What to look out for when sourcing a lighting supplier....
Making the transition to LED lighting can be a rewarding and cost effective solution saving you money, improving the quality of your lighting, reducing your carbon foot print and should assist you in saving time and effort on maintenance, as we've all heard LED's can last 50 000 hours. However, this is not always the case.
Solid state lighting has seen tremendous growth over the last 5 years opening the flood gates to a bevy of new entrants. Like any other industry some of these participants are serious and well informed suppliers and others are ill advised opportunists with a view that LED lighting is easy and all the same, blindly offering long term warranties on inferior quality products which are often incorrectly designed.
To help familiarize yourself with key insights to important information regarding key aspects of quality solid state lighting, we have listed a set of points below outlining key considerations you should take into account before purchasing LED as an end user or reseller.
Thermal Management - The key prerequisite for an LED luminaire to perform effectively and meet it's expected life span is effective heat dissipation. As we all know LED's biggest enemy is heat. Our focus at eLighting is to remove the heat from the source (LED on pcb) and dissipate it into the surrounding atmosphere and as far from the electronic driver and LED's as fast as possible. We have used aluminum extrusions to achieve this which have resulted in a light weight cost effective luminaire which easily achieves its expected warranty covered 5 year/ 50 000 hour lifespan while retaining it's original lumen and colour maintenance.
A full polycarbonate extrusion has made its way into the market to appeal to a more price senstive audience but often resulting in a higher outlay by the purchaser. Plastic and glass led tubes dissipate heat poorly, quickly resulting in product failure, poor colour and lumen maintenance. Thermal management is the most important aspect in LED lighting longevity.
Power Factor Correction-Power factor is the difference between working power and apparent power. A high power factor is beneficial to the user of the LED light, as well as the power utility supply electricity on an already constrained grid. A high power factor will improve the voltage supplied to the LED lamp, fully utilize the current carrying capacity with reduced losses resulting in a more efficient lamp saving on electricity. An LED lamp with a poor power factor is not efficient and is the cause of poor product performance and higher utility bills for the poor utilization of power.
Be sure to check on a prospective lamp's power factor to ensure you are getting an efficient product. It must be noted that a high power factor is not a comprehensive indicator of the lamp's electronic driver quality but rather it's effiecency. Your lighting supplier should have this information available for you.
CRI-Depending on the environment the LED lamp is being used in, an important consideration may be CRI: the colour rendering index. CRI interprets how correctly a light source reproduces the wavelengths of the visible part of the electromagnetic spectrum, in essence thus the quality of light produced. Incandescent lights typically produce a CRI close to the maximum score of 100RA.
A decent quality led lamp should achieve a minimum of 80RA. Your lighting supplier should have this information available for you.
Luminous Flux-Luminous flux is the quantitive expression regarding the amount of light emitted from a light source measured in units referred to as lumens. Luminous flux measured over a square meter is referred to as lux. Lux levels are important in determining the number of lamps required to achieve the desired quantity of light on a specified surface. This information is generated through the use of photometric testing equipment.
It is not uncommon for a lighting supplier/ manufacturer to exaggerate the amount of light dispersed by their lamps. Be sure to request photometric data files on the lamps you are interested in, to try and substantiate the legitamcy of the specified lamp data. eLighting has a state of the art testing facility and would be more than willing to assist you with light testing.
SDCM- Large scale production of LED chips, deviations in photometric properties occur within differing manufacturing batches as it does in most things i.e paint. The binning process sorts the LEDs of a particular batch. LEDs are sorted in compliance with the ANSI standard (American National Standards Institute). Since 09.01.2013, the European Union has specified a colour constancy of < 6 SDCM (Standard Deviation of Colour Matching). LED lamps of identical types must therefore not differ from each other by more than 6 steps of brightness and colour temperature.
eLighting achieves an average value of < 4 SDCM. Improved binning of LEDs is more costly but provides colour consistency and uniform light.
Colour rendering is of particular importance for indoor LED lighting. The SPD of a typical white LED is very different from that of typical indoor lights whether incandescent or fluorescent. Consumers may be disappointed to find that the colours of familiar objects in their home or office will change significantly when they install new LED light bulbs.
REFLECTED LIGHT : The situation changes when we view reflected light from two or more metameric light sources. In this case, the spectral content of the light coming from each source is modified by the spectral reflectance properties of the object the light is reflecting off of before it enters our eyes. A blue object for instance reflects at the blue end of the spectrum, roughly 400-500 nm, and absorbs the medium to long wavelengths. Consequently, the SPD of the reflected light now is a function of both the SPD of the source and the spectral reflectance properties of the illuminated object.
This complex interaction means that the colour of an object can shift dramatically when illuminated in succession by two metameric light sources. Even though the two light sources have the exact same chromaticity coordinates, if their SPDs differ then the SPDs of the reflected light will also differ. Moreover, the SPDs of the reflected light will most likely not be metamers of each other and consequently will appear to shift in colour at least slightly if not dramatically.
CRI is calculated by comparing the chromaticity coordinates of a set of standard patches or colour swatches when illuminated by the light source you are testing and by a reference light source. These standard patches have specific spectral reflectance distributions, meaning the percent reflectance at each wavelength for each patch is specified. Specifying the chromaticity coordinates of the patch is meaningless without specifying a light source illuminating the patch or more precisely specifying the source’s SPD. CRI uses fourteen patches in all. Eight are used to derive the actual CRI value and the additional six provide individual measures for specific colours of interest. The patches were chosen to be representative of common materials.
The lighting industry has faced this problem in the past and developed the colour rendering index (CRI) metric to quantify the colour rendering properties of a particular light source compared to that of an incandescent source for low colour temperatures (< 5000K) and to daylight for high colour temperatures (> 5000K). If the match is perfect, meaning that colours appear or render the same under the light source in question as they do under daylight or an incandescent source, then the index will be an even 100. CRI is not perfect as a predictor of the colour rendering performance of a light source, but it does do a reasonable job.
ACHIEVING HIGH CRI WITH LEDS :
CCT TUNABLE LAMP : Unlike both sunlight and incandescent light bulbs, LEDs have a fixed colour temperature. To make an LED source that imitates sunlight as it changes colour temperature throughout the day or an incandescent bulb as it is dimmed, we have to mix together the light from two or more different colour temperature LEDs. The chromaticity coordinates of a mixture of two colours will fall on a line in the CIE Diagram between the chromaticity coordinates of the two colours being mixed. Since the Planckian locus is a curve and not a straight line, a two LED mix will only approximate the Planckian locus for a relatively short span of the locus. Fortunately the region from 2700K to 6500K can be approximated quite well with a straight line between x = 0.464, y = 0.409 (~2700 K) and x = 0.312, y = 0.323 (~6500 K)
CONVERSATION STARTERS :
UNDERSTANDING THE DIFFERENCE BETWEEN LED RATED LIFE AND LUMEN-MAINTENANCE LIFE :
The rated life of an LED light source is different from the lumen-maintenance life, and is an essential reliability value that is required by luminaire makers and end users.
RATED LIFE : The rated life of an LED light source is explained, per ANSI/IES Rp-16, as the life value designated to a particular type of light. This is commonly a statistically - determined estimate of median operational life.
The rated life in hours of an LED light, specified by the manufacturer, applies under certain operational conditions and for defined failure criteria.
The statistical measure for the rated life is designated Bp and is measured in hours, where p is a percentage.
EXAMPLE : A B50 rated life of 1000 hours means that 50% of the tested products have lasted 1000 hours without failure.
B50 is also known as the products RATED AVERAGE LIFE.
If a product has a B10 rated life of 1000 hours, this means that only 10% of tested products failed within 1000 hours, so the product
should last much longer than a product with a B50 rated life of 1000 hours.
LUMEN - MAINTENANCE LIFE : For LED light sources, LM-80 explains lumen- maintenance life as the elapsed operating time over which an LED
light will maintain the percentage (p) of it's initial light output.
Rated lumen-maintenance life is measured in hours with associated percentage of light output, Lp.
EXAMPLE : L70 of 30,000 hours means that the LED light source produces 70% of the initial light output of 30,000 hours.
If an LED has L50 of 30,000 hours, it's lumen output dies faster than one with L70 of 30,000 hours.
For LED light sources, failure can be defined when the LED can no longer produce a certain percentage of the initial light output value.
Failure might be defined when the light output of an LED reaches 70% or lower of the initial light output (including if the LED's light output is zero).
If an LED produces insufficient light or no light, for a given period of time, the LED is considered at failure.
Using this definition of failure criteria, the statistical measure can be defined with the defined durability measure. The combination of lumen-maintenance life (Lp) with statistically - measured failures (Bp) is the LED light source's rated life, or BpLp value.
EXAMPLE : If an LED light source is believed to have B50L70 of 30,000 hours, then 50% of tested samples should have a lumen-maintenance life of
To obtain the rated lifer LED's, the statistical failure measurement can be integrated with lumen-maintenance measurements during the life test.
When 50% of the tested samples reach a light output equal to 70% of initial lumens, including the samples that failed to produce light, then B50L70 (in hours) is obtained.
RETINA FUNCTION : The retina has two types of light - sensitive receptors called rods and cones.
Cones are responsible for colour and high resolution vision.
Rods are responsible for night or low-light vision and for motion detection primarily in the periphery.
There are three varieties of cone receptors, each with their own range of spectral sensitivity curves that overlap each other, but are yet distinct.
Although the spectral response of each cone type corresponds roughly with the coloursred,green and blue, visual psychologists prefer to call them
"L", "M" and "S" cones, which stand for long, medium and short wavelengths respectively. The relative ratio of the response of these three cones receptors to a given SPD determines the colour we perceive.
At the shortest wavelengths when only the S cones are being stimulated we see the colour violet. As the wavelength increases the M cones start to respond in addition to the S cones and violet turns to a deep blue. As the response of the M cones increases further relative to the S cones the colour perceived shifts to a greenish-blue, then to a bluish-green and finally to a pure green when the ratio of the M cones to the L and S cones is at its highest.
The process continues as the wavelength increases to the point where the L cones start to respond significantly to stimulation. Yellow occurs at roughly the point where there is a balance between the M and L cones. As the wavelength continues to increase, the response of the M cones decreases rapidly causing a rapid shift from yellow to orange and then to red. From 640nm and above, the response of the M cones is so low that the colour perceived is a deep red that changes very little in appearance, regardless of the wavelength since only the L cones are responding. From this analysis, it is easy to see that the color of any wavelength is strictly determined by the ratio of the cone responses at that wavelength.
COMBINING CONE RESPONSE :
The neural signals leaving the cones undergo further processing before entering the optic nerve for transmission to the visual cortex at the back of the brain. The L, M and S responses are combined into three channels, one that encodes brightness ( intensity or luminance ) information and two that encode for colour. The L and M channels are added together in a weighted average to produce the luminance signal. The S cones contribute little and possibly nothing to the luminance channel.
The combined response of the cones that constitutes the luminance channel results in the well-known photopic curve, V(λ) under daylight conditions. Under nighttime illumination levels the cones are too insensitive and contribute little to vision. The rod cells which are much more sensitive than cones take over and dominate. Rod cell contribution to the luminance channel is characterized by the scotopic curve designated by V’(λ). All photometric measurements such as intensity (candelas), flux (lumens), illuminance (lux) or luminance (nits) use either the V(λ) or V’(λ) curves (depending on illumination level) to weight the SPD of the light being measured. Photometric flux for instance is calculated by integrating the SPD weighted wavelength by wavelength by the V(λ) curve over the visible spectrum.
The two colour channels consist of an L cone minus M cone channel and an L plus M minus S channel. These two channels are often referred to as the red-green channel and the yellow-blue channel and are widely thought to be responsible for the opponent colour nature of red/green and yellow/blue, meaning that we do not see colours that are reddish-green or yellowish-blue.
What happens when light is broadband rather than a single wavelength? In the case of broadband light falling on the retina, the response of each cone type is the integral of the SPD of the light source multiplied by the spectral response of each cone on a wavelength by wavelength basis.
SPD OF A SOURCE :
Two light sources with differing SPDs, but which nonetheless appear the same colour are called Metamers.
Metamers are very important in colour science, particularly in colour printing, colour displays, photography, paint matching and in LED illumination. When a colour is reproduced by a colour display for instance, the display does not have to reproduce the exact SPD of the original colour that is being reproduced. The display only has to produce an SPD that is a metamer of the original SPD. LCDs use three coloured pixels of red, green and blue, generally referred to as the display’s primary colours or primaries. When mixed together in the correct proportions, these primaries produce metamers to a wide range of possible SPDs and their associated colors. The SPD reproduced by a mixture of the primaries only has to stimulate the L, M and S cones in the same proportion as the original SPD does to accurately reproduce the colour of the original SPD.
As you might imagine it is not a simple task to find the right combinations of the three LCD primaries to create a metamer for a known SPD. The history of colour science is essentially the history of solving this problem. In the early days of colour science at the beginning of the 20th century, the spectral response of the cones was not known. In fact, the number of cone types was not known, but was assumed to be three because three additive or three subtractive colours are all that are necessary to mix all of the basic colours. The first step in the process was to characterize the spectral response of the eye.
James Clerk Maxwell was the first to empirically measure the ratio of mixtures of three light sources to create arbitrary colours. He even created a colour triangle that foresaw the CIE Chromaticity Diagram. Several scientists refined his techniques over the next 70 years culminating in experiments that formed the basis of the CIE 1931 standard. These experiments, known as colour matching experiments involved a monochromator that generated a test light consisting of a single wavelength illuminating one half of a small screen that subtended two degrees from an observer’s viewpoint. For each test light wavelength in the range from 700 nm to 400 nm the subject would adjust the intensity of three other monochromatic sources or primaries illuminating the test light half of the screen until the mixture of the three primaries matched the colour of the test light.
COLOUR SPACE TRANSFORMS :
The results of colour matching experiments are three curves or functions, which show the relative intensity of each primary required to match the colour of a light of a single wavelength over the visible range of wavelengths. If the primaries are changed to a different set of monochromatic sources, then the experiment results in a different set of colour matching functions. We will refer to a set of colour matching functions from a given colour matching experiment as a CMF. How are two CMFs from experiments with different primaries related to each other? It turns out that all CMFs are related to each other by a linear transformation. The below equation represents such a linear transformation in matrix form. It transforms the CMF consisting of a(λ), b(λ) and c(λ) into a new CMF, a’(λ), b’(λ), c’(λ).
All CMF's and their associated colour spaces are linear transforms of each other.
The CIE 1931 standard is just one of an endless number of possible colour spaces, all of which can be derived from any other set of colour matching curves with a linear transformation. The CIE 1931 standard was derived from data from two independent colour matching experiments performed by W David Wright and John Guild in the 1920s. Each of these colour matching functions were transformed into a new colour space that corresponded to a different set of idealized primaries. The results after the transformation were nearly identical for both Wright and Guild’s data as would be expected because as stated above all colour matching functions are linear transformation of each other. The differences were due to experimental error and natural variability amongst the observers participating in the experiment.
TWO - COORDINATE COLOUR :
The simplicity of identifying a colour with just two numbers makes the CIE Chromaticity Diagram a very useful tool for specifying the colour of an LED in a universal standardized way across the industry. More importantly though by studying the origins of the CIE 1931 standard we gained a better understanding of the science underlying colour vision. In the process three fundamental concepts of colour were introduced that provide powerful insights into how we see colour. These three concepts are 1) that colour can be explained by the relative cone responses, 2) metamers and 3) all colour matching functions are linear transformations of each other. When these concepts are properly understood they can be applied to great effect to a wide variety of problems facing the LED engineer.
APPLYING THE CIE DIAGRAM :
The CIE Diagram has many uses, the most common in the LED industry is the specification of colour or chromaticity bins by LED manufacturers. It has several important additional features that require some explanation:
A colour’s hue is essentially its basic colour on the spectrum, from red to orange through yellow, green, blue and violet. Purples are mixtures of red and blue and fall along the purple line. The colour of narrowband light always appears to be more pure in hue than light of the same hue with a wider spectral bandwidth. The wider the spectrum the farther the x, y chromaticity coordinates will be from the spectrum locus. Moving from the spectrum locus towards white in the central region of the diagram, results in a more desaturated or pastel hue and eventually off-whites and white. White (and grey for that matter) is often described as an absence of colour. The saturation level of a colour is related to how pure the colour is. The farther a colour’s chromaticity coordinates are from white the more saturated the colour is. Colours that fall on the spectrum locus are fully or maximally saturated.
If we wanted to exactly follow the Planckian locus we would need to mix in a third LED so that the gamut of the three LEDs would completely encompass the Planckian locus from 2700K to 6500K. A third LED at x = 0.35, y = 0.4 added to the mix would expand the gamut enough to fully encompass the Planckian locus over the desired range. In practice this approach is not warranted, because we cannot buy any LEDs with those precise chromaticity coordinates. The eye doesn’t discriminate colours that closely so as long as we are close to the Planckian locus the results will be satisfactory for all but the most demanding applications.
Colour rendering refers to how the colour appearance of illuminated objects can change when illuminated by different light sources. Naturally we expect colours to change somewhat when illuminated by light sources with different correlated colour temperatures (CCT). We are aware of the change in colours of objects outdoors from noon to sunset on a sunny day. You may have even had the experience of choosing a paint colour at the store under natural or fluorescent lighting and then being disappointed in the colour of that paint as it appears on your walls at home under incandescent lighting. Both of these examples of colour rendering are within our common experience, are to be expected, and in part can be explained by different CCTs of the light sources.
What is less apparent, though, is that the colour of an illuminated object can change significantly when shifting from one illumination source to another, even when both sources have the exact same CCT and even when they have the exact same chromaticity coordinates. To understand how this can happen, let’s review and apply our first two principles of colour vision.
Our first principle of colour vision states that the colours we see are directly related to the relative response of the three types of cone cells in the retina to the spectral power distribution (SPD) of the light falling on the cone cells. This means that two light sources with widely divergent SPDs can nonetheless still result in the same relative response of the three types of cone cells and hence look like the exact same colour. This is also why a mixture of the light from a red, a green and a blue LED can have the same colour as a 3000K incandescent lamp, even though the SPDs of the lamp and the LED mixture are vastly different. As long as the cone responses to the two SPDs are identical, then the two sources will look identical in colour. This phenomenon is called metamerism, and is our second principle of colour vision. Any two or more SPDs that have the same chromaticity coordinates are metamers of each other.
Natural light sources are for the most part thermal sources and therefore are at least approximately blackbody radiators. The surface of the sun is also approximately a blackbody radiator, but the SPD of sunlight at the surface of the earth is modified by absorption and scattering of the atmosphere. The atmosphere scatters blue wavelengths predominantly which explains why the sky is blue. At noon sunlight passes through the least amount of the atmosphere while at sunrise and sunset sunlight passes through a significantly larger cross section of the atmosphere, removing most of the blue light. Consequently direct sunlight at sunset as well as clouds backlit by the setting sun, both turn to the deep reddish orange hues of a sunset. The SPD therefore of direct sunlight changes significantly throughout the day.
As you might already be aware there is no single white. The closest thing to a single white is the equal energy white, E. It is defined as the x, y coordinate of an SPD that has equal intensity or energy at every wavelength in the visible spectrum. The CIE 1931 color matching functions are each normalized so their integrals over the visible spectrum are all equal. This means that the tristimulus values for equal energy white (E) will all be equal, (X = Y = Z) which results in x = y = 1/3. Equal energy sources are of theoretical interest only since they do not occur in nature and would be prohibitively expensive if not impossible to create artificially.
There are two technical terms relating to hue and saturation that have explicit definitions with regard to the CIE diagram, namely dominant wavelength (hue) and purity (saturation). The dominant wavelength of an LED is defined as the wavelength on the spectrum locus intersected by a line from E through the x, y coordinates of the LED. The colour purity of an LED is determined by the position of the LED’s x, y coordinates along this line. Purity is 0 if its x, y coordinates are coincident with E. It increases as it moves along the line toward the spectrum locus, reaching a maximum of 1 at the spectrum locus.
ADAPTING TO LEDs : It is impossible to buy LEDs with exact chromaticity coordinates. LED manufacturers sell LEDs in chromaticity bins and generally will not sell just one bin, but a grouping of bins. This inherent variability in the chromaticity coordinates of the two LEDs used to create the new mixture will result in a corresponding variability in the chromaticity coordinates of the mixture.
THE IMPTORTANCE OF CRI IN SSL : CRI is primarily important for indoor lighting and is less important for outdoor lighting. High-pressure sodium (HPS) street lights for instance, have a very low CRI, in some cases as low as 20. This poor colour rendering is offset by exceptional luminous efficacy, which can be as high as 150 lm/W. Colour rendering in this application is generally considered unimportant when compared to energy efficiency. Some HPS lamps do have slightly higher CRIs, but at the sacrifice of lower luminous efficacy.
The only outdoor lighting application where high CRI is important is architectural lighting, such as wall washers and floodlights used to illuminate façades and landscapes. A low CRI in an architectural application can significantly detract from the aesthetics of an illuminated building or landscape.
In indoor lighting, CRI is particularly important in residential, retail, and restaurant lighting. Colour rendering in office environments is of less importance, because office lighting is designed to provide the best lighting for performing tasks and less so for aesthetics.
LED-based retrofit lamps and to a lesser extent SSL fixtures are starting to make inroads into the residential lighting market. For this market penetration to continue, the cost of the LED lamps and fixtures must continue to come down, while the quality remains high. The higher cost of LED lighting products compared to CFLs and incandescent bulbs can be offset to some degree by the long lifetime of LEDs and by the continuing increases in LED energy efficiency. The quality of LED products though involves more than just reliability. The quality of the light produced by an SSL lamp or fixture is also important, especially to residential customers. The quality of light in SSL is essentially the colour rendering quality of the LEDs themselves. Since CRI is our only objective standard to quantify colour rendering, it becomes an important product specification along with reliability and luminous efficacy.
White LEDs are actually blue LEDs coated with a phosphor material. The phosphor absorbs a portion of the blue light from the LED with the rest passing through the phosphor. Some of this light absorbed by the phosphor excites electrons in the phosphor molecules to a higher energy level. As these electrons fall back to lower energy states they emit photons. The spectrum of the light emitted by the phosphor is broadband in nature ranging from 500-700 nm with a peak typically around 550 nm.
If the phosphor were to absorb all of the blue light it would glow yellow. Since it doesn’t absorb all of the light from the blue LED, the transmitted blue light and the yellow light emitted by the phosphor combine to create what appears to be white light. If the mixture has more blue light than white, it will be a cool white with a high CCT. If the mixture has more yellow light from the phosphor than blue light from the LED, it will be warm white with a lower CCT.
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LED manufacturers often face competing goals, such as cost and performance. Manufacturing high CRI LEDs with high luminous efficacy especially for warm colour temperatures is one such challenge. The reason for this has to do with how white LEDs actually create white light.
THE PLANCKIAN LOCUS : The Planckian locus is the plot of chromaticity coordinates of blackbody radiators over the range of approximately 1000K to infinity. The colour of a 1000K blackbody radiator is a deep red. Below 1000K there is negligible radiation in the visible spectrum. As the temperature rises the colour shifts to orange, yellow, white and finally a bluish white.
If a light source is not a blackbody radiator, but its chromaticity coordinates lie close to the Planckian locus, we can characterize its colour by its correlated colour temperature, CCT. The CCT of a light source is the colour temperature on the Planckian locus that the chromaticity coordinates of the light source are closest to perceptually.