Truth Lies and LEDs


As an emergent technology within film and broadcast over the last six years, LEDs have generated strong opinions for and against, adjudging their capabilities and relative merits, when compared to traditional light sources. Development has seen them grow from the small coloured pinpoints of light on your stereo or TV, to a level where they can produce a volume of usable white light.
Like all technologies, the path of true development has not run smooth, and arguably some views currently expressed are products of both the state of development at the dawn of the high brightness technology, and a very mixed bag of product quality. Mix into this some good old fashioned FUD (Fear, Uncertainty and Doubt), from purveyors of other lighting technologies who perceive a threat from "the new kid on the block", and it can become difficult to sift the information available in order to make properly informed decisions in regard to their use.
So what is the truth about their capabilities? Are there issues with their safety? Do they produce a sufficient quality of light to be regarded as a serious tool when used for image capture?
There are a number of fundamentals in relation to their physical capabilities which can be reviewed first, before looking at issues relating to their output.
Efficiency
Power efficiency of an LED is currently about 5-6 times that of a traditional tungsten light source. In the league table of efficiency they rate more highly than both tungsten, HMI (including all strike sources, CSR, MSR etc), Halogen, and are slightly more efficient than fluorescent tubes.
The only real method of assessing efficiency accurately is through use of a light meter and knowledge of what a comparative light source would produce for the same power consumption. It is important to measure like for like, factoring beam angle, rather than relying on optimistic peak photometric readings which are often unreliable. Interestingly power efficiency is equally impressive with large LED fixtures. The biggest LED fixture currently in existence will outperform a 6 kW space light in black skirt mode on both peak output and volumetric distribution. The power requirement to achieve this is less than 1 kW, giving an efficiency compared to a conventional fixture of over 6 to 1.
Types of LED
The first LEDs used commercially as indicator lights in our home appliances and then in first generation film and broadcast fixtures were of the 'through-hole' type. To look at, they are smallish clear pimples which have two wire tails that pass through the circuit board and are then soldered on the back.
The most common soft panels use this type of technology still (including the many that now herald from China). This is not to knock them; like many things in life, you generally get what you pay for. If there is an issue with this iteration of the technology, it is in its colour accuracy and level of output (they tend to lack punch), all issues which are better addressed with the later generation of surface-mount LED technology which are now state of the art.
Surface-mount LEDs are like many other modern solid state components in that they affix directly to the face of a circuit board. Most commonly they are seen now in ever increasing numbers of cars as side and tail lights. All the continuing investment by the core technology providers is in surface-mount devices.
With both through-hole and surface-mount technology, colour and consistency are achieved entirely by the lamps originator having a suitable binning arrangement with the LEDs manufacturer (‘binning’ is simply a test and classification process that sorts finish product, as it is impossible to predict the exact finished characteristics of the devices through manufacture). This ensures that only LEDs which conform to certain parameters of efficacy and colour temperature accuracy are delivered. The reality of this to a high-quality fixture builder is that something less than 1% of any type of LED produced are of suitable standard for use in film and broadcast, which naturally has a cost implication for the final product. Both the cost and quality issues can be negated by sourcing LEDs to a much wider tolerance but quality of output is then inevitably compromised.
At the cutting edge of development in LED technology is the multi-chip array. This is a surface-mount LED which has within it the ability to host multiple colours; these are mixed to produce the final colour temperature output, unlike through-hole and simple surface-mount LEDs which are reliant on the colour space of a single device.
Whilst still fundamentally an LED, this represents a sea change in technology on a variety of levels.
Firstly colour temperature output is the controllable product of a mix of devices creating a broad spectrum (mixed wavelength) of light controlled by calibration and on-board software, and not simply a suitable binning arrangement with the manufacturer.
Secondly, output from a single multi-LED chip can be much higher, allowing for the first time the production of focusable lamps that produce a hard shadow, much the same as a tungsten-based Fresnel.
Thirdly, because there are multiple components on one LED chip, software control can allow the introduction of a colour-changing dimension. In practical terms this extends to the production of whites between about 2900 K and 6500 K, as well as saturated colours.
The highest realistic output of single array LED lamps incorporating this technology, is approaching direct comparison with a 1 kW tungsten source.
Dimming
LEDs can be driven using two basic methods, either by constant or discontinuous current.
Both have issues in their use which have implications for use in image capture.
Lamps driven by constant current are dimmed by reducing the voltage that the LED receives to reduce intensity. There is unfortunately an unassailable physical fact which relates to LEDs: if the input voltage on an LED is changed, its colour temperature of output will change. The greater the change in voltage, therefore, the greater the colour temperature shift. This is problematic in image capture, where gel may be required to maintain colour temperature of a dimmed fixture which in turn reduces intensity yet further.
The alternative method of driving an LED is with a discontinuous current. The device therefore flickers at a rate which is not visible to the naked eye and in a high-quality fixture, the camera.
To dim a fixture discontinuously often involves a technique known as pulse width modulation (PWM) which entails rapidly switching the LEDs on and off. If the gap between the LED being on in the cycle is lengthened, the eye and camera see the lamp as being dimmed.
The benefit of this is that, at all points, the LED is powered at a consistent voltage, minimising any colour drift as it dims.
This can, however, introduce another issue should the fixture not be designed for use with a camera. For the camera not to see the flicking of the LED (at 25 frames per second or more), it requires a switching rate of about 800 Hz. If the lamp is to be used in high speed applications then a rate of 35 kHz and above is required, which will allow shooting to a rate of between 7,000 to 8,000 frames per second. High frequency ballasts on fluorescent lamps are at around 20 kHz which allows shooting rates of up to 1,000 frames per second.
Examples of LEDs operating at a cycle discernible by the camera are sometimes seen when architectural LED based fixtures are used in TV set designs. These produce interesting colour, but, once in shot, appear to flicker because they are running at a switching frequency of only 200-300 Hz.
The more recent multichip array technologies, because of their broad spectrum characteristics use a combination of software algorithms and voltage change to maintain colour consistency whilst dimming. As dimming is not a linear process, accuracy can be maintained by the complicated process of on-board number crunching.
Life-expectancy
This is one topic in relation to the technology where it would be useful to define some terms before making any claims. As an attempt at doing this we could perhaps state the following:
Life-expectancy for an LED should be defined as its continuing ability to produce usable levels of light output with no variance in its original colour temperature. This is often expressed with terms such as 'L70', the associated value of which expresses the time elapsed to a point at which the LED output drops to 70% of its original.
The problem with which we are faced is that the relative age of the technology, coupled with some of the early claims made within the market place (50,000 to 100,000 hours of use), make realistic life-spans hard to prove at this juncture.
Bearing in mind the above definition, a more realistic life-span would probably be in the region of 20,000 hours. Before throwing your hands in the air at the inadequacy of this, the maths extrapolates this to eight hours per day, five days per week for 10 years. How many 10 year old tungsten light bulbs or fluorescent tubes do you own that have been in virtually constant use?
Once again the better multi-chip array technologies provide longevity by the use of the ubiquitous software control in combination with closed loop feed back, to boost colour channels that start to under perform, as a consequence of degradation through age. Total life is again difficult to predict as the clever sums are compensating for the loss in output, so comparing this technology with simple ‘L70’ devices is not easy, but on the up side a guarantee of 100% output is achievable and arguably more useful. This does make the provision of warranties by manufactures in relation to colour temperature accuracy and level of output, far more realistic.
Heat
Despite their apparent cool running, LEDs do produce heat, which needs to be accounted for and managed with good fixture design. Unlike traditional fixtures there is no heat transmitted in the light beam (if shooting ice cream is your thing, LEDs are definitely for you). The heat produced does, however, need proper dissipation if the lamp is not to risk colour shift as its temperature rises. (Temperature is a significant factor in the performance and life of LEDs).
In comparison to traditional fixtures, the heat produced is minimal because of the improved efficiency over conventional technology as discussed earlier. The knock-on benefit in a studio therefore is a greatly reduced requirement for environmental control such as air-conditioning. Major broadcasters use an air-conditioning metric of 2.5:1 which, when compounded with a conservative technology efficiency of 5:1, gives an overall power saving of 12.5:1.
Safety
Research has been done into the safety of LEDs and the possibility that they can cause retinal burn. The claims have been made based on research carried out by the National Physics Laboratory in Teddington, Middlesex.
The concern relates to 'cheaper' through-hole LEDs of a lesser quality. Fundamentally an LED is powered by a blue pump at 450 nm. Output is then passed through a suitable phosphor to produce a broader spectrum and the final colour temperature output. Fluorescents work in much the same way.
In the case of poor quality LEDs, the quality of the phosphor and its early degradation can leave a blue spike in the spectrum of light produced. The blue spike can potentially disable what is known as the blink-away function of the eye, which is a reflex preventing damage from harmful wavelengths of light.
LEDs of this type would be of little use in the world of image capture because of the way the light produced would render on film or digital formats. Once again it is the superiority of the LEDs used within fixtures which is immediately discernible in their quality of output.
One other concern mooted in an allegation in the US is the possible damage that can be done to the eye as a consequence of multiple point sources of light. LED lights like any other form of film or broadcast lighting should appear as single source to avoid unwanted shadow and other aberrations in image capture. A high-quality fixture therefore will have suitable diffusion, ensuring even rendering on the subject, thus negating the possibility of any potential eye damage to say nothing of the avoidance of poor images or 'sharp' tiring light for subjects.
It can also be said that fundamentally any source of bright light can damage the eye – isn’t that why we don’t stare at the sun?
The issues raised thus far all relate to the technical characteristics of LEDs. But what of the practical capabilities of fixtures and the artistic potential they enable.
Versatility
In practical terms, LED lamps can now provide both hard and soft sources of high-quality light.
Their low power requirement, hence low voltage requirement, means that in many instances they can be run on batteries or any of the generally available voltages found on a film set (12 to 40 V).
This makes them highly portable, and given their solid state components, highly resilient without need for consumables such as bulbs or tubes.
Further, in the case of multi-chip array technology, they are colour tunable which negates the need for gels or the time taken to apply them, as their colour can be altered on-board or remotely via DMX.
All of the above can lead to a reduction in the volume of equipment needed, as well as a reduced infrastructure to support it. In the case of a spacelight replacement, the cable to run the lamp is reduced dramatically as the current requirement drops sixfold from 25 A to 4 A in the UK. A staggering 11,000 A would be required to run 200 traditional spacelights in the US. The benefits either in the studio or on location therefore are many.
Colour Spectrum
The acid test for any fixture manufactured for film and broadcast is the quality with which it renders subjects during image capture, on its own and in conjunction with lamps based around other technologies.
Simply put, if it is not in the source, it will not end up in the camera.
First off, LEDs, like fluorescents, are what is known as a discontinuous light sources. The Sun produces a broad spectrum of light that can be depicted as a spectral power distribution curve, which expresses the intensity of the various colours which go to make up the final temperature of white light experienced. When represented as a curve it has a smooth arc as it traverses from blue through green and yellow to red.
Imagine a dark thunderous day with heavy rain showers. If the sun breaks through, refraction can occur in the form of a rainbow. The make-up of the visible spectrum of light is dissected in a moment to the naked eye. What nature provides insight into in this instance is the spectral breadth of the various constituent colours which provide us with natural daylight. This is more than just red green and blue; it is a whole palette of colours.
Any light source can be broken down in this manner and it is the nature and 'completeness' of the spectral curve resulting from its output which dictates the ultimate quality of the light source.
What a discontinuous light source does is produce colour at far fewer points along the spectral curve, tricking the eye and camera into rendering the resulting output as a single shade of white light. The perfect discontinuous source for image capture would have an infinite number of peaks between 400 and 750 nm and no gaps in the spectral distribution.
The quality of the light produced by the LED is determined very much, therefore, by the quality of the phosphors and dies contained within it, which dictate the final colour of white light emitted.
One obvious method of checking colour temperature is to reach for a meter. But there is a problem. Calibrated to work with a continuous spectrum of light (such as a tungsten lamp), when confronted with an LED, meters cannot interpret the gaps in the spectral curve, because the final colour temperature is constituted of a limited number of colours. Consequently they provide wildly inaccurate readings. In short, with LEDs (and fluorescents) meters won't work and cannot be trusted.
The best test of colour quality is to look at images that have been shot with a particular LED light source to see if there are any obvious aberrations. Does the light in the images look blue, green or magenta for example. Is the LED light source directly comparable to or not discernible from another trusted tungsten or daylight source.
It must be said that, as a quality issue, colour inaccuracy is not confined to LED technology. As an example, lesser-quality fluorescent tubes can render noticeably green, and HMIs can look very blue. As we know, HMIs shift in CCT for every hour of use which makes matching lamp to lamp difficult, and other sources will shift CCT when dimmed.
Any issue which might exist with the colour quality of a single-colour LED is very much negated by the cutting edge multi-chip LED array. More advanced LED chips of this type will contain, for example, seven different colours. In addition to the obvious red, green and blue, there are intermediate hues which create a much more complete spectrum of light. The spectral curve of the output produced is therefore something far more approaching what one would expect from traditional light sources such as tungsten. It is also more complete or balanced than the outputs of either fluorescent or HMI light sources.
The requirements of the camera to interpret the output accurately are more than satisfied, as is their ability to work in conjunction with other light sources. Moreover with multiple colours on the chip, the ever-present software can modify the relative intensities of each to alter the colour temperature of output. This can be especially useful if matching other less-than-perfect sources elsewhere on the shoot.
Summary
LED technology is now more than capable of illuminating a subject, for the purpose of capturing quality images either digitally or on film. As with any other technology, it is the quality of the light source which has a direct bearing on the final result.
The truth surrounding the technology, like most lighting technology that has preceded it, can be prone to misinterpretation as a consequence of gaps in our understanding. As we become more confident in its use, however, and more discerning of quality, a point will be reached where we forget that a lamp is an LED and it becomes just a lamp again. Then we will know the truth about LEDs.

Tags: Guild of British Camera Technicians | gbct | iss048 | led | lighting | dimming | life expectancy | hmi light sources | N/A
Contributing Author N/A

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