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New to LED Lighting? Take a look, "LEDs for Dummies"

LED Lowdown

LED is nowadays considered to be a mainstream lighting technology. Hugh King of Thorn Lighting explains everything you ever wanted to know about LED but were afraid to ask.


Despite its extensive use for more than 40 years the LED remains a mystery to its millions of users. Even those who know that an LED is a solid-state semiconductor device that emits light when an electric current passes through it, probably only know a few of the attributes that contribute to one of most remarkable growth patterns the lighting world has ever known.

Apart from continued improvements in brightness, life, colour quality and the range of shapes and sizes, the efficacy of current LED light sources for general lighting are now above 100 Lm/W compared to the limited red indicator of the late 1960s and its useful life has been extended to around 50,000 hours. In short, it is now considered a mainstream lighting technology.

How they work

The LED consists of a diode chip made of semi conducting material, encased within an epoxy, plastic, resin or ceramic housing and attached to the electrical circuit. This housing may be in a variety of shapes and sizes and helps determine the optical characteristics of the LED. Generally a second optical controller is used in the form of a lens mounted on the epoxy housing.

As well as the optical control of the system, designing using LEDs requires careful thermal management.

Whilst the ratio of light to heat produced by LEDs is much higher than for an incandescent lamp they do still produce heat, not as infrared radiation (IR) – the light beam is cool – but conducted through the back of the chip.

Without adequate heat sinking or active cooling, the junction temperature of the internal diode will rise, shortening lifetimes. Finally, to operate LEDs requires a regulated direct current supply, usually supplied by a self-contained “driver” which converts the AC mains electricity to the correct DC voltage. This driver must be correctly matched to the LED it is powering as incorrect voltage and current will at best provide poor light, and may severely reduce the life of the LED or cause instantaneous failure of the system.

The driver can provide a quite advanced level of control, allowing dimming down to 0%, and with a cluster of different coloured diodes and the use of technology such as DMX protocols linked to a light mixing console extremely complex lighting effects may be produced.

So the LED is not a true “lamp”, generally being supplied as a complete electrical and optical system, which is then embodied into a housing – hence the term “solid state lighting” as the light is emitted from a block of semiconductor material rather than from a vacuum or gas enclosure.

How do LEDs produce white light?

The light produced by an LED is monochromatic and the colour of the emitted light depends on the material used in making the LED and varies from red through orange, yellow, green and blue.

To produce white light two methods are used. The best, in terms of quality of the spectrum of light, is produced using a blue or UV LED with a phosphor coating (typically yellow). This is termed a phosphor down conversion and in principal is similar to a fluorescent lamp.

LED packages may also be configured to produce mixed or blended light, either through the use of three or more different coloured LEDs (i.e. red, green and blue) or through a multicolour

LED that incorporates two or more different colour chips within the same epoxy package.

What is the quality of the white light?

The colour-rendering index (CRI) is a measure of the ability of the lighting source to reveal the colours of an object. Higher numbers are better, up to a maximum of 100. Current LEDs used for interior lighting have CRIs around 90+.

What is binning?

An additional consideration is that the process for producing LEDs cannot accurately reproduce LEDs with identical colour appearances, especially for white LEDs.

To overcome this a process called binning is used, in which the LEDs are sorted into groups of similar colour appearance.

The level of precision required depends upon the application. Basically, the shorter the throw of the light before illuminating a surface or where numerous light sources are in the field of view, the more critical the binning.

Hence, for an application such as wall washing, it’s essential for all the LEDs to match closely to give a consistent appearance.

However, if floodlighting a statue with one or two projectors from an adjacent building the binning process is less important, as individual differences would be less obvious.

What is the lifetime of a LED?

A benefit of LED technology is the relatively long life of the systems, manufacturers quote upwards of 50,000 hours life with 70-80% lumen maintenance, but other factors should be considered. Whilst an LED may produce light for a long period the amount produced will deteriorate over time rather than suddenly “burn out”. This is mainly due to discoloration of the epoxy housing of the LED over time.

Lumen depreciation and LED life varies between manufacturers, depending on how the LED is integrated, its operating temperature and the drive current, so makers data should be consulted.

How is an LED luminaire used?

Arrays of LEDs have proved ideal for signage, guidance and effects lighting and their recent use now extends from streets and building surrounds to offices and even residential developments. Additionally, as the light produced is “cold” it has major benefits in museums and retail outlets where heat may damage displays. LEDs continue to build the future of light.

Key attributes

  • Saturated colours and high quality white light
  • High lumen output with high efficacy
  • Good optical control – directional light from a small source
  • Low maintenance – extremely long operating life. Slow failure rather than sudden outage
  • Durable – shock and vibration resistant
  • Cool light – no IR or UV emitted
  • Instant light with dimming and frequent switching capability
  • Absence of toxic substances (no mercury)

Source

 

Source: SELECT - CABLEtalk March 2010,

http://newsletter.voltimum.co.uk/onlineversion/?82dfe4d013f66794b928abx1308263x289

 

I graduated with honours in Electronic and Electrical Engineering a while ago now!. Since then, I have spent many years in design and also as an FAE

6 Aug 2010, 11:22

Comments

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0 Votes

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0 Votes

March 16, 2013 16:32

Footprint 48LQFP 7 X 7 X 1.4 Anyone have? Thanks

0 Votes