When selecting LED lighting for industrial, marine or oil and gas applications, buyers are often bombarded with a plethora of technical information from suppliers. Buyers should therefore benchmark suppliers based on key technical parameters, says Alan Mellows
LEDs have now reached a level of cost and performance that make them attractive in many lighting applications. Market studies forecast that by 2020, almost 50% of all new and replacement light source unit sales will be based on LEDs.
Therefore, it is critical that buyers fully understand how to assess and compare the key technical parameters of LED luminaires (and traditional light sources) from different suppliers. This article outlines the most important parameters that should be considered.
Light output and luminous efficacy
The performance of an LED is often measured in terms of lumen per watt or luminous efficacy. The efficiency of luminaires with fluorescent tubes is explained using the Light Output Ratio (LOR), which indicates how efficient the optics are. For these luminaires, the installed power in watt is often used as a measure of the luminaire’s light output.
However, LED luminaires only use the total luminous flux. The rated lumen value from an LED module may therefore give an inaccurate picture of how much lumen you actually get from the luminaire.
Life of an LED fitting
When comparing different lighting products, consider the life of the complete LED fitting, ie both the LED driver and the LED module. The life of the LED driver is often the ‘bottleneck’ of the system design. For example, if the driver lifetime is limited to 50,000 hours at the rated ambient temperature, and the life of the LED module is longer at that same temperature, the life of the complete LED fitting is defined at 50,000 hours.
Real lifetime curves
The temperature of the LED can become very high, which causes the LED to gradually emit less light and reduce the lifetime of the LED. Proper heat management is key to controlling performance and LED lifetime. In luminaires, the LED is cooled by a heat sink, whose size and design determine the life of the LED. It is therefore important to request ‘real lifetime curves’ at different ambient temperatures from your supplier.
Colour quality and tolerance
Not all LEDs have a full colour spectrum and so special attention must be paid to their colour quality and colour rendering capabilities. Otherwise, a lighting installation with visible colour differences may be the outcome.
Colour temperature indicates whether a light source is perceived as warm, neutral or cool white. LEDs can be produced in all colour temperatures, which are characterised by the dominating wavelengths. The colour temperature of an LED is defined by the blue and yellow peaks in the spectrum.
LEDs are manufactured with many different colours and colour temperatures. LEDs are grouped into ‘bins’ that share the same colour characteristics.
One process for defining the colour tolerance of an LED relates to the human eye’s colour sensitivity via the MacAdam ellipses model. The ellipses are mapped onto the binning structure in the colour diagram and their size corresponds to the LEDs colour tolerance. The size is measured in steps: the more steps, the bigger the tolerance and the easier it is to spot a difference in colour. Generally, a three-step ellipse is considered a good colour tolerance.
Colour rendering capability
Light sources render colours differently depending on the colour of the light already present in the light emitted from the source. This effect is measured by the colour rendering index (CRI or Ra), which is the average value of the light source’s ability to render eight standard colours on a scale from 1 to 100, where 100 is the best.
For indoor applications, a Ra of 80 is considered good. Look for a supplier that is very specific about colour quality and rendering. This is important if an LED luminaire needs replacing after five years and you require an identical replacement.
Perceived lux levels
The perceived lux levels from LED luminaires may differ from the actual light measurements. The standard method of calculating and measuring lux levels is based on light levels during the day. However, luxmeters do not read the full spectrum of light.
As a result, a lighting installation may have more LED fittings installed than is necessary to achieve the required (perceived) light levels. Conversion factors should be used which vary depending on the type of light source. This conversion factor is multiplied by the lumen output or lux level of the light source to arrive at the perceived light level for a given installation.
Total cost of ownership
Too many purchasing decisions are made based on the capital costs of a lighting installation, ignoring the operating costs over the life of the installation.
By considering the total cost of ownership ie initial investment, energy costs, lamp replacement and maintenance costs, etc, buyers can estimate the payback period. In most cases, the total cost of ownership of an LED lighting solution will be lower than a conventional lighting solution.
Alan Mellows of Glamox International.