Get street wise on LEDs

Selecting LEDs with the right characteristics, in the right format, for the target application is important.  Cost should not be the sole driving factor.

By Lance Hemmings, Global Product Group Manager, RS Components


LEDs are making significant inroads into many lighting applications, both indoors and outdoors.  For new builds and total refurbishments in sectors such as retail spaces, refrigerated displays, restaurants, museums and galleries, and even aircraft, LEDs are invariably considered and widely adopted.

The retrofit market has been a little slower to recognise the benefits.  Some sectors, such as street lighting in ‘smart city’ initiatives, have found that lower running costs and longer life are sufficiently compelling arguments to make the switch to LEDs, despite a higher initial outlay.  The regular man in the street, faced with a €20 price tag for an LED light bulb to replace a €2 incandescent, is not so easily swayed by vendors’ calculations of cost savings over its lifetime.

While cost remains an issue, the quality of light produced by LEDs, has begun to turn the tables and stimulate further adoption of LED lighting.  Light quality is determined by a number of factors, including colour temperature, colour rendering index (CRI), brightness and consistency.

Colour temperature is what we now recognise as warm, neutral and cool white light. The first point to make is that pure white is created from a mix of red, green and blue light. The human eye is highly sensitive to colour change, more so than to small changes in brightness.  By allowing more blue light in the mix, the resulting white becomes ‘cooler’, represented by a higher correlated colour temperature (CCT) figure of 5000 to 8500K.  Less blue or more red, will give a ‘warm’ white, measured as a lower CCT figure (2600 to 3700K).  Neutral or natural white is typically 3700 to 5000K.

The colour rendering index (CRI) is a measure of the ability of a light source to reproduce colours faithfully.  Most of us have probably purchased an item of clothing in a shop, only to find it appears quite a different colour outside in daylight.  This is due to the poor colour rendering of traditional fluorescent light sources.   A high and consistent CRI is particularly important to ensure skin tones remain natural as the light level is reduced.

For reference, natural light has a CRI of 100 while those old yellow sodium vapour streetlamps have a CRI of 0.  Incandescent lamps have a CRI above 95, rendering most colours, with the exception of dark blue, reasonably well.  Halogen lamps come in around 90 or better.  Cool white fluorescent lamps have a CRI of 62, and compact fluorescent lights around 80.

The CRI of LEDs varies, depending on the technology, and has been improving steadily in recent years.  Companies such as Osram, Philips can achieve a typical CRI of 95 for warm white LEDs, up to 4000K, for example.  At 5000K the same vendors, plus companies including Cree, Rohm and Sharp, are producing LEDs with CRI figures of 80 or better - a few up to 90 to 93.  But for cool white LEDs, 6500K upwards, CRI can be lower, with 80 to 82 being about the maximum.

Colour consistency is another factor important in LED lighting.  Manufacturers produce LEDs to meet a specified CCT, but it cannot always be guaranteed that chips from different batches will emit precisely the same colour.  Natural variations in material and process can affect the chromaticity and luminous flux.  For the best consistency, vendors will sort their output into ‘bins’ containing devices within a very close range of a specified CCT.  Devices from larger bins, broadly dubbed warm, neutral, cool white, for example, are less expensive.

Colour can change at higher operating temperatures too.  Again, some manufacturers will rate their products to a performance level at higher temperatures, typically 85 degrees C.  The price premium may well be acceptable for the right application.

The LED market may appear daunting, with hundreds of vendors worldwide and tens of thousands of device families as well as modules.  The key specifications to consider for any new lighting design project are: CRI, CCT, luminous flux (light output), efficacy (lm/W), forward voltage and viewing angle.  Some of these aspects are highlighted in a selection of applications outlined below.


The art of lighting

In applications, such as museums and art galleries, accurate colour rendition is critical.  Typically, museum lighting should deliver a CRI of 90 or better, with a CCT between 2700 and 3000K, and importantly, it must be dimmable. An ideal light source will deliver the same spectrum regardless of brightness.

Food displays and refrigerated units benefit from LEDs with selected CRI and CCT to make food and packaging more appealing.  Cooler colours around 5600K are typically used for freezers with warmer whites (3000 to 4000K) for displaying meat.  Vegetables look best under high CRI lighting.



Figure 1: LEDs can help make food displays more appealing, as well as reduce running costs for the retailer.  Source: Philips Lighting.

Note that incandescent bulbs typically have a yellow hue, and reducing the power tends to increase the yellow effect.  Further, incandescent bulbs give out UV and IR, which can damage sensitive artwork and cause food to spoil faster. Filters can be applied, but add to the cost.  Conversely, today’s LEDs deliver a more uniform spectrum, while dimmable LEDs maintain the same colour at low intensity.

Now consider hospitality environments, such as hotels, restaurants and bars, even homes.  When halogen lights are dimmed, the CRI stays the same, but the colour temperature changes, giving a warmer, more relaxed feel.  People expect lights to dim in this way.  Typically, when an LED is dimmed, there is little change in colour temperature.  Therefore, for this type of application, LEDs and dimming schemes need to be selected that will change the colour temperature when the lights are dimmed.



Figure 2: In hospitality environments, LEDs allow architects to create different moods for different areas.  Source: Osram 

Various solutions are emerging, such as colour mixing using a tuneable array of die with different colour temperatures and carefully matched secondary optics.  Compact emitters are said to produce the best results.  Of course, the driver electronics are essential to control the dimming and colour tuning.


Flying high

A key attraction of LEDs is their ability to be controlled and tuned dynamically, so that light levels and colour temperature can be adjusted as necessary.  It is also a major contributor to reduced power requirements in many applications, such as street and outdoor area lighting, emergency and security lighting.

The aircraft industry has adopted LEDs, proving especially attractive in the passenger cabin.  Controllability of light levels and CCT brings improved aesthetics and passenger comfort.  Consider a long haul flight starting in the afternoon, with cabin lighting close to natural daylight.  By dinnertime, this will have been dimmed to produce a warmer, lower intensity lighting.  As the flight nears its destination, and the cabin crew need to prepare passengers for landing, the cabin lighting can be gradually brightened to simulate dawn.  Much more pleasant than the rude awakening of sudden bright illumination!

Studies have shown that this approach contributes to reducing fatigue and the effects of jetlag.  It is also being used in hospitals to aid recovery from operations, while LED light boxes that simulate daylight to counter seasonal affective disorder (SAD) are becoming more popular.



Figure 3: LEDs can be programmed to simulate natural light levels during a day.  Applications include aircraft cabin lighting, hospitals and healthcare, and general wellness.  Source: Philips Lighting.


Narrow beam, wide range

Now let’s take a look at some typical LED products and the applications they may serve.  While narrower beam devices may be best for some applications, particularly where they are used in arrays, some lighting scenarios, such as floodlights, require a wide viewing angle that gives a more uniform radiation pattern.

Osram Opto Semiconductor has developed a range of LEDs with 150o viewing angle, for use in floodlights or in combination with secondary reflector optics. The Oslon SSL 150 is a 1W high power series, with CCT ranging from 2700K to 5000K, and CRI typically 90 to 96, although some lower CRI versions at 70, 83 are available.   Offering high efficacy up to 125lm/W, the devices can be purchased individually on reels, or mounted in arrays and clusters.

For applications requiring the highest CRI figures, a number of LEDs can be selected from Osram Opto’s Oslon range featuring a CRI of 95, covering CCTs from 2700K through to 4000K.  For a CCT of 5000K, Cree and Rohm offer devices with a CRI figure of 90 or 93.  Cree’s Xlamp series of specialised tightly binned 5000K parts can be supplied with forward voltage up to 36V and efficacy up to 117lm/W.  In contrast, Rohm’s low cost LEDs for more general applications feature an efficacy of 61 or 75lm/W and a forward voltage of 3.3V

Mid power (0.1 to 0.5W) LEDs, derived from low-end, low-cost devices, are now emerging in ‘lighting quality’ formats, and are commonly being used to create linear lighting modules.  Mid-power LEDs arranged close together create a more even light compared to fewer high power LEDs spaced further apart.

The Osram Duris range is a classic example, with the original Duris E3 devices targeted at cost-sensitive retrofit T5 and T8 linear fluorescent tubes.  With a 120o beam angle, these low cost devices are available in CCTs from 3000 to 6500K, producing a homogenous light source.  The 5000K version, for example, has a CRI of 72 and efficacy of 110lm/W.  The Duris P5 range delivers higher efficacy versions for professional indoor and outdoor applications.  While the Duris S5 range is a more compact variation.

To gain more insight into the range of LEDs available with the characteristics required for your next project, take a look at the RS Components website.  Filters can be applied to narrow down your choice, including, colour, CCT, luminous flux, forward voltage, and, of course, by vendor.