When I first saw an LED intended for general-use lighting in 2001 I thought it was a bad joke. At that time it was a ribbon strip of LED diodes, that were not bright and were of such a cold colour temperature that it seems more appropriate for a sci-fi film. I told my boss and mentor at the time, that “this is not the future of lighting! Its a toy…and nothing more”. I have since eaten those words a million times over…
LED stands for Light Emitting Diode.
Every lamp (geek speak for a light bulb) in the general-use lighting market, be it halogen, incandescent or fluorescent consists of an inert gas illuminated by an electrified filament. Except of course LEDs, for which there is no filament or gas. This is the reason they last so much longer than traditional lamps, there are no mechanical parts (filament) to fatigue and break.
For this reason, I used to describe every light fixture, no matter how exotic in its design, as a life support system for a lamp. As the weak link in the fixture was typically service and maintenance related to lamping. Currently, good LEDs make redundant the service and maintenance of light fixtures, beyond dusting.
So what Does makes a good LED?
NB: For the sake of brevity we are going to talk strictly about “white light producing” LEDs in this blog.
LEDs intrinsically make an immense amount of light in a small area immediate to the diode. In it’s infancy the LED manufacturers did not have access to focusing lenses a.k.a. “optics” to throw the light produced at nadir (more geek speak, it means the origin point) to where it is needed. Nowadays optics exist that can throw the light produced by a 3 watt LED (equivalent to a 35 watt halogen lamp) 25 linear feet with great accuracy.
Type of LED
A good quality LED is sorted and categorized by voltage and color. When an LED is ‘born’ it can be controlled for color to a range of 100 degrees K (Kelvin, is alternate temperature measurement to Celsius or Fahrenheit). The temperature range for visible light is 10,000 degrees K, so the tolerance for controlling the light output is within 1%. For the manufacturers, it requires some discriminating quality standards to ensure that the lights produced in a batch are all of the same colour temperature. Accuracy in the manufacturing process though now a fine science, was in the infancy of LED product manufacture a relative gamble, and still is today for low cost manufacturers.
All white LEDs are born blue and are tinted with phosphorus to warm them up to the appropriate color temperature. The more you tint the LED’s the less output they have This can help explain why back in the LED’s infancy, many looked very blue and had unreliable color temperature matching.
The colour temperatures in greatest demand are:
3200K – incandescent-like warm white slightly yellow light output;
4200K – is the most popular currently as it is the best marriage of light output and agreeable color temperature. It is the standard in the industry; and
5000K – being the most bright while still staying out of the blue spectrum, but can be challenging on the eyes.
Most LED diodes on the market allow multiple driving (see next section for a discussion of driving) option and can be run at 1, 2 or 3 watts. The option utilised ultimately effects the output and longevity of the LED.
The chart below shows approximations for life span based on driving (a.k.a. power input):
|Power Input||Electricity Consumption||Approx. Lifespan (hours)||Approx. Lifespan (years with 24/7 usage)|
|350 mA||1 watt||100,000||11.4|
|500 mA||2 watt||50,000||5.7|
|700 mA||3 watt||10,000||1.1|
It is also important to note that 2w doesn’t mean it is twice as bright as 1w, nor is 3w 3x times the brightness. The calculation is 2w = 1/3 brighter and 3w = 2/3 brighter, than a 1w LED. Based on this we are usually better off running more LED’s at 1w than less at 3w.
An important note is that an LED rated at 100,000 hour lifespan, doesn’t mean it will die at the 100,000 hour mark, think of it more like a bad rockstar….it just slowly fades away, while continuing to operate at a diminished output.
Driving (Power Input)
LED’s run best at fixed amperage or voltage, depending on the type of LED. We will discuss the two most typical types here:
Tape lighting (long cuttable strips) run best at 24v DC constant voltage. The amperage fluctuates based on the wattage used, but voltage is CONSTANT, allowing for even distribution of power throughout the strip.
LED diodes and clusters of small LEDs run best at fixed amperage. Meaning that each diode must receive the same amperage from the transformer whether there are 3 or 14 LED units.
Running an LED at an inappropriate voltage/current guarantees it will not meet its maximum life expectancy.
LED’s have been doubling in power every 36 months since the 80’s, a similar development curve to computer chips (see Moore’s Law). A 1w Diode in 2002 was putting out 45 lumens per watt. Today a 1w diode outputs 90 lumens and can be driven at 500-700 mA up to 132-165 lumens.
The most neglected component in LED product design is thermal management. almost without exception, a heatsink is used to remove the heat produced by an LED.
To be clear LEDs make heat, not a lot compared to the old conventional lamps but the little heat the LED creates MUST be managed to ensure the LED can meet its maximum life expectancy. 45 degrees celsius, (113 Fahrenheit) is the temperate you want to stay below if you want an LED to stay cool enough to achieve its maximum lifespan.
There are LEDs that use fins, fans and even liquid coolant to capture and diffuse heat. However, a high quality metal heatsink is the most reliable methodology.
A good high quality heatsink is a piece of metal that makes direct contact with the LED diode and conducts the heat away from the diode. They are usually made of aluminum and brass. The brass increases exponentially the metallurgical heat dissipation qualities of any heatsink.
In the last century, when incandescent lamps first came on the scene people retrofitted gas lights and ran wiring instead of gas piping. After some time designs came out that specifically catered to the needs and uses of the A frame incandescent lamp.
A similar retrofit has been happening in the last several years as incandescent and fluorescent lamps are being replaced with LED versions. In many ways this type of retrofitting is like taking a station wagon and turning it into a pick up truck. It can be done but efficiency is lost in the transition.
When an intrinsically LED product is designed with the correct driving option, the appropriate thermal management and the minimized dimensions marrying to the scale of the Illuminating emission, it produces what we at DDM Lighting like to call a Harmony of Light design.
What does the future hold for LED’s?
My crystal ball predictions are as follows:
– 120v LED’s will be the norm, the technology is available and is starting to be developed en masse. We are specifying fixture currently that have a handle on this and are bright and reliable;
– Within the next decade the majority of the public lighting globally will be LED. Imagine a world of ‘many smaller lights’. It is happening mostly in new construction projects but it is only a matter of time. Just as railroads replaced steam engines, so LEDs will replace old style lamping; and
– A greater marriage of solar and LED’s in pre construction projects. If you are running a house with 75 halogen pot lights (the current industry standard) at 50w each, you require 3750w of power. However, the same 75x pots with a 3w (equivalent to a 50w halogen) LED would require only 225w and could easily be powered by a 250w solar panel, which are readily available and inexpensive in today’s market.