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Five things you should know about LED lights

LEDs are a new form of lighting and different to incandescent and fluorescent lighting in several ways. This guide is

designed to help you understand some of those differences so that you can make better informed choices about yourlighting.

1. Light output

In the days of incandescent bulbs we didnt think too much about light measurements we simply knew fromexperience whether we needed a 40W or a 75W bulb. As CFL (Compact Fluorescent) lights came in, we relied on the

labels on the box to say that a certain CFL would replace a 60W bulb. Now with LEDs its different again.

The total light output of a lamp is called the luminous flux and is measured in lumens. The US GovernmentsEnergy Star website has a table giving the typical light output of incandescent bulbs in lumens, which is copied

below:

How much light do I need

Incandescent Bulbs (watts) Minimum Light Output(lumens)

40 450

60 800

75 1,100

100 1,600

150 2,600

(source: http://www.energystar.gov/index.cfm?c=cfls.pr_cfls_lumens)

To add to this, a 50W halogen downlight produces a minimum of 900 lumens, and a 4-foot (1200mm) T8 fluorescenttube produces about 2850 lumens.

However, what is more useful is to get an idea of how much light will fall on a working surface. This measurement,

called illumination, is the light detected at a particular distance and angle from the light source, and is measured in Lux.

LED lights are likely to produce a higher illumination compared with an incandescent or CFL with the sametotal light output, because unlike incandescents or CFLs, LED lights are directional.

2. Beam angle

LED lights are directional. This means that if the light source is mounted on the ceiling, more of the light will fall

directly below the lamp. Incandescent and fluorescent lamps can also be made directional if they are placed in a

parabolic reflector.

The advantage of directional light is that more of it goes where it is most useful the working surface or living areas.

However, it can mean that the ceiling itself is darker as a result.

Most LEDs mounted on a flat surface have a nominal beam angle of 120. This will typically be the case for LED tubes

and downlights which dont use a lens. With a lens, the beam angle can be focused to anywhere from 15 to 90

depending on the application.

In some cases it is desirable to increase the beam angle for example to get more uniform illumination in offices wheretubes are used, or when replacing incandescent or compact fluorescent bulbs in bedside lamps or suspended lamps. This

can be done to a limited extent by using diffusers or to a greater extent by mounting multiple LEDs on a curved surface.Some LED bulbs have multiple flat surfaces pointing in different directions giving close to 360 illumination.

3. Colour

When buying LEDs two measurements of light colour will be important: Colour temperature (measured in Kelvin) andcolour rendering (measured as CRI or RA).

LEDs for general lighting will usually come in either a Warm White colour or something like Pure White or Cool

White. The colour temperature in Kelvin corresponds to the light emitted by a black body radiator when heated. Justlike with a rod of iron in a fire, at lower temperatures you get a reddish glow and at very high temperatures you get a

pure white or blue-green colour. Thus Warm White usually means somewhere in the 2700K to 3500K region, Daylight

White is in the 4000K to 5500K region and Cool White is in the region above 6000KIf you are used to the slightly yellowish light from an incandescent globe or a halogen downlight, you will find the

Warm White LEDs most similar. But comparing the light colour of LEDs with other light-sources is notstraightforward. Visible white light is made up of many different wavelengths of light corresponding to different

http://www.energystar.gov/index.cfm?c=cfls.pr_cfls_lumenshttp://www.energystar.gov/index.cfm?c=cfls.pr_cfls_lumens

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colours. When passed through a prism (or many rain droplets) you can see these different colours as the colours of therainbow. If you look at the relative power of these different wavelengths you get a spectral distribution curve.

Here are some of the spectral distribution curves for different light-sources:

Note that daylight has a fairly even curve, with a bias towards the blue end of the spectrum. Incandescent is also fairly

even with a steady bias towards the red end. Fluorescents and LEDs tend to be a bit more spiky. This can affect howwell colours are perceived, as a colour is only visible in the light of that corresponding spectrum. Fluorescent tubes

typically have a Colour Rendering Index (denoted as CRI or RA) of between 60 to 80, where 100 is the maximumpossible, corresponding to daylight. A good quality LED should also have a CRI of over 70.

4. Heat management

LEDs will, over time, get dimmer. The speed at which this occurs is very much affected by how hot they run. Thecooler the LEDs, the longer they will stay bright. To run the LEDs cool, two things are important - an efficient heat-

sink to release heat into the atmosphere, and a good route for the heat to travel from the LED to the heat-sink. LEDswhich are mounted directly on the circuit-board - SMD (Surface Mounted Device) and COB (Chip On Board) tend to

have better heat-management than DIPs (Dual Inline Polarity) for this reason. (See Chip Types below)

For larger luminaries such as flat panels or tubes, heat is usually managed fairly well as there is a large surface area. Butit can become an important issue for smaller lamps - especially down lights which often have to generate a large amount

of light in a confined space.

5. Chip types

As mentioned above, the LEDs come in different forms. The most common types are SMD and DIP.

SMD chips are usually small square yellow chips which are mounted directly onto the circuit-board in the same way

that computer chips are mounted. The machinery involved in mounting the chips is the same as for computers and is

not particularly cheap. However it does mean a good and very consistent contact with the board with little variation.

DIPs usually appear as round glass-covered chips raised up from the board. Before mounting on the board they appear

like a tiny chip in a glass bubble with two wires poking out. These wires poke through holes in the circuit board and aresoldered on the rear of the board - either by hand or by machine.

More recently there have been more and more COB based luminaries appearing. These appear like a continuous yellow

area on the board and are made up of many tiny individual LEDs joined together under a layer of triphosphate.

DIPs can offer a slightly better beam angle than SMDs, but tend to have less effective heat management than SMDs andCOB LEDs.

Copyright Mike Lowe, PowerUp Green PTY Ltd.

email: mike@powerupgreen.com.au website: www.powerupgreen.com.au

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