LED Lighting Notes

Comments on LED Lighting.

 

What They Are:

 

When electric current is passed across the junction of two dissimilar crystals light and heat is given off.    This is is the basis of Light Emitting Diodes (LED).

However if too much heat is created the crystals will break down and in practice this means that the power of an LED light is limited and that they will operate satisfactorily only below an ambient temperature of about 40 degrees C.

 

Power of  LED Lights available end  2011

 As the current is increased more light and heat is generated but the current technology cannot dissipate unlimited heat and this limits the power of LED chips.

For replacement bulbs such as an MR16 the maximum power  is 6 or 7 watts unless mechanical cooling such as a fan is used.

To get more power, a light fitting can be designed to use multiple LED’s or with more elaborate heat sinks that are able  to keep the LED chips cool.  Our most powerful downlight is 25 watts.

 

Life of an LED

 

There are only a few companies making LED chips such as Cree, Osram, Phillips, etc and good quality LED light fittings and bulbs use chips from these manufacturers.

The chip manufacturers expect up to 100,000 hours life but this is reduced dramatically by the current applied to the chip.

350 milliAmps will give maybe 50,000 hours but higher wattage units may be applying 900 milliAmps and the life will be reduced to 20,000 hours.   This is the theoretical time for the light output to decay to an unacceptable level, but unless the chip is damaged it will continue to emit light.

 

Brightness

 This is where the LED story becomes confusing.    The amount of light given off at a source is measured in Lumens, but  the amount of light hitting a surface is depends on the distance between the light source and the surface and is measured in Lux.

The Led chips currently used can convert 1 watt into about 80 Lumen.  Quite often the power lost in the driver is added to the lamp's rating so not all the watts going into the lamp are being converted to light. ie a "6 watt" lamp may only have 5 watts actually reaching the crystal.  This is not always admitted so a more reliable measure of the brightness of the lamp is the lumens claimed.

For example MR16 halogen replacement lamps give off between 200 and 450 lumen.  For comparison a 50 watt halogen lamp gives off 950 lumen. So however you package it a change form 50 watt halogen to 6 watt LED bulbs will not give the same amount of light in a room

However if the light output is focussed into a narrow beam then the LED lamp can produce an area with same Lux as a halogen, so the claims that a 3 watt LED is equal to a 50 watt halogen are sort of true if you only want a one meter spot of light.

 

Use of LED’s.

 These are extremely useful in commercial applications where the high capital cost is offset by avoiding the cost of changing lamps and high hours usage gives excellent power savings.   The payback time can be reasonably short. 

Our 15 watt and 25 watt downlights are used mostly in this area, and the light levels and quality of light is enhanced.

 For the home LED’s are excellent for use in ambient lighting and garden lighting.  They are ideal for stair wells, corridors, and to provide mood lighting in lounges and tv areas.  The feedback is that the light quality is superior to halogen lamps

Using LED bulbs in standard garden fittings can reduce the power used dramatically.  A garden can be lit with less than 100 watts, so the garden or drive lights can be left on every night with the power use of one old style bulb.   5 watt LED bulbs can be fitted directly into existing fittings as the 12 volt wire wound transformers will supply the correct voltage.  Only 5 watt is recommended as more power will overheat the bulbs in the watertight fittings and cause  them to fail