FEATURE
Issue: 

Standards of illumination

Stephen Mounsey looks at the regulated world of LED lighting, and the photonic test and measurement technologies that enable LED developers in this rapidly growing market

Rules, regulations, and standards seem to have a huge influence over the LED lighting market; international and EU standards cover many aspects of how to quantify the output of an LED, which can vary hugely in terms of colour and brightness, and even regulations covering the safety of LED light sources have been introduced. LEDs offer much greater efficiency than their incandescent predecessors, and more than the current generation of compact fluorescent lamps (CFL) high-efficiency bulbs. For this reason the adoption of LED lighting is likely to be driven by further government regulation, as incandescent bulbs are phased out and building codes altered to favour new, low-power illumination.

Robert Yeo, a director at UK-based measurement specialists Pro-Lite Technology, cites the move towards energy-efficient light sources as a significant driver for the industry. Although LEDs are replacing traditional bulbs in luminaires, he says, the industry now faces a problem in terms of consistency, and this is where a standardised test and measurement process becomes vital. The factory specifications of LED chips are determined under idealised conditions, he says, and can therefore be misleading in real world use, even when coming from leading and reputable companies such as Phillips and Osram (although more so when dealing with products from unknown companies).

LED luminaires and light fittings contain many LEDs in a small space, operating at a much higher temperature than that at which they were tested. At 25°C they give out 100 per cent of their rated light, but when operating at 70 to 80°C their light output is decreased by about 30 per cent.’ These discrepancies, Yeo says, have led to some dissatisfaction with some of the products already on the market, as they fail to produce the brightness expected by consumers. The onus falls increasingly on the companies manufacturing completed lighting products (luminaires), rather than the LED manufacturers themselves, who may be in the Far East. ‘In the UK we have a momentum going with the lighting and luminaire industry, but they need a standardised way of determining output light.’

Leslie Lyons, technical support manager at Bentham Instruments (Berkshire, UK), adds that standards are important to produce a level playing field for companies competing within the industry. ‘Many of the justifications of new standards cite the fact that reported data sheet values are often incorrect. Instead of LED manufacturers reporting inflated performance claims, let’s give realistic values.’ The standards, he says, tie the manufacturers’ claims back to the physics of their components.

Despite the perceived need for more meaningful definitions in this fledgling industry, Yeo notes that an appropriate EU or UK standard for the measurement of LED light sources has not yet been created. ‘There’s a general lighting standard for the photometric measurement of luminaires (BFEN 13032), but it lacks any reference to solid state lighting. This matters, because solid state lighting varies so much depending on temperature and on how the device is driven.’ The standard should, Yeo says, mandate letting the LEDs warm up and stabilise before taking a reading, which can take up to two hours.

One of the difficulties, he says, stems from the fact that the output of an incandescent bulb (on which the standards are based) doesn’t vary with operating temperature – 100W is about 2,000 lumens, and will be regardless of what luminaire the bulb is fitted in. Fluorescent lamps have only slight and easily accounted-for temperature dependence, but LEDs have great dependence, with the luminous output reducing as the operating temperature rises, from a colour temperature of 5,500K (a clean white light) at 25°C to 8,000 to 9,000K (a blue tint) at an operating temperature of  70 to 80°C.

Keeping safe

Demand for LED lighting has driven changes in other areas of regulation, such as safety, where the existing framework proved insufficient for the needs of a commercialised technology. ‘In the past, the optical radiation safety of LEDs was considered under the standard for laser safety (IEC 60825),’ Lyons says. ‘That was never a satisfactory situation: lasers, for example, have very low divergence, and moving further away from them doesn’t reduce the risks of their high output power. LEDs on the other hand are very divergent, and moving further away from them greatly reduces risk.’

In 2007, he says, the IEC technical committee responsible for laser and optical radiation safety removed LEDs from the scope of the laser standard. While looking for another way to consider LED photobiological safety, they adopted CIE [European] standard S009, which was then published as IEC document EN 62471 on the photobiological safety of lamps. This has been adopted as a very generic standard that covers all lights and lamp systems – LEDs, compact fluorescent lamps, halogen lamps, etc. While the change in standards was driven entirely by the demand from the LED lighting market, the manufacturers of any product that produces light in the 200 to 3,000nm region (regardless of whether or not that’s the main function of the product) must give consideration to optical radiation safety.

‘Safety measurements are very much dependent on the end products,’ Lyons continues, ‘and the focus is increasingly on luminaire manufacturers to perform these tests rather than the LED producer, who has no idea how the luminaire manufacturer is going to use the component.

Additionally, the new standard can require warning labels to be displayed on luminaires in certain instances, and so lamp manufacturers do their own tests in order to try to avoid having labels on their products. If they’re fitting a room with LED lighting, for example, where can they display a label that says “these are risk group 2 LEDs; do not stare at the lamps?”’ The rapid adoption of such a complex standard, Lyons notes, has lead to a great workload and some confusion in the industry, as luminaire producers struggle to understand how best to apply it.

The darker side of LEDs

What is the reason for all of these safety regulations? What possible threat can LED lighting pose to the health of, for example, an office worker exposed to it every day? Part of the hazard stems from the way in which some LEDs work, with a blue/UV LED exciting a phosphor coating, which then re-emits white light. Allen Carey, sales and marketing manager at Ophir-Photon (Logan, Utah, USA), notes that one industry concern is that some of this UV can leak through the phosphors, and particularly the dangerous UV-A in the 360 to 380nm wavelength range. ‘Those are the wavelengths that the FDA is in upheaval about because of sunscreens not having UV-A protection,’ he adds. ‘SPF gives information about UV-B, but UV-A is now known to have just as much potential to cause skin damage as UV-B, although it won’t cause sun burn.’

Bentham’s Lyons explains that an additional risk exists in the form of what’s known as a blue light photochemical hazard. ‘There are some very specific wavelengths that can cause damage to the retina, and there’s a hazard weighting function peaking at approximately 435 to 436nm. White LEDs have a blue LED pumping a phosphor, and that blue LED happens to peak at around 440 to 450nm,’ he says. Any blue light leaking through the phosphor, regardless of its UV content, could therefore cause retinal damage. To account for this, new standards are being developed that will necessitate colour measurements.

Aside from the alarming prospect of retinal damage, lighting may also have more subtle health altering effects: ‘LED output peaks at 450nm, and then the phosphor output peaks at 550nm in the green,’ explains Lyons. Research has found a non-visual photoreceptor in the eye, believed to regulate the circadian rhythm (wake-sleep cycle), and it is proposed that this photoreceptor’s response peaks at around 470nm. A recent report by a French research organisation (ANSES) suggests that LED illumination produces less output at 470nm than other light sources, meaning that it could potentially have a negative effect on the wakefulness of people using these lamps for everyday illumination. ‘One thing to consider here is that manufacturers of lamps specifically designed to treat seasonal affective disorder (SAD), who are aiming to stimulate the same non-visual photoreceptor to improve the outcome of their patients, use blue LEDs specifically because they’re close to that 470nm wavelength,’ notes Lyons. ‘Who is correct? I’m not sure, but it’s certainly an interesting aspect of lighting design.’

In terms of safety measurements, both irradiance and radiance are required for the standards. For the uninitiated, the subtle difference between the two can be a little confusing.

‘Irradiance is power per m2, whereas radiance measurement is taken using an imaging measurement – power per m2 per steraidian, which is a unit of solid angle,’ says Lyons, adding that radiance is equivalent to retinal irradiance. ‘It tells you how much light an optical system would couple when looking at the source.

‘Knowing how much irradiance falls at the level of your eye does not tell you how much light is being coupled to the retina. For this standard we don’t care so much about what the source outputs in total, but more about what the source couples into a given area – namely the retina.’

In the blink of an eye

Robert Yeo from Pro-Lite explains that the safety standards looking at the radiance and irradiance caused by a certain light source at a certain distance require spectroradiometric measurements, whereas data produced by photometric measurements is what actually differentiates luminaires in practical and aesthetic terms. Again, the difference is a subtle one.

‘Radiometric data is about absolute amounts of power at each wavelength, in W m-2 nm-1, or units of irradiance – the absolute amount of light present. Photometric output, on the other hand, is the amount of light as perceived by the human eye, taking account of the very distinct colour sensitivity of the eye,’ he says.

He gives an example: ‘If we had three LEDs, emitting red, green, and blue, and if each was radiometrically producing the same amount of luminous flux, we would all agree that the green one looked the brightest by many times. This is because humans are most sensitive to green wavelengths, and so we perceive it as brighter.

‘Plotted as a graph, these colour-dependent perception differences resemble a Gaussian distribution with a peak at 555nm. It’s called the observer function of spectral sensitivity (or a photopic response), and any measurement equipment designed to take this into account – to gauge brightness as a human eye would see it – is called a photometer.’ Luminaire producers must take detailed photometric readings of their products in order to sell them into markets such as commercial or office lighting, where illumination design can be a sophisticated process.

Growing light fittings in a growing market

As might be expected, international standards exist that describe the way in which both photometric and spectroradiometric measurements can be taken.

When using an integrating sphere to obtain a measurement of the total luminous flux, for example, the standards dictate that the sphere must be a certain size relative to the luminaire it contains.

Because LEDs are hot and have a temperature dependence, LED luminaires must be large enough to dissipate heat, and so integrating spheres used to measure them are tending to get larger and larger. Pro-Lite has installed a 2m diameter sphere, with a 3m diameter product under development.

Too many rules?

Allen Carey of Ophir-Photon says that although the company has produced systems for LED measurement, uptake in the interior and home lighting market has so-far been slow, probably due to the high costs. Another part of the problem, he says, lies with building regulations: ‘The government rushed to mandate compact fluorescent lamps, and they were rushed into building codes as it were. Now, in any new building we have to put CFLs in for the inspectors. Few people are going to bother to swap them out for LEDs later on.’

Although LED lighting is undoubtedly going to be one of the most disruptive technologies of the next 10 years, regulators must be careful not to tie it up in too much red tape.