Today there are many manufacturers offing a variety of LED solutions. The push for awareness has moved from ‘why’ to ‘how’, and it is now about choosing what is right for the job. What do you need and how do you compare when choosing LED lights? Let us consider some of the options.

What do you need?

A quick search online will get you a 50W 50,000-hour LED lamp with a claimed CRI of 98 for less than $25! You are set! For less than a tenth of the price of a replacement HMI bulb you can buy enough lights to shoot a movie. Well…. yes, you can… but will it do the job and is it what you really need for you to express your lighting creativity?

First and foremost, you need to decide what you want to do. Are you after direct replacements, additional features, are you the only operator or do you have crew, are you after gag-light, or do you need a fixture that comes out of the truck day in and day out? Just like any other fixtures in the truck, the more versatile the light, the more it will be used and the better your return will be on any investment.

Look beyond Colour Quality Index numbers

Typically, quality numbers are the old favourites of Colour Rendering Index (CRI) and Television Lighting Consistency Index (TLCI) - the definition of which are very well covered by in-depth articles you can find with a simple internet search. In simplified terms, CRI and TLCI try to quantify the spectral light quality output to a single number within the range of 0-100 where 100 is “a perfect black body illuminator” and 0 represents “a failed power-supply”.

So, the greater the number, the better the quality of the light spectrum, right? Just like megapixels in cameras, base numbers can be misleading and do not reveal all that one needs to know about a fixture.

First, there are the ‘static’ effects. For instance, the CRI number does not always take into account green and magenta bias - often having an unwanted magenta bias can actually increase CRI. Also, look at homogeneity across the angle of spread of a fixture. How does the colour temperature change with beam angle?

Also, look for consistency between fixtures. Are the fixtures calibrated or are specifications a generic figure? This usually comes down to selection and specification of the LED illuminators themselves. There can be significant consistency differences between cheap LEDs and those of reputable quality.

Then there are the ‘dynamic’ effects. Issues such as changes in LED junction temperature affect ‘drift’ (a change in colour temperature) and ‘droop’ (a drop off in light output). Life expectancy can also be affected. A 10 degree increase in junction temperature can have a 10,000 hour change in life expectancy. A well-engineered LED light will take all these factors into consideration.

In reality, anything with a CRI above 90 is a fair indication of an acceptable starting point; then you need to look at the other factors. Most of these are the same selection criteria you would use for choosing any lighting fixture. Just because it is LED, don’t ignore the obvious.

Look at other issues such as the fall and wrap of the light, punch, focus. Are you after only one type of illumination source, or do you need felicity in how the light can be controlled – from hard to soft?

See how the product dims

It is all well and good to be blown away by the brightness of a fixture given the power input, but it is important to know what happens once it comes off full power. With new highly sensitive cameras, low power or dimmed operation of lamp is very significant to its overall use. Dimming to figures even under 5% of full power (4 stops) are quite common.

How does the fixture handle subtle dimming at low levels? Colour mixing fixtures can behave very poorly at low levels if not calibrated to handle this. In a full colour mix the drift can result in completely different output colour, never mind a shift in colour temperature. How is the TLCI or CRI now? How linear is the response? What happens to magenta/green offsets?

Look for flicker

Many LED lights are dimmed using a technique called Pulse Width Modulation (PWM). This involves pulsing the LEDs at a high frequency and varying the ‘on time’ or ‘off time’ of the LED to make it appear brighter or dimmer. If the PWM frequency is too low, then this pulsating can be appear to a camera as visible flicker. A rough rule of thumb is that the PWM frequency should be 30x the frame rate you want to shoot - e.g., if you want to shoot at 1000fps, you want a PWM frequency of at least 30kHz. At least one product has a ‘high speed’ mode that boosts the PWM frequency to over 50kHz, allowing for very high frame rates. 

Dropped your light – is it still working?

Dropping a fixture is not what you want or intend to do, but trust me, someone hired for the job will do it. Lights take all sorts of punishment in the name of getting the shot. There’s no point having 100,000-hour LEDs if the fixture breaks the first time it encounters some kind of physical stress.

Check if the fixtures have been built to a standard or to a price. Look at things such as robustness of build and the materials used in construction: are they plastic, steel, alloy or something else? Ask for a reference from someone that has used (abused) them in anger! Check IP ratings and look for issues such as protrusion and protection of control buttons. Could they be sheared off with a knock? Can the fixture be grabbed and moved by holding different parts of the fixture? Are there sufficient points to rig and secure the fixture? These will all be tested at some point during the life of an LED light.

Connectivity

Finally, it is important to look at interfacing. Does the device interface with all the controllers you use? As a minimum it should have DMX control. Can you dowse or flash your lights remotely? Can the lamp be synchronised to camera shutter for lighting or other flash effects? Can you trust set-points on the back of the fixture or the DMX control point to match the actual set-points of the fixture?

LED fixtures are the here-and-now, and most of the decisions you need to take are the same as for any other fixture. Up until now, everyone has been fixated on the quality indexes and the complication of acronyms and marketing speak in the product specification. Be careful to choose your fixture based on what you need – creatively. Otherwise any light output quality index number might well just be MIPS (Meaningless Information Provided by Salesmen).