Color Rendering: Narrow-Band Emitters Give Designers Another Tool

SPRING 2010 shop-built, four-lamp, low-pressure sodium, open-face luminaire luminaire open-face sodium, low-pressure four-lamp, shop-built, high-pressure sodiumlampson stage, a complete apicture of with Lighting unique. Max Keller in hue, saturation, intensity, distribution, andmovement. color rendering tool to inaddition asadesign theusual tools of using sources, incandescent from light against lamps discharge vapor mercury and sodium, high-pressure sodium, low-pressure and lamps fluorescent from light Toelstede played Hans designer lighting as alive more or less becoming scenes saw nevertheless they but makeup, and costumes actual the of palette color tight the performances anything didnotsee quite of sostartling, because one. red a wearing now was skirt black a wearing been had who member chorus a up, but faded was light stage stayed thesameintense yellow incandescent asgelled front monochrome. black—completely or street clothes, onwearing stage wasyellow, everything yellow-gray, fromlight low-pressure sodiumlamps. und Aron N Twenty years ago give designers another tool another designers give emitters Narrow-band rendering:Color ew Toelstede’s low-CRI lampson stage wasunusual useof but not Y ork City O City ork has several pages describing the use of low-pressure and and low-pressure of use the describing pages several has saw sight.yellow asurprising stagewith The waswashed pera production of productionpera of Light Fantastic:Light the Art Stage of andDesign people watching people atech the rehearsal of A A rnold Schoenberg’srnold nd then colors emerged! The The emerged! colors then nd A A udience members at the the at members udience s the chorus rushed ons thechorus rushed Moses Moses SPRING 2010 SPRING 8 strange colors if they are they not. colorsstrange if properties, by spectral ortheir peculiar to unpleasantly be surprised are todesigners they aware usecolor rendering tool if asadesign of LEDs alsoare narrow-band emitters. LEDluminaires allow are designer nolongerlighting limited to gas-discharge sources: emitters are limited to gas-discharge lamps. narrow-band of choices the if productions, of majority vast the for color rendering tool isjust asadesign too difficultand limitedtoo lamps. these with luck of out is vapor, she mercury color other thanyellow, or any white other thantheblue-green of white. wantsto adesigner play colorof If with rendering inany version blue-green one only but white, is vapor Mercury different. much yellow-amber—not is sodium yellow. High-pressure only it’s yellow, but except color all suppressing for great is which light, yellow monochromatic virtually a gives lamp sodium low-pressure limited. is do can they what Furthermore, imagination. the of and theluminaires thatusethem aren’t by any spotlights stretch don’tThey on instantly, turn can’t they electronically, dimmed be gas-discharge sourcesusing theseindustrial on stage isnoteasy. a20-inchthat needed mechanical douser to dimit. However, However, narrow-band thechoices emitters of for atheatrical By l G.Karl Rul ing Mike a nd Wood W orking with with orking Photo Cour A tesy CarolRoegg relative levels of stimulation of the L and M cones are the same, the Color perception perceived yellow will be the same. A cone has no mechanism for The human eye has two sets of receptors: rods and cones. The signaling that its stimulus is any particular wavelength; it can only rods work at very low light levels and essentially give us no color signal stimulation or the lack of it. information. The cones work at higher light levels and are the The system of three cones also allows us to see colors for which color receptors. They come in three types, often labeled L, M, and there is no single corresponding wavelength of light, magenta S—long, medium, and short—to describe the wavelengths of for instance. That is the color we see when the L and S cones are the parts of the spectrum to which they are sensitive. The actual stimulated by wavelengths of light at the long and short wavelength photonic energy absorption curves for the cones do not align ends of the spectrum, but there is little or no light in the middle of with what we normally call the primary colors of light and have the spectrum to stimulate the M cones. Magenta gels are sometimes significant overlap, but there is a lot of signal processing in the called “minus green” because they are. retina and brain that improves the color resolution—and obviously the system works. Metamerism and metameric failure Any particular color can be described by how much it stimulates the three types of cones—its tri-stimulus values—but for colors in the middle of the spectrum, several combinations of wavelengths of light, several spectral power distributions, can create a particular set of tri-stimulus values, a particular color. Colors that look the same but that actually have different spectral power distributions are called “metamers.” The yellow of the low-pressure sodium lamps and the yellow of the gelled front lights on Moses und Aron are one example, but metamers abound in everyday life. It is a safe bet that anytime you see something painted to match a dyed fabric you are seeing metamers; it is unlikely that the paint pigment is reflecting exactly the same spectrum of light as the dye. It only looks the same when observed under what we call “normal” light, be that daylight, incandescent light, or some other general- purpose white light source. Figure 1 – Normalized responsivity spectra of human cone cells. Metameric failure happens when objects that look the same [The graphic is from the Wikipedia “Cone cell” article, and its use is under one lighting condition do not look the same under another. licensed under the Creative Commons Attribution 3.0 License.] The skirt on the chorus member in Moses und Aron, which matched the jeans worn by another chorus member under the low-pressure A look at the sensitivity curves shown in Figure 1 can make it sodium but that didn’t match under the gelled incandescent light, is clear how we see color. A pure red light at 680 nm, for example, one example of metameric failure, although probably most people will stimulate the L cones, but stimulation of the M and S cones wouldn’t call it a “failure,” just what you can expect with low- is virtually nil. The signal from the L cones and the lack of signals pressure sodium. However, metameric failure really is a failure when from the others is interpreted by the brain as “red.” automobile upholstery matches the paint under daylight but doesn’t The mechanism for perceiving a color at an extreme end of the under streetlight. spectrum is fairly easy to understand, but the perception of colors Metameric failure happens when a wavelength that a paint or dye in the middle is more complicated and interesting. For example, is using to create its color under one set of lighting conditions isn’t the yellow of low-pressure sodium is produced by light of two very there under another. For example, we could have a teal paint that closely spaced wavelengths: 589.0 and 589.6 nm. That stimulates reflects a band of light peaking at 505 nm and a different teal paint the L and M cones, and the signals from the cones are processed in made with pigments that reflect a spectrum of light with peaks at the retina and the brain to yield the perception “yellow.” However, 470 nm and 525 nm. They can both look the same if illuminated the same sensation could be produced by light of two more widely by full-spectrum white light or incandescent light filtered through separated wavelengths, for example, 550 nm and 630 nm. The a cyan or princess blue gel, which lets through a broad range of perception of the same yellow also could be created by a band wavelengths and will surely cover 470 nm through 525 nm and of wavelengths centered around 589 nm—the situation with the more. However, the results are much less predictable if we illuminate gelled incandescent front light on Moses und Aron. As long as the the two paints with LED color-changing luminaires that use colored SPRING 2010 SPRING

9 protocol SPRING 2010 camera iswhite balancedcamera to theincandescent, but, for allowing this, balanced to white. The final three all show a blue tint because the L sources: incandescent; cool white fluorescent; amber and cyan the mixare notasdramatic. L white with rendering color variable of photographs but phosphors. Hence, thetechnical issues are much pretty thesame, rendering dependent LEDsandmixof isstill on thechoice of color the and isn’tflat, distribution power spectral the but white, L monochromatic of output the mixing from get you than spectrum continuous more a is It phosphor. the and L blue the from output the of mixture a is emerges that white L blue a over phosphor fluorescent yellow broad-band white LEDsinthemix. are These usually made by puttingafairly argument.of S our examples to luminaires thatusecolored LEDsfor simplicity apure white projection screenof asourreference. inmany colored stripes differentpiece with colors fabric of infront levels comparable, but nocolors were touched. sampleisa The contrast were tweaked slightly inPhotoshop to make thelight weretimes adjusted and between photographs. brightness The was left locked to incandescent color balance andonly exposure that inorder to asclean get acomparison aspossible, thecamera differentrendering with monochromatic LEDmixes. Pleasenote LED luminaire does acolor fade to deep blue. change differently,certainly will notinhue then inintensity, if the if LED luminaire them illuminating includes cyan LEDsor not, and LEDs respectively. two The paintsare likely the to different look if nm and525are thedominant wavelengths blue andgreen of nanometers isthedominant wavelength acyan LED, of and470 colorsLEDs to make including of white. arange Five-hundred five E Figure 2 Let’s take atafew alook examples to thedifferent illustrate color Ds balanced to a white; and finally red,green, and blue L shows the result when the subject is lit by four different ome color-changing LEDluminaires alsoinclude Figure 2

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RG The B doesjob on agood thewhole, but we losethe pink now shows as different shades of amber, and the green has gone pleasing, until you realize that everything that was yellow, red, or overemphasizes the blue. The amber and cyan mix is visually very the other hand, the fluorescent does a poor job of the yellow and the bottom of the photo, which the fluorescent picks out well. rendering thedifferent near job of poor pinkandmagenta stripes incandescent, blue wavelengths, but, itslack of of because itdoes a W hich one is correct? Figure 3 shows an example of two metamers of acolor, twoshows metamers of anexample of inthis Y ou might initially be tempted to say the Figure 3 Y ou can see thatthecolorou cansee on the O n LEDs and color rendering The above discussion does not mean that LED luminaires are bad for color rendering, but it does mean that they can make color rendering a design tool, with the advantage of being easier to use than gas-discharge lamps. A designer can use them for accurate color rendering, or a designer can use them to create forced, fantasy colors, or to suppress color, as Toelstede did in Moses und Aron. Designers should keep in mind, however, that even when they are not trying to consciously manipulate color rendering, the Figure 4 color rendering on stage will be affected by the LEDs used in the luminaires; luminaires using different sets of LED colors may be The final example, Figure 4, shows just how little light of a able to make the same range of whites and virtually the same colors wavelength is needed to give your eyes the clues they need. In this on a white surface, but will render colored objects differently. case both halves of the image have the same primary illumination— Obviously, an LED luminaire that uses red, blue, and green LEDs blue and green LEDs—but the right image has the addition of a very is not going to give a designer many choices of how to make any small amount of light from a red LED, less than 10% of the other. particular color. Yellow is going to be made by turning on some Amazingly, that tiny amount of red is enough to make the reds pop mixture of red and green LEDs. That’s it, there are no other options. out. Look at the white background; the red isn’t enough to make any However, not all RGB LED luminaires use the same red and green real difference to the overall hue, nor is it enough to turn the two LEDs. Some, to get a bit better yellow and color rendering with apparently green stripes to the yellow they should be, however your pastels, use red-orange LEDs (615 nm) rather than red (625 nm). eye latches onto that red and uses it where it can. Others, to get more intense blues, will use royal blue (460 nm) rather than blue (467 nm). The choices all work, but a designer using luminaires with different LED color sets will find that the SPRING 2010 the same, even if the color of the light light the of color the if even same, the coloredrendering isnotquite objects of a preset desk. Many advanced lighting parameters by sliderslight moving on moving asmaddeningit canbe assetting control gives great control, adesigner but rendering quality. 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