Temperature and its control have a significant impact on the quality and lifespan of an LED. To ensure LEDs operate at their optimum capabilities, effective thermal management is essential.
The principal role of thermal management is to extract the heat from the LED module and dissipate it into the surrounding air. This can be done through conduction, convection and radiation and different approaches are being taken to this issue across the industry, with varying degrees of success.
Optimum thermal management is achieved when the number of thermal conductive interfaces between the LED and its heat sink are reduced and the thermal resistance between these interfaces is minimised. In addition, careful consideration needs to be given to the heat sink material, its surface area, geometry and roughness as well as the management of airlow around the LED as a whole.
The key to creating an LED lighting scheme, that looks good for years to come is in ensuring that, over their lifespan, all of the lamps are performing within an acceptable tolerance in terms of colour deviation. To define ‘acceptable tolerance’ from lamp to lamp, LED manufacturers have adopted the MacAdam ellipse and SDCM (Standard Deviation of Colour Matching) measurement of colour consistency. MacAdam EllipseM
The MacAdam ellipse is a system of colour measurement. It measures how much colour variation is possible around these axes, before the human eye detects a colour change. A series of ellipses can then be drawn around any target colour, and the closer any given lamp is to the target, the less colour deviation will be experienced when these lamps are placed side by side in an installation.
The distance from the target point in each ellipse is measured in SDCM. An SDCM of 1 step means that there is no colour difference between LED chips, 2-3 SDCM means that there is hardly any visible colour difference. Colour consistency of 7 SDCM is accepted by the market and in line with Energy Star requirements.
The Color Rendering Index (CRI or Ra) is a quantitative measure, which rates a light source’s ability to reproduce the colours of objects faithfully. In order to objectively compare the colour rendering properties of any light source, the CIE’s standardised measuring method operates on a scale from 0 to 100 (poor to excellent). The colour change of 14 standard colours is calculated when an object is exposed to a specific light source and then this is compared to a reference illuminant of the same colour temperature (a black body* is used for colour temperatures up to 5000K and daylight above that). The CRI for a pair of light sources can only be compared if they have the same colour temperature.
Life & Lumen Maintenance
LED lamps are different to Incandescent, Halogen and Fluorescent lamps. These older technologies base their life declarations on average rated life at 50% survival (this is a legal European requirement under IEC 60064). This has been the standard approach for some time and does not take into account the amount of light those lamps emit, only the number that survive to a certain point.
However, the semi-conductor industry has always rated LEDs differently, and since the recent boom in LEDs for mainstream lighting there has been some confusion over these differences. Megaman is now seeking to clarify its position for its ranges and categories of lamps.
Currently Megaman offers an Economy LED Series and Professional LED Series in the UK.