some, the overall response of the camera may be fine. However, with some colors, LEDs may not render the scene as naturally as a black-body luminary such as incandescent. In the last few years, the ability of LEDs to render scenes effectively has been vastly improved.

Early LED instruments were simply arrays of LED chips, which meant the luminary could not be focused like a fresnel instrument. However, LEDs now can be put into small, efficient and focusable luminaries that allow much more lighting flexibility.

Another important factor is the life of the “bulb.” Incandescent sources may last 1000 hours with proper care. However, LEDs should last 25 to 50 times as long, which makes their initial higher cost much more acceptable. In studio settings, where air conditioning is needed, LEDs provide a second benefit because they produce considerably less heat. Most broadcasters are trying to be good environmental stewards, which makes the lower power consumption of LEDs a large benefit.

John Luff is a television technology consultant.

Send questions and comments to: john.luff@penton.com



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