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Home , Color temperature, Planckian locus

SPRING 2008 depends only on its and the of producedlight temperaturecolor itstheof dependsononly and hypothetical,emittedaby light incandescent perfect blackbodyof beforereachingfinallybluish- a beforemelts.just it familiarmannerpassingthroughred,a in , yellowwhiteand tungstenheatedcold,fromfilamentisits changes blackstatecolorit completelya incontinuousradiationemits spectrum. the As incandescenttheincandescent isan and lamp filament lamp commonlyentertainmentmostinthe used farby still source is that changing. is that all of expansion rapid the problem.With one-dimensional simple relatively a been has differences the with charts. paint in see you white of shades 27 those like bit a of range broad a mean actually can white that and isn’ttrue that known long we’ve lighting entertainment is light White Out of the Woodof Out When whitelightisn’t Whydo I sayit’s one-dimensionala beenproblem? Well,light the w hite light isn’t it? it? isn’t light hite BY

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WOOD LE S till, in the main, dealing dealing main, the in till, Ds as viable light sources sources light viable as Ds I n the field of of field the n T he colorhe SPRING 2008 SPRING 14 MSD. position,measuredKelvinin Color (K),its as sourcereferlightwhite andthatto a colortheof describe tolocus colorchart.a on line) blackbody Wethe thispositionalong the use alwayssomewhere Planckianwill lie(oftenthe locusalongcalled resembles that of the source.the of that resembles closely perceivedmost colorwhose radiator Planckian the of ( temperature colorcorrelated the wheretemperature colorcorrelated conceptof the use can we that locus the to enough close usually are points locus.Planckian the on exactly fall usually don’tpoints color output close,their get to strivemanufacturers H are familiar very are we which with lamps of group Another temperature color correlated Thesecond-dimension— 5600K. of temperaturecolor term the that such out averagethese often we purposes lighting 5300K. normal Foraround lower at little a clouds,fromis diffusion the and sky the from contribution the alone,without is.Direct sky overcast howthat and sky the in is the high how on depending 6000K upwardsof anything be can which temperaturecolor source, a body has a robust.more and cooler little a is filament the where range 3200K to 2500K the in typically anymetal). of highest (the point melting that’sits as totungsten close with go can you as high as much pretty is 3700K light.bluer producea will 3700K,it to lamp.the as color same the about of producelight would 3200K of temperature a to heated metal a that problem.one-dimensional the as to referring been marked. approximate( temperaturescolorwith chart CIE igh igh Figure 1 Figure When we say a lamp has a of 3200K, we mean 3200K,we of temperaturecolor a has lamp a sayWhenwe I T D) such as such (HID) lamps Discharge ntensity hese lamps have discontinuous spectra and, although the and,although havespectra discontinuouslamps hese T he position along that curve is the single value that value single the is curve that along position he illustrates the on the familiar 1931 familiar the on locus Planckian the illustrates c or CCT or Tc N ) of a source is defined as the temperature the as defined sourceis a of ) atural , which is also very close to close very also is daylight,which atural ntertainment incandescent lamps are lamps incandescent Entertainment daylight I f that same metal is heated is metal same that f H commonly refers to a toreferscommonly owever those colorowever those

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) binning Ds. LEDs. LEDs you now GB luminaire and LEDs is an inexact his question has T f we can’t have fan have exact we can’t I LED die with a LED dies mounted in a single LEDs we use in our will LEDs and sort them into he size of each of those bins T he phosphor technique is becoming | When | white light isn’t white T his is particularly obvious when using T t’s a question witht’s more than one answer LED manufacturers deal with this by I NEMA ANSLG NEMA C78.377-2008, SpecificationsANSLG for nGaN) or UV (GaN) he LEDs give us a discontinuous spectrum, and often T he first technique just mimics the R LEDs? T he standard also discusses perhaps tightening this to a T ED is white, LED is white, Ds. LEDs. HID sources LED luminaire which uses a mix of , and blue emitters We now have to ask now have a ourselves question. We Whichever way you make white Whichever light from your he development and manufacture of these owes a lot o make matters worse the manufacture of these Figure Figure 2 – Temperature Correlated Color Out Out of Wood the 4-step 4-step ellipse in the althoughfuture but, not better too than that’s 7, A tightsingle a ellipsestep MacAdam tolerance a either. represents region plotted on a diagram showing where colors are to be perceived the same a by the 4- viewer average logically, and, step ellipse is four times larger—in other a words that is four times more than the minimum we color difference can and the subsequent range of colors each one encompasses is critical in determining how close a match the to have each other and thus how much color variation we are going to see between units. how much color variation match, are we to prepared accept between similar white depending on what you are lighting and how. been studied extensively with lamps fluorescent and there is now an ANSI standard ( the Chromaticity of Solid State Lighting Products for Electric Lamps a recommending series of bins based on ellipses7-step MacAdam as criterion the for color-tolerance solid a state huge lighting—that’s variance! lamps and, lamps althoughand, they are theygetting better suffer every day, from many of the same withproblems gaps in the spectra and a propensity to appear green. not only to have worry about the first-dimension of correlated color temperature but also the second-dimension of green/ shift. T science with a wide variation in the product coming off a single production line. My but what about yours? Like one with even bigger gaps. to a produce white; but is also the case when using so called white there Currently are LEDs. two common techniques for producing white package to mix to an white approximation light, while the second uses a deep blue (I coating to create white light. very common and is now pretty much the norm for white T to the R&D done on the similar phosphors used in fluorescent an where where they measure the output of the various or ranges, before sale. bins, uses multiple ofwavelengths different ocol t 15 o pr D lamps, it’s not HID it’s lamps, owever the owever almost that two lamps (shown “ H V and film practitioners O) the color temperature. T Figure Figure 2

LEDs as the ubiquitous light source for the the lines of equal correlated color

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his green correction is a second-dimension of correction and, he use of the wholecorrelated color temperature isn’t story his is ofan approximation course but one that works well in T T T f there has to be an error it is usually better for a light source to err t’s not uncommon to see t’s this problem with discharge lamps used

much color variationHow are we prepared to between accept similar ? “ 21st century will us force to deal with this problem all the time. although we know about unless it, you are working in the television or film areas of our industry and using large on on the magenta side than the green one. something we are dealing with every day. the problem. A green the tint problem. in what is supposed to be white light is usually to be at avoided all our costs, eyes are particularly sensitive to green and it makes skin tones in particular look very unnatural. I are very familiar with using “minus green” gels to are verytry familiar green” with“minus and correct using Planckian locus, Planckian one could the locus, have and above same one below, correlated color temperature but still lookA, veryOne, different. would appear slightly greenish B, magenta and or the pinkish. other, I in followspots or automated lights and though, though, especially if the are color coordinates a long way from the can see from You Planckian locus. as points A and B) whose fall color co-ordinates either side of the using gels to correct color temperature up and down the Planckian locus to increase (CTB) or decrease (CT B are both on the 6000K Once line. we are back on the Planckian locus then we can use our earlier definitions and treat the source as if it were a radiator and we can be comfortable with practice. practice. are the short lines crossing the Planckian locus and points A and inevitable takeover inevitable by takeover SPRING 2008 SPRING 2008 SPRING 16 white color-binning currently offered by offered currently color-binning white approximately what a 4-step MacAdam ellipse looks like on the CIE the on like looks MacAdam 4-stepellipse approximatelya what the same color.same the being as classified be would ellipse that chart. Any sourceswithin in plotted ellipse red the example LEDs.an As for worksdoesn’t fluorescentit lamps for mean (somewhat) works for use we should what see.But ellipses. ellipses. MacAdam step six and four between vary sizes bin these emitters, binning, available currently of example an As white for criteria color-binning recommended establish to range. tolerance color the on scene (CC temperature color correlated spectrum, level, light of impact the investigated between difference color just-noticeable a see would observer human normal a point what white for criteria color-tolerance study, the from permission with quoted extracts (text follows as reported and criteria these determine to study a on results I T implies. that variation natural the all and perception human with E The Out of the Woodof Out nstitute has taken on the task and recently published preliminary preliminary published recently and task the on taken has nstitute “ stablishing these criteria is not a simple task—we are dealing dealing are task—we simple a not is criteria these stablishing he he skin tones ..tones . skin for awful but efficiency for great is Green L ighting Research Center of the Rensselaer Polytechnic Polytechnic Rensselaer the of Center Research ighting

study LRC I s this good enough? good this s L RC researchers conducted experiments to develop develop to experiments conducted researchers RC T ), and visual complexity of the illuminated illuminated the of complexity visual and ), Figure 3 – MacAdam Ellipse MacAdam – 3 Figure “ LE D light sources. light D | When white light isn’t white isn’t light white | When Ds? Just because a standard a Just because LEDs? LE T Ds. his information was then used used then was information his T L he criteria define at at define criteria he umileds for its its for umileds T Figure 3 Figure heir study also also study heir Figure 4 Figure L RC). illustrates illustrates shows the the shows L I uxeon LE n Ds. SPRING 2008 f this I LED ighting a display case L “

| When | white light isn’t white RC RC were not specifically considering L Color willconsistency become a ifmajor concern, not a headache, for us all. “ would argue that, for many of our manufacturers manufacturers are responding with moves in the right direction and Osram recently multicolored scene. and multicolored objects or paintings are some examples. entertainment lighting of course but I applications particularly when lighting critical skin tones, we fall in the first category and should be looking at 2-step very ellipses. Until MacAdam recently this was a much tighter tolerance than the binning offered by the major LED the however manufacturers; Out Out of Wood the announced announced a new initiative to to move 3-step binning for white LEDs which they claim is the finest binning in the industry. ocol t 17 o pr he T D panels, LE f subjects saw Ds: I B LE G RC RC proposed the L Figure Figure 4 – Luxeon binning his same value was used as the T B filters, and a variety of white —For —For applications where the white —For applications where the white G RC RC researchers built a display cabinet L D fixtures) are placed side-by-side and are D fixtures) are not directly visible, or LE LE aking all this into account, T Ds (or white Ds (or white 4-step MacAdam ellipse 2-step MacAdam ellipse an achromatic (white) Accent lightingscene. a white wall and lighting a white cove are some examples. when these fixtures are used to illuminate a visually complex, directly visible, or when these fixtures are used to illuminate LE LE

o carry out the tests n n As might be expected the type of visual background has a major T he test compartment then changed color systematically in light color before we notice a color difference than a plain white background. with two side-by-side compartments. Using R MR16 halogen lamps with R impact on the color tolerance range. Complex visual backgrounds using different colors allow for much greater variations in white a color difference, they commented on how different the two compartments appeared. starting point for the adjacent test compartment light source. T incremental At steps. each subjectsstep, were asked whether they saw a color match or noticed a color difference. reference reference compartment showed a constant white light color at a specific x, y chromaticity value. and multicolored backgrounds, test subjects compared the color of white light in one compartment against the other. following two criteria for color binning white SPRING 2008 Out of the Woodof Out improve even more. even improve perhaps and follow will manufacturers successful is initiative I ’m sure the other other the sure ’m Figure 5 – Osram “Fine – White”5 Figure binning | When white light isn’t white isn’t light white | When eight rectangular bins (black quadrilaterals) quadrilaterals) (black bins rectangular eight the on superimposed structure binning Figure 5 Figure shows the proposed Osram Osram proposed the shows SPRING 2008 SPRING 18 recommendedin restricted to direct view and display and view direct to restricted much pretty was lighting entertainment tones…. skin for awful but efficiency watt! per lumens most the gives it as be tolikemanufacturers the that’sso side where green the on efficient most fluorescents, forbe to tendused those white for used phosphors the side.Unfortunatelygreen the on errors than eye the toalwaysacceptablemore are locus the of side magenta the on we thing one wewhatever earlier,locus. discussed Planckian As the of side green the on are bins those manyof Youfrom see also can are.bins those big how just shows and 2008 Treasurer and Immediate Past President of ESTA. of President the Past is Mike the Immediate of and industry, veteran Treasurer technology 30-year standards, A R&D, Property. entertainment strategy, entertainment Intellectual the and technology on within industry companies to support Wood Mike Technologies (ASSIST). and Systems Illumination Solid-State for Alliancethe by sponsored was which research their from extracts quote to permission for PolytechnicInstitute,Troy, Rensselaer the NY of CenterResearch Lighting the to Thanks second-dimension! minds.our of forefrontthe isn’t questionin the usually so lamps incandescent with it about worry Weto having not toaccustomed have been all.us headache,for a not concern, major if a become source, consistencywill color light white a as used be to and performers light to used be to enough bright are LEDs concern. of was this of colors,none so deep in time the pieces,of much set and backings lighting for and applications C onsulting onsulting Only a year ago the use of of use the ago year a Only LLC is President of Mike Wood Wood Mike of President is do which provides consulting consulting provides which don’t T want in a white a in want ime to learn about that about learn to ime MA ANSLGNEMA C78.377- want is green—errors is want Figure 4 Figure G n reen is great for great is reen Ds in LEDs LEDs,like just how just D the LED N ow