lighting design


Like so many technology-based industries, lighting has seen some significant changes in recent years, supporting the need for a more systematic approach to lighting specification. Richard Strode, managing director of Tridonic, explains

Of course, changes in lighting technology are nothing new; they have been happening since before the invention of the candle. The pace of change, however, has increased dramatically, as evidenced by the rapid growth of LED technologies in lighting in just the last few years.

Given the potential cost-savings and environmental benefits of LEDs this is clearly a welcome change but I would also suggest it is a more profound change than most of the other changes we have seen in the last couple of decades. Consequently, lighting specifiers need to fully understand the depth of change if they are to deliver maximum benefits to the end client. Of particular importance is the need to consider LED lighting as a system from the early design stages, rather than a collection of disparate elements – as has been the case for many lighting installations in the past.

The differences, and the extent of the changes, can be illustrated by comparing it to the evolution of fluorescent lighting that has been prevalent in so many areas of lighting for such a long time. In terms of the light source and control gear, T12 linear fluorescent lamps with magnetic ballasts were the ‘standard’ for many years, gradually evolving through T8 & T5 linear fluorescent and compact fluorescent lamps. The evolution of these light sources was accompanied by a similar evolution in control gear with the introduction of high frequency electronic ballasts.

These changes have been quite gradual, marked by relatively small steps within what is still a single ‘family’ of light sources that use fundamentally the same technology – albeit more efficiently. As a result, in specification terms, making the transition from T12 to T5 (for instance) has been relatively straightforward. The key has been to understand what has changed and design systems accordingly.

In contrast, the differences between traditional light sources and LEDs are ‘revolutionary’ rather than ‘evolutionary’.

Not least of these is the fact LED light sources have integral lenses so they are inherently directional. This impacts on the whole optical system of the luminaire, with less emphasis on the use of optics to ensure light leaves the body of the luminaire. It also removes many of the restraints that were imposed on traditional luminaire designs and opens the 
door for more imaginative use of light when using LEDs.

However, it also necessitates a different way of looking at basic factors such as light outputs. Traditionally we have spoken in terms of light output ratios (LORs) as an indication of how much light leaves the luminaire – the result of the combined lamp and optical system. In most cases, given a certain wattage of lamp and a roughly similar luminaire design, there would be relatively little variation in LOR from one luminaire to another. Certainly, the lamps themselves would have very similar lumen outputs, assuming they were from one of the major, quality lamp manufacturers.

LEDs are different! The lumen output of two LED fittings with the same wattage from different manufacturers can vary considerably, so that thinking about lumen output and LOR is no longer meaningful with LED lighting. Instead, specifiers need to think in terms of the lumens that will be directed to the lit space – often referred to as ‘useful lumens’. Of course, this also means they also need to have confidence in the performance figures quoted by the manufacturer!

Thermal management

Performance isn’t just down to the LED light source, however. Of the electrical energy that is consumed by an LED, around 25% of it is emitted as light, the rest as heat. Furthermore, LED circuits are very sensitive to heat and if they are allowed to overheat this will greatly reduce both their light output and their longevity. This makes thermal management particularly important and emphasises the need for a ‘system approach’, rather than a ‘component approach’.

A degree of thermal management is built into the LED light source itself, ensuring that heat is conducted away from the sensitive electronics. It is then essential to ensure that the ‘system’ provides mechanisms for dissipating that heat so that it is not allowed to build up in the fitting, typically using heat sinks built into the body of the fitting, with a design that encourages convection currents to pass through or past 
the fitting.

This has now become an essential element in the design of LED lighting products and is something that specifiers need to explore when selecting products. A great many of the LED lighting products that haven’t delivered on expectations have failed through a lack of attention to thermal management by the manufacturer. In reality, this is often a cost-cutting exercise by some manufacturers and reinforces the need to source products from reputable manufacturers that invest in quality.

Colour tuning white light

One of the key benefits of LEDs over most of the discharge lighting they are replacing is their enhanced controllability which, in turn, creates opportunities for energy savings over and above those achieved by reducing the installed electrical load.

There are the obvious examples, of course, such as dimming or switching the lighting in response to occupancy or natural daylight levels. There are also opportunities for scene setting in meeting rooms, retail applications etc. To facilitate this, LEDs can be used with a range of dimming options including DALI, 1-10V, DMX and wireless control protocols.

Also, because the LED sources are very compact, it is relatively easy to combine LEDs with different colour temperatures in one fitting, facilitating the use of ‘white light colour tuning’. The principle of white light colour tuning is that natural daylight changes colour through the day, so this is the situation that humans have evolved with and this colour change has an influence on our natural biorhythms.

Given that most of us spend most of our time in buildings under artificial lighting, it is clearly desirable to be able to be able to change that artificial lighting in a way that mimics natural light. In the past, though, the opportunities for doing this have been limited by the lighting technologies available. Typically, such attempts would involve the use of colour filters or controlling layers of warm and cool lamps separately to achieve different mixes of ‘white’.

The compact nature of LED light fittings, combined with the controllability referred to above, makes it relatively easy to include separately dimmable arrays of warm and cool white LEDs, or even different coloured LEDs mixed with the white LEDs, to achieve a wide range of colour temperature options. Currently, these techniques are finding considerable favour in healthcare applications, where an improved sense of wellbeing from more natural lighting underpins improved patient outcomes.

Colour tuning can also be used to mimic natural daylight ‘out of sync’ with what’s happening outside the building. This is particularly useful for shift workers in factories, call centres, dealer rooms etc. where light with a high blue content has been shown to increase alertness and productivity. In industrial applications it can also reduce the risk of accidents. This is a clear example of how we can exploit the ability of the latest LED products to do more with lighting than has been practical in the past.

The system approach

All of these issues support a ‘system’ approach to lighting, an approach that may be unfamiliar to some lighting specifiers. Certainly, the traditional approach has been to think of lighting as an assemblage of components such as light source, control gear, reflectors, luminaire bodies, diffusers etc. – often coming from different manufacturers and brought together in a complete luminaire. The downside of this approach was that, though those components will have been checked for compatibility, this isn’t the same as a system where the elements enhance each other’s performance.

In a true ‘lighting system’ the sum of the whole is greater than the sum of its parts with light source, control gear and control strategy all working in harmony to ensure optimum performance.

This more holistic approach is reflected in the latest version of Part L of the Building Regulations, which allows the Lighting Energy Numeric Indicator (LENI) to be used as a measurement of luminaire efficiency. The key element of LENI, in this context, is that it takes account of both the efficiency of the lighting and the way it is controlled.

Fit and forget

So far this article has focused on the differences between LED lighting and more traditional installations, with just a passing reference to the cost and carbon benefits – not least because these have been covered very thoroughly in Electrical Review in the past. However, it’s worth noting that as end clients become more accustomed to these benefits their expectations will change.

For example, in addition to saving energy most end clients will anticipate a significant reduction in the maintenance costs associated with re-lamping. Indeed, this is frequently factored into return on investment calculations. In fact, LED lighting is coming close to being perceived as a ‘fit and forget’ solution in the eyes of many building operators.

As a result, if we as an industry are to provide maximum value to end users, all of these LED products need to be supported with a comprehensive after-sales toolbox that includes a generous parts and labour warranty as well as ongoing technical and site support. This is something that specifiers will need to consider when ensuring they are providing the best solution for their client throughout the lifetime of the system they have designed.