Published: 9 October 2014
Category: Technical articles
As LEDs become the light source of choice for many different lighting applications it is increasingly important to understand their characteristics. John Gorse of Philips lighting explains:
LEDs have now made major inroads into commercial lighting and are increasingly in demand in domestic applications. In many cases the electrician will decide which LEDs to use – and will take the flak if those LEDs don’t perform as expected. Consequently, it is very important to be able to differentiate between the LEDs on the market.
Two new standards
One problem in the early days of LED availability was the lack of ‘joined up’ information and standards by which to check the quality of product. Fortunately that is all changing now and for those who really want to know the detail you can look to two new IEC standards, IEC 62717 and IEC 62722. These are LED module and luminaire standards for a full range of aspects such as performance, testing, measurements etc.
In most cases looking out for the indication that these standards have been followed will be good reassurance that products are of a higher quality and designed to a recognised standard.
In retrofit lamps there is also European legislation found in the Domestic Implementation Measure (DIM 2) and Label Design Regulations introduced in the last few months. Easier to digest than an IEC standard, they effectively harmonise the basic requirements for performance of LED light sources and what can and should be printed on lamps and packaging.
They are a legal minimum standard, so most LED lamps on the market should now follow these requirements. The National Measurement Office is charged with responsibility for checking this and carrying out market surveillance to enforce the requirements.
However, if you don’t want to go through all these specification and regulation documents line by line, then the key thing is to be aware that LED light sources are very different from the more familiar lamps that have been in use for many years.
How LED lighting works
A typical LED system is made up of a power supply that converts mains to a constant voltage (e.g. 24V) that is then supplied to the LED driver. The LED driver converts the constant voltage into a constant current to the LEDs to ensure a constant light output. Light output of LEDs is proportional to the current applied and most high brightness LED sources will operate across a range of currents.
Energy is lost at each conversion stage, so the power supply efficiencies of different manufacturers’ LEDs can vary from 90% to less than 50%. This is an important performance aspect to check when selecting LED lighting.
The power losses described above also result in heat generation, and this needs to be managed efficiently as high temperatures can damage the temperature-sensitive electronic components in the driver. Just a 5°C increase in temperature can reduce light output by as much as 50% while also shortening system life.
Consequently, when sourcing retrofit LED lamps, or LED luminaires to replace existing fittings, it is important to ensure they incorporate efficient thermal management.
Optics
The optical design of a retrofit lamp or LED luminaire will also have a significant influence on overall performance. The primary optics are frequently part of the LED source; with high-power LEDs this will typically be in the form of a simple dome that increases light output.
Secondary optics may then be utilised in directing light to produce a specific pattern, distribution or beam. LEDs are almost ideal point sources with no infrared in the beam, so light can be controlled with high-grade plastic optics which minimise losses and spill light. However, all optics will introduce some light losses.
Another characteristic of LEDs is direction of light output. Conventional sources spread light in a full hemisphere. A large proportion of the light, therefore, does not hit reflective surfaces and is uncontrolled, producing spill light and reducing efficiency. LEDs emit light in a semi-hemisphere so a standard collimator will distribute almost all of the light in the right direction, resulting in increased efficiency.
Longevity
As well as their efficiency, LEDs are valued for their long life. However, high temperatures, moisture infiltration and lack of surge protection can result in premature failure. The system must therefore protect against all three.. Usually the housing will contain the heat sink and control chip temperature, while a sealed housing provides ingress and humidity protection and the driver protects against electrical surges. The housing should also protect against vibration damage.
LED lighting manufacturers use Highly Accelerated Lifetime Tests (HALT) to predict product life, ideally with additional tests to gauge resistance to vibration, voltage variation, electrostatic discharges and high temperatures. Not all tests relate to lamp life and reliability though. For example, CE and ENEC/UL approvals only relate to safety. Consequently, HALT and environmental tests are the only real lifetime tests.
In practice electricians will rarely have time to explore all of these criteria. It is far more practical to select products on the basis of the quality and reliability of a known manufacturer with a well-established reputation for LED lighting.
