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my reply to Mr. Walerczyk. Thornton’swork strongly suggests thatvision has three invariant visualchannels. Thornton has studied thistopic from several points-of-view,including brightness perception, 5,6chromaticity errors, 7-9 color preference,10 and metamerism. 11-14Thornton has called the spectralregions near 450, 530 and 610 nmthe “prime color” regions of thespectrum. His work suggests thatthese three regions are uniquelyefficient at creating white lightswith exceptional brightness perwatt and excellent color preference.Thornton’s work fully embracestrichromacy, which is the idea thatvision is fundamentally regulated bythree spectral channels. Thorntondid not originate the idea of trichromacy,but he was the first person tomake a practical light source basedon trichromatic principles. In 1979he was recognized as the USNational “Inventor of the Year” bythe Association for the Advancementof Invention and Innovation forhis patent on the triphosphor fluorescentlamp. 15 Trichromacy has along history in vision science. It wasalluded to by Aristotle and can betraced through history in the workof eminent philosophers and scientists:“...the rainbow appears withthree colors. The rainbow hasthree colors, and these threeand no others.” 16 Aristotle 350BC“Each ray of light is compoundedof three other raysonly...” 17 George Palmer, 1777“From three simple sensations,with their combinations,we obtain seven primitive distinctionsof color; but the differentproportions in which theymay be combined afford a varietyof tints beyond all calculations...” 18 Thomas Young, 1845“The theory which I adoptassumes the existence of threeelementary sensations by combinationof which all the actualsensations of color are produced...”19 James Clerk Maxwell,1856“When we speak of reducingthe colors to three fundamentalcolors, this must be understoodin a subjective sense as being anattempt to trace the color sensationsto three fundamentalsensations.” 20 Herman vonHelmholtz, 1866Despite the fact that trichromacyis a given for vision, Dr. Bermanoffers no comment on this topic. Heinstead states that I am “confusedabout the relationship betweenpupil size and room brightness.” Myoriginal article and response to Mr.Walerczyk focused on what Ibelieve are more important considerations.Dr. Berman dismisses as irrelevantthe 29 references I cited thatare related to spectrally derivedmetrics. He states that all of themetrics I quoted are “about subjectivepsychophysical measures notas fundamental as those that providethe basic scotopic and photopicspectral sensitivity functions.”First, allow me to comment on the29 references. Mr. Walerczyk askedme to cite recent research. Iresponded to his question with a listof references as evidence that it ispossible to create an infinite numberof derived spectral metrics. The29 metrics I cited may be irrelevantto Dr. Berman and his S/P agenda,but they are not extraneous to thisdiscussion. It is a fact that Dr. Berman’sS/P ratio is just one of manyspectrally derived metrics. Second,Berman’s assertion that the photopicand scotopic functions areintrinsically fundamental is only halfthe story. It is generally acceptedthat the photopic function is basedon the L and M cone response (notsimply the cone response asBerman states) and that the scotopicfunction is based on the rodresponse. By considering just scotopicand photopic functions the Scones are (apparently) ignored.When I asked Dr. Berman about thisspecific topic he replied that Scones do not appear to be importantfor vision. 21 This reply strikesme as implausible. Dr. Berman’sS/P model implies vision has twochannels. A two-channel model isinconsistent with hundreds of yearsof thinking and more recent evidencethat favors trichromacy.Consequently, it is difficult toaccept a two-channel model as“fundamental.”More than 50 years ago Polyakdescribed the (then) current stateof knowledge about the retinalstructure and color vision. In hisconclusions he stated “The specialfeature of this paper is the dethronementof the photoreceptors—therods and the cones——from their role of absolute rulers inthe visual hierarchy. The physiologyof vision, especially of color vision,was and still is too much under thespell of these structures.” Rods andcones, critical though they are in initiatingvision, are just the startingpoint. It is the complex neurologicalweb and post retinal processingthat ultimately regulates vision. Ibelieve that trichromacy resides notin the retina, but in the visual systemas a whole. Berman’s S/P ratiois one limited way to characterizethe retinal response, but this is fundamentallydifferent than characterizingthe gestalt visual response.Dr. Berman and I also disagree onthe essence of what constitutes awell designed spectral power distribution.He promotes high S/Pratios because he believes thesewill constrict the pupil and enhancebrightness perception. If lamp companieswere to embrace this conceptthey would design electric lightsources that produced monochromaticenergy at 507 nm (the peakof the scotopic function); this ispoor guidance for spectral optimization.I endorse spectral power distributionsthat embrace trichromacybecause trichromatic optimizationleads to white light sources withexceptional color preference andhigh brightness perception perwatt.In closing I wish to restate themain points of my November 2002“Lighting for Quality” article. Lampcompanies rely on metrics such asthe lumen and CRI to quantify thespectral performance of their lightsources. Both V() and CRI havelimitations, particularly when theimpression of brightness is animportant design criterion andwhen exceptional color preferenceis desired. Nevertheless, V() andCRI are numerically objective andeasy to use, and will likely remainthe tools-of-the-trade until markedly22 LD+A/May 2003 www.iesna.org
etter metrics are agreed upon.Foremost questions are: How canwe use light to improve the visualexperience of building users? And,how can we do this while reducingenergy consumption? A path consistentwith the above objectives isto develop light sources that fullyembrace trichromacy.References1. Ten Doesschate, J., andAlpern, M. “Response of the Pupilto Steady-State Retinal Illumination:Contribution by Cones”.Science. 149(3687): 989-991.1965.2. Trejo, L.J., and Cicerone, C.M.“Retinal Sensitivity Measured bythe Pupillary Light Reflex in RCSand Albino Rats”. Vision Research.22: 1163-1171. 1982.3. Kovalevsky, G., DiLoreto, D.,Wyatt. J., del Cerro, C., Cox, C., anddel Cerro, M. “The Intensity of thePupillary Light Reflex Does notCorrelate with the Number ofRetinal Photoreceptor Cells”.133(1): 43-49. 1995.4. Lucas, R.J., Douglas, R.H., andFoster, R.G. “Characterization of anOcular Photopigment Capable ofDriving Pupillary Constriction inMice.” Nature Neuroscience. 4(6):621-626. 2001.4. Thornton, W.A. “A System ofPhotometry and Colorimetry BasedDirectly on Visual Response”. J.Illum. Eng. Soc. October: 99-111.1973.5. Thornton, W.A., Chen, E.,Morton, E.W., and Rachko, D.“Brightness Meter”. J. Illum. Eng.Soc. October: 52-63. 1980.6. Thornton, W.A. “Toward aMore Accurate and ExtensibleColorimetry, Part I. Introduction.The Visual Colorimeter-Spectroradiometer.Experimental Results.”Color Research and Application.17(2): 79-122. 1992.7. Thornton, W.A. “Toward aMore Accurate and ExtensibleColorimetry, Part II. Discussion.”Color Research and Application.17(3): 162-186. 1992.8. Thornton, W.A. “Toward a MoreAccurate and Extensible Colorimetry,Part III. Discussion (continued).”Color Research and Application.17(4): 240-262. 1992.9. Thornton, W.A. “A Validation ofthe Color Preference Index.” J.Illum. Eng. Soc. October: 48-52.1974.11. Thornton, W.A. “MatchingLighting, Metamers, and HumanVisual Response”. Journal of Colorand Appearance. 2(1): 23-29.1973.12. Thornton, W.A. “Lamps forAssessing Metamerism”. J. Illum.Eng. Soc. October: 11-18. 1974.13. Thornton, W.A. “Intersectionsof Spectral Power Distributionsof Lights that Match”.Color Research and Application.18(6): 399-411. 1993.14. Thornton, W.A. “Intersectionsof Matching Spectra: Applications”.Color Research andApplication. 18(6): 412-421.1993.15. Thornton, W.A. “Method andDevice for Efficiently GeneratingWhite Light with Good Rendition ofIlluminated Objects.” US Patent No.4,176,294. 1979.16. Aristotle. Meteorologica. (c.350 B.C.) Translation by E.W.Webster (Oxford: Clarendon Press.1923). MacAdam, D.L. ed. Sourcesof Color Science. The MIT Press:Cambridge, MA, pg. 9. 1970.17. Palmer, G. Theory of Colorsand Vision. (London: Leacroft,1777). MacAdam, D.L. ed. Sourcesof Color Science. The MIT Press:Cambridge, MA, pg. 40. 1970.18. Young, T. “On the Theory ofLight and Colors”. (PhilosophicalTransactions of the Royal Society ofLondon, 92: 20-71. 1802). Mac-Adam, D.L. ed. Sources of ColorScience. The MIT Press: Cambridge,MA, pg. 51. 1970.19. Maxwell, J.C. “Theory of thePerception of Colors”. (Transactionsof the Royal Scottish Societyof Arts, 4: 394-400. 1856).MacAdam, D.L. ed. Sources ofColor Science. The MIT Press:Cambridge, MA, pg. 63. 1970.20. Von Helmholtz, H.L.F. PhysiologicalOptics. (1866). Mac-Adam, D.L. ed. Sources of ColorScience. The MIT Press: Cambridge,MA, pg. 96. 1970.21. Private communication.IESNA Annual Conference. NationalGallery of Canada: Ottawa, Ontario.August 6, 2001.Kevin W. Houser, Ph.D., LCUniversity of Nebraska-LincolnOmaha, NEwww.iesna.org
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