PerceivedPerceived BrightnessBrightness inin EntertainmentEntertainment LightingLighting CanCan wewe quantifyquantify it?it?

MikeMike WoodWood HighHigh EndEnd SystemsSystems

Perceived Brightness 1 LDI 2003 WhatWhat isis Brightness?Brightness?

LUMINANCE is what is measured by a light meter – it’s an objective measurement.

BRIGHTNESS however is a completely subjective attribute of light to which humans assign a label between very dim and very bright.

Brightness is perceived, not measured.

Perceived Brightness 2 LDI 2003 FactorsFactors thatthat affectaffect PerceivedPerceived BrightnessBrightness

„ ColorColor TemperatureTemperature

„ FieldField FlatnessFlatness

„ ‘‘RedRed RingRing’’

„ RelativeRelative contrastcontrast

„ DazzleDazzle oror RawRaw PowerPower

„ TimeTime –– adaptationadaptation ofof thethe eyeeye

Perceived Brightness 3 LDI 2003 ColorColor TemperatureTemperature

„ WithinWithin reasonablereasonable limitslimits BlueBlue (cool)(cool) lightlight appearsappears brighterbrighter thanthan RedRed (warm)(warm)

„ ColorColor ofof thethe backgroundbackground amplifiesamplifies thethe effecteffect

Perceived Brightness 4 LDI 2003 Color Temperature

Perceived Brightness 5 LDI 2003 Color Temperature

Perceived Brightness 6 LDI 2003 Color Temperature

Perceived Brightness 7 LDI 2003 FieldField FlatnessFlatness

„ AA CenterCenter HotHot SpotSpot maymay helphelp blendingblending butbut itit doesndoesn’’tt helphelp thethe brightnessbrightness

„ AgainAgain thethe colorcolor ofof thethe backgroundbackground amplifiesamplifies thethe effecteffect

Perceived Brightness 8 LDI 2003 Field Flatness

Perceived Brightness 9 LDI 2003 Field Flatness

Perceived Brightness 10 LDI 2003 Field Flatness

Perceived Brightness 11 LDI 2003 RedRed RingRing

„ AA veryvery commoncommon combinationcombination ofof thethe lastlast twotwo –– colorcolor temperaturetemperature andand flatnessflatness

Perceived Brightness 12 LDI 2003 Red Ring

Perceived Brightness 13 LDI 2003 ContrastContrast

„ AreaArea wherewhere therethere isis mostmost potentialpotential forfor foolingfooling thethe eyeeye andand brainbrain

„ ManyMany illusionsillusions useuse thesethese effectseffects

Perceived Brightness 14 LDI 2003 Contrast

Perceived Brightness 15 LDI 2003 Simultaneous Contrast

Perceived Brightness 16 LDI 2003 Simultaneous Contrast

Perceived Brightness 17 LDI 2003 EffectsEffects ofof ShadowShadow

„ Shadows also affect our perception of contrast

„ We are used to shadows in the real world and we ‘know’ that anything in a shadow is really brighter than it appears, so we compensate.

„ This happens all the time on stage, but most of the time we are not aware that we are making this unconscious compensation

Perceived Brightness 18 LDI 2003 Shadow and Contrast

Perceived Brightness 19 LDI 2003 DazzleDazzle

„ DifficultDifficult toto demonstratedemonstrate inin aa PowerpointPowerpoint slideslide

„ TheThe higherhigher thethe luminanceluminance thethe lessless sensitivesensitive thethe eyeeye becomesbecomes

„ OverloadOverload

„ DoDo coloredcolored strobesstrobes everever reallyreally looklook anythinganything otherother thanthan white?white?

Perceived Brightness 20 LDI 2003 CanCan wewe quantifyquantify thesethese effects?effects?

„ ByBy definitiondefinition thisthis isis aa qualitativequalitative procedureprocedure –– attemptingattempting toto applyapply empiricalempirical valuesvalues toto aa perception.perception.

„ WouldWould bebe usefuluseful toto bebe ableable toto establishestablish somesome repeatablerepeatable quantitativequantitative figuresfigures toto predictpredict whatwhat thethe eyeeye willwill see.see.

Perceived Brightness 21 LDI 2003 SimpleSimple FormulaFormula

„ StartStart byby assumingassuming thatthat thethe perceivedperceived brightnessbrightness cancan bebe representedrepresented byby aa simplesimple setset ofof factorsfactors appliedapplied toto thethe luminanceluminance value.value.

„ ForFor simplicitysimplicity beginbegin withwith justjust twotwo variablesvariables –– ColorColor TemperatureTemperature andand FieldField FlatnessFlatness

Perceived Brightness 22 LDI 2003 SimpleSimple FormulaFormula

Brightness = Lumens x (Flatness Ratio)a x (Color/5600)b

Where a and b are power indices derived empirically from observation by multiple observers

Flatness Ratio is the Edge luminance divided by the Center luminance

Color is arbitrarily referred back to 5600K. This value was chosen because the human eye is optimized for daylight. Other values such as 3200K could also be used.

Perceived Brightness 23 LDI 2003 RealReal DataData –– ExampleExample 11

Edge/ Color Field Center Temp Fixture Center Temp Perceived % Lumens a = 0.1 b = 1.4

Studio Spot 250 4159 lm 0.33 8500K 6669 100%

Studio Spot 400sa 8503 lm 0.42 5600K 7804 117%

This matches reality well – the 400W fixture ‘looked’ 20% brighter than the 250W, not 100% brighter

Perceived Brightness 24 LDI 2003 RealReal DataData –– ExampleExample 22

Edge/ Color Field Center Temp Fixture Center Temp Perceived % Lumens a = 0.1 b = 1.4

Studio Spot 575 8165 lm 0.15 5600K 6772 100%

Studio Spot 575/2 8165 lm 0.15 7200K 9628 142%

Nothing changes here except Color Temp – prediction of 42% brighter! Again, this matches reality very well.

Perceived Brightness 25 LDI 2003 WhereWhere dodo wewe gogo fromfrom here?here?

„ To make this data meaningful and repeatable would need a much larger statistical study to check that all observers see the same results.

„ If this is done it would be possible to predict how ‘Bright’ a fixture would actually look in practice.

„ Caveat – the above simple formula didn’t allow for Contrast or Time effects. Contrast in particular is very significant and would need to be controlled if comparisons are to be useful.

Perceived Brightness 26 LDI 2003 Perceived Brightness 27 LDI 2003