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Why use LEDs?
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What is an led?
A light-‐emi)ng diode (LED) is a semiconductor light source. The LED consists of a chip of semiconduc;ng material doped with impuri;es to create a p-‐n junc(on. As in other diodes, current ﬂows easily from the p-‐
side, or anode, to the n-‐side, or cathode, but not in the reverse direc;on. Charge-‐carriers—electrons and holes—ﬂow into the junc;on from electrodes with diﬀerent voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. This eﬀect is called electroluminescence and the colour of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. LEDs present many advantages over incandescent light sources including lower energy consump;on, longer life;me, improved robustness, smaller size, and faster switching. One of the key advantages of LED-‐based ligh;ng sources is high luminous eﬃcacy. White LEDs quickly matched and overtook the eﬃcacy of standard incandescent ligh;ng systems. In 2002, Lumileds made ﬁve-‐waQ LEDs available with a luminous eﬃcacy of 18–22 lumens per waQ (lm/W). For comparison, a conven;onal 60–100 W incandescent light bulb emits around 15 lm/
W, and standard ﬂuorescent lights emit up to 100 lm/W. Current technological developments have seen commercially available LED chips with luminous eﬃciacy of up to an ini;al 150lm/W and higher in lab condi;ons. Vexica
Key benefits of using leds
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Efficiency: LEDs emit more light per watt than incandescent light sources. Their efficiency is not
as dependant on shape and size and have and element of flexibility in the product design. Colour: LEDs can emit light of an intended colour without using any colour filters as traditional
lighting methods need. This is more efficient and can lower initial costs.
Size: LEDs can be very small and are easily populated onto printed circuit boards.
On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness
in under a microsecond. Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps
that fail faster when cycled often, or HID lamps that require a long time before restarting.
Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering the
forward current.
Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that
can cause damage to sensitive objects or fabrics.
Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt failure of
incandescent sources.
Lifetime: LEDs can have a relatively long useful life. Almost all chip manufacturers quote
50,000 hours of useful life, though time to complete failure is usually longer. Fluorescent tubes
typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use,
and incandescent light bulbs at 1,000–2,000 hours.
Shock resistance: LEDs, being solid state components, are difficult to damage with external
shock, unlike fluorescent and incandescent units which are fragile.
Focus: The solid package of the LED can be designed to focus its light. Incandescent and
fluorescent sources often require sizable reflectors to direct light output as usable light. In
most cases LEDs are able to achieve this in a more effective and system efficient manner using
optical lenses or LED reflectors.
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LED reliability data
Solid state devices such as LEDs are subject to very limited wear and tear if operated at low currents and at low temperatures. Typical life;mes quoted by the LED Chip manufacturers are 50,000 hours but heat and current seZngs can extend or shorten this ;me signiﬁcantly. The most common symptom of LED failure is the gradual lowering of light output and loss of eﬃciency. To quan;ta;vely classify life;me in a standardized manner it has become widely accepted to use the terms LM80 and LM70 which is the ;me it will take a given LED to reach 80% and 70% light output respec;vely, although the higher LM80 standard is fast becoming the norm. The LM80 test procedure addresses only one factor in the life of an LED luminaire – lumen deprecia;on of the LED device over the prescribed test period. When LEDs are installed in a luminaire or system, there are many addi;onal factors that can aﬀect the rate of lumen deprecia;on or the likelihood of catastrophic failure. These include temperature extremes, humidity, moisture incursion, voltage or current ﬂuctua;ons, failure of the driver or other electrical components, damage or degrada;on of the encapsulant material covering the LEDs, damage to the wire bonds that connect the LEDs to the ﬁxture, and degrada;on of the phosphors. LM-‐80 prescribes uniform test methods for LED manufacturers under controlled condi;ons for measuring LED lumen maintenance while controlling the LED’s TC or case temperature, the DC forward voltage and forward current to the LED. LM-‐80 requires 55 °C, 85 °C and one other TC chosen by the LED manufacturer. It also requires lumen maintenance data out to at least 6,000 hours of constant DC mode (not pulse mode) opera;on. Based upon LM-‐80 data, LED manufacturers then extrapolate lumen maintenance out to tens of thousands of hours and while LM-‐80 does not specify the method, many LED manufacturers use more conserva;ve exponen;al extrapola;on due to the behavioural nature of LEDs and most electronic components. The SSL SubcommiQee is working on TM-‐21 which will standardize this extrapola;on method. Vexica
Designing a reliable
LED product
Most people are now aware of the increased life span of LEDs and the beneﬁts associated with them. However, this does not mean that all ﬁnished luminaires automa;cally mirror the predicted degrada;on curves of the chips they are based around. When LEDs are installed in a luminaire or system, there are many addi;onal factors that can aﬀect the rate of lumen deprecia;on or the likelihood of failure. These include temperature extremes, humidity, moisture incursion, voltage or current ﬂuctua;ons, failure of the driver or other electrical components, damage or degrada;on of the material covering the LEDs, damage to the wire bonds that connect the LEDs to the ﬁxture, and degrada;on of the phosphors. Bearing this in mind there are several key factors to take into account to ensure longevity and reliability when designing an LED based luminaire: •  Voltage and current management •  Thermal management •  Adequate environmental protec;on •  Use of high-‐quality of components from reputable suppliers •  Proper manufacturing processes •  Suitable provision for intended installa;on method and opera;ng environment Component Quality Manufacturing Consistency Driver Electronics Reliable LED Products Mechanical Design Thermal Management Vexica
Colour consistency
As semiconductor products, LEDs are sorted in a process known as binning, to ensure consistency of colour and quan;ty of light so that designers can specify precisely the colour and brightness they need for their par;cular applica;ons. Targe;ng the smallest binning quadrangle at the LED ini;al test point normally 25 C at 350mA (t0, or beginning of the life cycle) is a cri;cal requirement to maintaining ligh;ng colour consistency in the ﬁnished assembled products. Selec;ng a realis;c shippable bin region that is both economical and consistent for the applica;on requirements can help ensure the design colour consistency for the life of a product. Colour binning selec;on is one of the most important decisions designers need to factor into their LED ligh;ng design. How are bins speciﬁed? In 1931, the CIE (Commision Interna(onale de l’Eclairage, or Interna;onal Commission on Illumina;on) established a standard with respect to human colour vision (CIE 1931 x,y) that deﬁnes the colour metrics cri;cal to consistent ligh;ng quality. The standard is rather complicated and involved; for instance, metrics must be measured at a uniform luminance level, because the human eye sensi;vity to colour is reduced as the brightness of any light source is increased. LED manufacturers all have slightly diﬀerent binning methods, some being ;ghter than others, but with suﬃcient care it is possible for a luminaire manufacturer to ensure consistency of ﬁZng colours even when specifying diﬀerent makes and model of LED in the same product. However, ANSI binning systems are more commonly being used within the LED ligh;ng industry to support ligh;ng manufacturers. Vexica
Colour Consistency and
MacAdam Ellipses
An important factor to consider is that the human eye cannot dis;nguish minor varia;ons in colour, although LEDs can be binned very speciﬁcally. In a series of experiments in the 1940’s and 50’s David MacAdam (a colour scien;st who worked for Kodak) and a team of scien;sts discovered and documented this principal. MacAdam set up an experiment in which a trained observer viewed two diﬀerent colors, at a ﬁxed luminance of about 48 cd/m2. One of the colours (the "test" colour) was ﬁxed, but the other was adjustable by the observer, and the observer was asked to adjust that colour un;l it matched the test color. This match was, of course, not perfect, since the human eye, like any other instrument, has limited accuracy. It was found by MacAdam, however, that all of the matches made by the observer fell into an ellipse on the CIE 1931 chroma;city diagram. The measurements were made at 25 points on the chroma;city diagram, and it was found that the size and orienta;on of the ellipses on the diagram varied widely depending on the test color. These 25 ellipses measured by MacAdam, for a par;cular observer are shown on the chroma;city diagram opposite. These ellipses or “zones” are measured in seven steps of tolerance, the greater the number the greater the dis;nc;on between colours. It is widely regarded that anything within 2 steps is largely indis;nguishable to the human eye. Therefore when selec;ng bins of LEDs we must also consider MacAdam’s principle to ensure colour consistency. This is of par;cular importance where several diﬀerent manufacturers LED chips are used on the same project, as binning structures can vary signiﬁcantly between manufacturers. Vexica
Using optics & reflectors
PuZng together a func;oning ligh;ng solu;on always includes the speciﬁca;on of the op;cal performance of the product, LED technology being no excep;on, it may be very challenging to specify and choose the best components to fulﬁl the requirements set. The main reason is that there are no standards available. Lack of standards easily leads to varying interpreta;on of the parameters by diﬀerent suppliers and it may mean that a wrong or non-‐op;mal component is chosen. Because of these reasons it is important to consider the real content of the op;cal parameters such as FWHM, Eﬃciency, Materials and Life;me, to allow customers to beQer understand which op;cal component is best for which applica;on and how to choose it in a correct way. It is extremely important to use op;cal components which are made of documented, high-‐quality materials. Automo;ve or medical grade PMMA (acrylic) or PC (poly carbonate) are proven to be stable and reliable in all normal LED applica;ons. It is necessary to advise against using cheap lenses made of PS (Poly Styrene) or SAN (Styrene Acryl Nitride) or similar low cost plas;cs. Molecule chains can get cracked and molecules change their appearance and performance if the plas;c is not of suﬃcient grade. Similar considera;on must be applied to reﬂectors with metal coa;ng and protec;ve surface treatment. Very osen the ini;al cheap price may turn out to be very expensive in the long run, when lenses turn yellow, milky or the metal surface turns maQ and the eﬃciency and appearance of the light changes and falls outside of the original speciﬁed requirements. Vexica
The Future using LEDs
Ø  Less LEDs, more LED light
output for less wattage
Ø  Higher general efficiency
Ø  Improved Colour Rendering
Ø  Integration with Solar & Wind
Power
Ø  Integration of intelligent control Ø  Reduction in payback periods
systems
Ø  More application uses
Ø  The use of retro-fit lamps
Ø  Standardisation of LED
integration
Ø  The integration of LED modules
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For more informa;on please contact: Vexica Technology Limited [email protected] www.vexica.com Vexica