LED Luminaries

LED news and insight from our technology editor Steve Bush, who has been messing with LED lighting for more years than he would care to remember.

Why can LED not yet rival halogen?

The way LED bulbs give off heat, and the way halogen bulbs give off heat are completely opposite. Halogen bulbs give off heat from the light they output, warming up the rooms they light. LEDs give off heat from behind the light source, and if this is not dispersed correctly the LED will lose it’s brightness and eventually stop working.

The way LEDs disperse their heat is by use of a heatsink. The heatsink is a surface of metal, usually aluminium, attached to the LED. The heat is transferred into this heatsink and then into the air. The amount of surface area you need to disperse heat from an LED is largely debated, but the generally agreed rule is that you need 30cm2 per Watt of energy consumed by the bulb. For example, a 5 Watt bulb needs 150cm2 of surface area to disperse the heat.

The problem designers are having, is that to replace a standard halogen down lighter you need a 16W LED bulb. LEDs with this power do exist, however, you would not be able to fit an LED with a 480 cm2 heatsink into a halogen ceiling cavity. Until a new reliable way can be found to disperse heat produced by LEDs, which will fit into a compact area, lighting a home with LEDs will still require a larger amount of fittings.

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17 Comments

  1. humber
    April 27, 2009 06:51

    Why do not you install a fan between the PCB and LED circle plate, when the heat dispatch from the PCB, the fan blow the it to the air, and the LED circle plate will not be warmed up.

  2. GEOFF
    February 24, 2009 12:08

    Thanks Oliver… Interesting, especially in what is not said (as usual). I’m not an expert in this (more; as usual), but I will be pleasantly surprised if the rendering is better than what one sees from any other led array.
    Cynically I would expect that if you could make those tiny oleds cover the visible spectrum in cluster patterns of individual bands of colour. Who knows, say 20 bands from red to blue? Such that the overall effect was white light with a broad spectral coverage, then the efficiency would drop off and ruin the practical advantage (over QH, say)?
    I’m offering a simple circuit for panning an led white light generator from warm to cold to the circuit section. Sometimes we humans will accept a problem if we are allowed some control in its presentation.
    And another thing…. I don’t know how many out there might agree, but I like my lighting in two states: Full welly or a fixed dim level (usually alternative low powered lamps)… I never seem to want (or be bothered with) a variable dimming control!

  3. Oliver Sowerby
    February 20, 2009 11:37

    If you guys are interested, look up Organic LEDs. These do not give off just a concentrated spot of light, but a more even spread.
    I believe Philips have just finished manufacturing a flat screen TV using them. Below is a link to an image of a light designed by Ingo Maurer using them.
    http://www.luxurylaunches.com/decor/world_premiere_exclusive_table_light_with_organic_led.php

  4. Geoff Mitchell
    February 20, 2009 11:20

    Right Derek, let’s start a new blog on colour rendering! We all know how to get around it, but just when LEDs were getting affordable…..
    We can also throw in…. Anyone working with saving energy in lighting knows how a light diffuser behaves in reducing the light output of a once “efficient” source. Fluorescent lights get away with it because they have “area”, but LEDs are for all intents and purposes point sources.
    Where are the homogeneous LED strips or areas; flat and curved? Shadow is good, but it’s interesting how shadow focus affects our perception; too much and it tires the eye (good for depth detail); too little shadow and we drift into the scenery.
    So is our ideal relaxing light area the width of your thumb at arm’s length (sun and moon)?

  5. Derek Bond
    February 13, 2009 16:33

    The colour rendering of LED or flourescent lighting has not been given enough consideration in home lighting. The key point is that most home illumination is indirect. We are used to seeing by the black body radiation from the sun or from incandescent lamps. However if white light is synthasised from a few spectra then not only do the colours of some objects appear wrong the whole room may change colour depending upon the absorbtion or reflection coefficients of its contents. We are all getting fed up with the low light quality of the new low power bulbs for the same reason.The human eye and brain has spent thousends of years adapting to natural light, and does not respond well to being cheated.

  6. Geoff Mitchell
    February 13, 2009 11:31

    I’m sorry to upset this run, but solving heat sinking is relatively easy… just follow the rules. The major drag with leds is…. colour rendering.
    Remember those questions in science lessons at school about what coloured objects look like under different coloured lighting. Well if a particular frequency of light is not present then an object with that colour will not present its self correctly.
    A tungstan spectrum is broadly white noise whereas a white led is tuned to red green and blue peaks. Similar to flouresent tubes…. these are very good, but never quite as vibrant as a QH. All compared with the reference daylight.
    OK, street lighting, colour washes and torches no problem, but when it comes to living or working under LED lighting there is a “poorness” of quality which just bugs you. Roll-on the white laser! Now that gives me an idea…….

  7. chrisg
    February 11, 2009 17:56

    I think Jean-Louis has hit the nail on the head
    “You need to change the fixture, not the bulb…”
    LED lamps are just not suited to the small MR16 format. AR111 seems to be the fitting to use for LEDs. QLT make the U115-U lamp which gives 825 lumens for an input of 11W, nearly 74lumens/w, which is not far off the bare LED efficacy, and it has plenty of heatsinking as it is 4″ across http://www.qlt.it/public/Upload_PDF/PDF/8802%20componenti%20fabbricanti.pdf
    400 lumens for a 50W halogen came from Sylvania, one of the few to quote a lumen spec.
    http://ecom.mysylvania.com/sylvaniab2c/b2c/z_login.do but I have since found “special” high efficiency halogens that do give 600 lumens at 50W. Just 12 lumens/w compared to 6x as much for LEDs
    It is not the whole truth to say “Halogen bulbs give off heat from the light they output…”. Some of it yes, but most of the output is in the infra-red spectrum not in the visible. The lamps have dichroic reflectors so that the infra-red is deliberately not reflected into the beam but spreads into the light fitting. Neither people nor shop window displays take kindly to sitting under a focussed infra red beam.

  8. Oliver Sowerby
    January 29, 2009 18:03

    From any information I’ve ever read, an MR16 Halogen gives out 600 lumens at a height of 2 metres. 400 is way below that.
    The Luxon rebel has been able to produce 100 lumens per Watt for quite a while. The problem is getting reliable bulbs which emit this amount of power.
    The only MR16 LED bulb which I would feel confident in specifying at the moment is the Lamina 8W Sol series. This gives out 245 lumens. We have stocked bulbs from the company you have linked to before, and they last no-where near their claimed life expectancy.

  9. chrisg
    January 29, 2009 17:54

    This blog would make more sense if it were based on sensible numbers. Unfortunately it isn’t. A standard PAR16 50W halogen lamp outputs 400 lumens. To get 400 lumens from current high efficiency LEDs requires only 4W not 16W.
    http://www.seoulsemicon.com/en/product/prd/zpowerLEDp4.asp
    Look at the Solarox lamps here and you will see that they have cracked the problems of brightness/power/heat but not cost -yet. http://www.led1.de/shop/index.php?cName=led-spots-c-107
    EUR36.50 for 1 lamp is still a bit pricey but wait a while and the price will drop thanks to good old Cambridge Uni
    http://www.electronicsweekly.com/Articles/2009/01/29/45361/cambridge-r.htm

  10. Chris
    January 29, 2009 13:17

    A peltier effect heat pump requires yet more energy to move the heat which would ruin the LED’s overall efficiency. Same problem applies to any scheme for fan assisted cooling. Overall problem is not helped by the fact that it’s much easier to get rid of heat from something seriously hot like a halogen bulb than from something that needs to operate as close to room temperature as possible, like a LED.

  11. Oliver Sowerby
    January 29, 2009 12:58

    DC -
    The problem with adding extra systems to the cooling of LEDs is that these take up additional energy, making the LEDs less efficient.
    LEDs do exist which have a fan incorporated into them to cool down the heatsink. However, because these fans take up excess power, we found that the LEDs were not very bright for the amount of power which they consumed.
    Water is now being used to cool some ceramic heatsinks. This works both ways, as the water cools the heatsink and the heatsink heats the water.
    Raman – I believe copper is used in some, however the cost of manufacture is much higher than using aluminium. So to mass manufacture these would be a costly expense that consumers may not wish to pay. A colleague of mine found a copper/aluminium compound which had amazing thermal distribution properties, but again was very high in cost.
    Ceramic Heatsinks seem to be a likely alternative as they are expected to last longer in certain circumstances and can be used in very practical applications, as mentioned above.

  12. David Cutter
    January 29, 2009 12:53

    the other aspect is the power supply. When houses are wired with a low voltage supply, then conversion down to led levels will be cheaper and low power dissipation so as not to add to the heat dissipating problem. A 12V distribution system will be useful for the solar cells and battery technology that is fast approaching at reasonable cost (whatever that means).

  13. Raman
    January 29, 2009 12:46

    Why not use copper heat-sink instead of Aluminium?

  14. DC
    January 29, 2009 12:29

    Just thinking about the heat dissipation
    problem, has a peltier effect pump module
    been considered?

  15. Mikael
    January 27, 2009 21:53

    Yeah, that’s why computer heatsinks don’t work – all of those parallel fins just transfer heat back and forth without dissipating any into the air.

  16. Oliver Sowerby
    January 26, 2009 10:14

    The only problem with using circular plates as a heatsink for a downlighter, is that the plates will be horizontal to the LED.
    If the plates a placed horizontal to each other as well, then the heat given off from the plate below will heat up the plate above making it in-adequate.

  17. January 21, 2009 08:53

    Well, unless I am utterly mistaking, two circular platters of diameter 12.5cm will get you a total surface of 491cm2 and 5 platters of diameter 8cm will get you 500cm2.
    So getting the right dissipation surface is not really the issue here (assuming there is the appropriate air convection flow around the enclosure). The problem lies with the idea of using a “16W LED bulb”. Properly dissipating heat out of a retrofit LED lamp is not going to work.
    Retrofit LED lamps are just not flying. LED is a disruptive technology that require rethinking the fixture itself. After all, existing fixtures were designed around a particular light source (and lamp shape). So why should we stick to this design? Alternative LED downlighters are available on the market that are designed from the ground up around power LED, and have the adequate heatsinking. You need to change the fixture, not the bulb…

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