This is a very broad subject, so we will only be able to cover the basics here, but starting from the top:
The Light Emitting Diode
It is, as the name suggests, a diode - electricity flows from one substrate to another in one direction only. The clever bit is that, in doing so, it emits a photon - the basic unit of light. Sounds simple, but light emitting diodes have been around for over fifty years without troubling the market dominance of the incandescent filament based bulb. Why? Because producing light that is suitable for general lighting in offices and homes is a far cry from the early implementation of LEDs, such as red indicator lights on radios. Along the development path, various companies have filed a fair few patents. In general, the LED producers who hold the key intellectual property are able to produce the best LEDs.
So what is a good LED?
The key characteristics are efficiency, consistency, cost, colour temperature, CRI, optical characteristics and packaging.
LED efficiency is all about how much light, measured in Lumens, is produced per unit of electricity consumed, measured in Watts. So the measure of efficiency of an LED is expressed in Lumens/Watt and this is improving all of the time.
LED consistency is all about successful replication of all of the key performance parameters throughout production batches. LEDs, like wines, are sorted into bins. Each bin will meet specification tolerances and is priced by performance. So you can buy LEDs from a high price bin, where all of the LEDs in the batch are closely matched at the upper end of the production tolerance and these are expensive. At the other end of the spectrum, LEDs are available from bins which contain the production items which are at the outer limits of the specification. These are far cheaper.
Cost is self explanatory, except to say that you normally get what you pay for. If a product contains a high performance LED chip, but is very cheap, be suspicious. There is almost certainly some level of performance deficit.
Colour temperature is an indication of the colour of light which the LED produces. Expressed in degrees Kelvin, this measurement describes whether the light is very cold, towards the blue end of the spectrum, or whether it is warmer and closer to the red end of the spectrum. As a guide:
2500 - 3500K is warm white, similar in appearance to an incandescent light source
3500-4500 is approximated to day light
4500-6000K is cool white, which can appear very clean, but quite harsh
CRI is an acronym for Colour Rendering Index. It is a measure of the ability of a light source to accurately reproduce the colours of objects when compared with an ideal light source. It quantifies how accurately colours are represented. Low CRI lights make colours appear to be different from their appearance in natural daylight.
The optical characteristics of an LED chip are mainly defined by its primary optical packaging - that is, the lens system built on to the chip itself. When the diode produces light, the challenge is to project this light in an efficient way - and one which will be suitable for the final application of the chip.
LED chip packaging is the way in which the diode is mounted onto its substrate and prepared for use. As we will see in later articles thermal management is critical to performance over time and packaging has a notable effect on this. Other packaging aspects include primary optics, physical size and durability.
