Light. Measurement. What is Photometrics? Presentation Outline. What is Photometrics? What will you get out of today s workshop?

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1 All tet, figures, animations, schematics, drawings, and intellectual property contained in this document are Copyright 2001 by. This copyright applies to the paper and electronic versions of this document. All rights reserved. ELECTRONIC COPIES: This presentation may NOT be distributed in any electronic file formats, with one eception, ecept with the epress written consent of the author. In the event permission is granted, such copies must be distributed complete and unaltered. Authorship and copyright information must be preserved in all subsequent reproductions, including this copyright notice. The one eception is that electronic files may be freely distributed, without the author s permission, in Adobe s portable document format (PDF). Such copies must be distributed complete and unaltered, and must include this copyright notice. PAPER COPIES: Paper copies of this document may be freely distributed. Such copies must be distributed complete and unaltered, and must include this copyright notice. It is permissible to charge for copies up to but not eceeding actual duplication costs. Such costs must be reasonable, ordinary, minimal, and must not include any premiums, service charges, salaries, commissions, the like, nor anything else besides your minimum actual cost of providing copies. No part of this presentation, in either the paper or electronic version, may be used for profit. This applies to individuals, organizations, professional societies, and companies. For eample, it is not acceptable to accept an honorarium if this document is used as the basis for a presentation. In the event of any questions and/or doubt of compliance with copyright restrictions, please contact the author directly by at KHouser@UNL.edu or by phone at (402) , Ph.D., LC University of Nebraska-Lincoln Copyright, 2001 What is Photometrics? What is Photometrics? Light Measurement Photometrics can be thought of in terms of Photometrics for Light Sources (spectral issues) 2. Photometrics for Luminaires (directional issues) 3 4 What will you get out of today s workshop? 1. An understanding of the interrelationship between radiometry, the human visual system, and photometry 2. An appreciation of V(,) based photometry 3. An understanding of spectral weighting functions, including: photopic, scotopic, mesopic, tri-chromacy Presentation Outline 1. Review of fundamental quantities 2. Relationships between fundamental quantities 3. Radiometry vs. Photometry 4. Photopic Scotopic Systems based on Tri-Chromacy Systems endorsed by CIE 5 6 Page 1 of 19

2 Review of Fundamental 1 Quantities Luminous Flu Total amount of lamp light in all directions Measured in Lumens Symbol 7 8 Luminous Intensity Concentration of light in a particular direction Measured in Candela Symbol I Illuminance Density of light on a surface Measured in Lu (fc) Symbol E 9 10 Eitance Density of light off or through a surface Measured in Lumens/m 2 or Lumens/ft 2 Symbol M Luminance Concentration of light directed toward the eye Measured in Candela/m 2 Symbol L Page 2 of 19

3 Fundamental Lighting Units 2 Relationships between Fundamental Quantities Term Luminous Flu ( ) Total amount of lamp light in all directions Luminous Intensity (I) Concentration of light in a particular direction Illuminance (E) Density of light on a surface Eitance (M) Density of light off or through a surface Luminance (L) Concentration of light directed towards the eye Unit Lumen Candela Footcandle, Lu Lumen sq. foot or meter Candela sq. foot or meter 13 Footcandle Lu Candela Lumen Sq. Foot Lumen Sq. Meter Lumen Steradian What s a Steradian? 14 Plane Angle: Radians Solid Angle: Steradians θ r l θ l r Ω A r 2 θ ~ Radians Ω ~ Steradians Solid Angle: Steradians Solid Angle: Steradians r 1 r 2 W 1 L 1 W 2 L 2 Ω A r 2 (L)(W) steradian r Ω A (L)(W) steradian r 2 r Page 3 of 19

4 Relationships Between Units Relationships Between Units Consider a uniform one candela light source. Now consider this source at the center of a sphere of unit radius. 1 Foot or 1 Meter Relationships Between Units Relationships Between Units Now consider an opening of 1 sq. ft (or 1 sq. meter) at the surface of the sphere. 1 Foot or 1 Meter Q. What is the solid angle subtended by the opening? A. 1 steradian 1 Foot or 1 Meter Relationships Between Units Relationships Between Units Q. How many lumens are passing through the opening? Q. What is the illuminance at any point on the inside of the sphere? A. 1 lumen 1 Foot or 1 Meter A. 1 fc (or 1 l) 1 Foot or 1 Meter Page 4 of 19

5 Relationships Between Units 3 Radiometry versus Photometry Q. How many lumens does our uniform 1 cd source produce? 1. Radiometry Not weighted for human vision Measurement of radiant energy (watts) A. The sphere has a total area of 4π ft 2 (or m 2 ), and 1 lumen falling on each unit area. Therefore, it produces 4π lumens. 1 Foot or 1 Meter Etents are the entire spectrum of radiant energy 2. Photometry that is weighted for some aspect of human vision Etents are limited to the range of visible energy Counterpart Units Spectral Weighting Functions AKA: Luminous Efficiency Functions Radiometry Radiant Flu ( e ) Radiant Intensity (I e ) Watts Watts steradian Photometry Luminous Flu ( ) Luminous Intensity (I) Lumen Candela Definition: Any mathematical function used to weight the spectral power distribution of an illuminant. Irradiance (E e ) Watts meter 2 Illuminance (E) Lu Radiant Eitance (M e ) Watts meter 2 Eitance (M) Lumen meter 2 Radiance (L e ) Watts meter 2 sr Luminance (L) Candela meter Luminous Efficiency Functions Underlying Principle Luminous Efficiency Functions Underlying Principle two stimuli produce equivalent visual response Test, t Reference, r ratio between radiances defines luminous efficiency Subjects adjust the test field until it matches the reference on a criterion visual response Page 5 of 19

6 Luminous Efficiency Functions Underlying Principle Luminous Efficiency Functions Underlying Principle 100 Test, t Weighting t Radiance, r Radiance, t Radiance % 50 0 Ref, r Wavelength (nm) Luminous Efficiency Functions Underlying Principle Spectral Weighting Functions An Important Concept Weighting t Radiance, r Radiance, t There are many ways to quantify how radiant energy stimulates vision. Luminous Efficiency % 100 Ref, r 50 Test, t Wavelength (nm) Spectral Weighting Functions An Important Concept Range of Ways to Quantify Radiant Conditions that could be considered when developing a photometric system fall under two general categories: 1. Visual Response Conditions 2. Viewing Conditions Visual Response Detection threshold Brightness Spatial resolution (visual acuity) Temporal resolution (flicker detection) Spatial contrast Reaction time Visual search Attention Color & form recognition Viewing Condition Light level Adaptation level Eccentricity (foveal, parafoveal) Field size (point, 2, 10, full-field) Duration (steady, flash) Observer age (young, adult, aged) Surround conditions Page 6 of 19

7 4 Photopic Scotopic Systems based on Tri-Chromacy Systems endorsed by CIE Photopic vision mediated essentially or eclusively by the cones Defining the Base Unit of Light Defining the Base Unit of Light Base Quantities in the SI System Amount of a Substance Mole Electrical Current Ampere Length Meter Luminous Intensity Candela Mass Gram Temperature Kelvin Time Second Q: What is Measurement? A: The assignment of numbers to objects following some rule Defining the Base Unit of Light The V(,) Function AKA: Photopic Luminous Efficiency Curve Q: What is the rule for assigning numbers to light? Q: How is the Candela defined? A: By reference to the V(,) function (%) Wavelength (nm) Page 7 of 19

8 Defining the Candela I v K m Ie, λ ) 360 V ( λ dλ Where: I v luminous intensity in candela [note: cd lm steradian -1 ] I e,λ radiant intensity in W sr-1 nm-1 V(λ) spectral luminous efficiency for photopic vision K m maimum spectral luminous efficiency (683 lm W -1 ) and the limits of integration are wavelength in nanometers 43 Defining the Lumen Where: θ v θ v K m θe, λ ) 360 V ( λ dλ luminous flu in lumens [note: lm cd sr] θ e,λ radiant flu in W nm -1 V(λ) spectral luminous efficiency for photopic vision K m maimum spectral luminous efficiency (683 lm W -1 ) and the limits of integration are wavelength in nanometers 44 SPD for a 100W A19 V(,) Function SPD for a TL841 V(,) Function Lumens Resultant Lumens Resultant nm nm 46 SPD for a 70W HPS V(,) Function SPD for a 100W MH (NA/SC) V(,) Function Lumens Resultant Lumens Resultant nm nm 48 Page 8 of 19

9 SPD for D65 V(,) Function A Deeper Look at V(,) AKA: Photopic Photometry It is important to understand the eperimental contet for V(,) because of the practical implications on lighting practice. Lumens nm Resultant 49 Eperimental Contet for V(,): A B C Field of View Field Luminance D Wavelength Center of Gravity E Additivity Assumption Eperimental Methods 50 A Deeper Look at V(,) A. Field of View A Deeper Look at V(,) B. Field Luminance V(,) is based on a 2 to 3 field of view. A 2 field size is only about 0.01% of the total visual field that we see with both eyes. Often less than 10 cd/m 2 Field luminance in building interiors is more typically 50 to 200 cd/m A Deeper Look at V(,) C. Additivity Assumption A Deeper Look at V(,) D. Eperimental Methods Additivity law states that the luminance of a miture of lights will be equal to the arithmetic sum of the component luminances Only holds under restricted conditions Perhaps this assumption was driven by the technology available in the early 1900 s V(,) is (mostly) based on flicker photometry Page 9 of 19

10 A Deeper Look at V(,) E. Wavelength Center of Gravity Photopic Wavelength center of gravity is the wavelength that divides in half the area under the visibility curve. It was believed that the visibility curve should have a wavelength center of gravity for a Plankian radiator at 2077 K. Computed to be nm, the curve was balanced and smoothed to meet this requirement. Visual Response Detection threshold Brightness Spatial resolution (visual acuity) Temporal resolution (flicker detection) Spatial contrast Reaction time Visual search Attention Color & form recognition Viewing Condition Light level (~ 10 cd/m 2 ) Adaptation level Eccentricity (foveal, parafoveal) Field size (point, 2, 10, full-field) Duration (steady, flash) Observer age (young, adult, aged) Surround conditions (dark surroundings) A Deeper Look at V(,) Some Interesting Quotations V(,) works well for the conditions used in its derivation Problems occur when it is applied out of contet Although spectral sensitivity functions are adequate to predict the amount of energy required to stimulate vision, they are very poor at describing the perceived effect of suprathreshold lights. For eample, it is well know that perceived brightness differs significantly from photometric luminance under all but very restricted viewing conditions Dr. Alan Lewis Some Interesting Quotations Some Interesting Quotations... photopic illuminance alone does not adequately characterize the visual system spectral response, implying that lighting design for buildings based only on photopic spectral conditions does not capture an important and potentially valuable lighting attribute. Dr. Sam Berman It is worth pointing out that the CIE... have come to regard its standard luminous efficiency function, on which all of photometry is based, as an arbitrary wavelength function adopted for its convenience and utility rather than because luminance so evaluated correlates with what the eye sees. Dr. Dean Judd Page 10 of 19

11 Some Interesting Quotations Some Interesting Quotations... There are many instances in which light is measured in the routine way, with light meters and photometers, and the values recorded bear little or no relationship to the visual impression. CIE Publication it is clear that the normal visual system has definite spectral likes and dislikes (not indicated by the luminous efficiency curve) which bear on every light entering it. Dr. William Thornton Question: Q: Is V(,) a rational way to quantify light for typical illuminating engineering applications? A:... Scotopic vision mediated essentially or eclusively by the rods Scotopic Scotopic Scotopic V (,) Conventional view is that rods are active at light levels below cd/m 2 (equal to illuminance of fc on 80% reflective white paper) (%) Photopic V(,) Wavelength (nm) 66 Page 11 of 19

12 Scotopic Applications for Scotopic Photometry Scotopic Modern View of Rod Function (Driven by Dr. Sam Berman) Applicable when... the visual surrounding is at etremely low illuminance the rods dominate the visual function off-ais viewing (within central 20 field) If electric lighting is present, we re probably above the (traditional) scotopic range Rods contribute to visual functioning at light levels above the traditional scotopic range Rods (may) mediate the dilation and constriction of the pupil Smaller pupils lead to improvements in: Depth of field Visual acuity Scotopic Implications of Dr. Berman s findings for lighting practice Scotopic S/P Ratios for Common Light Sources Vision can be improved by providing short wavelength energy in the peripheral field of view Research suggests lamps with higher S/P ratios will result in better sight The S/P ratio equals scotopic lumens divided by photopic lumens 69 RE Sun Sky (D65) Daylight TL MH (na/sc) Incandescent Warm White W HPS eyes fully adapted to light levels between photopic and scotopic Vision with fully adapted eyes to conditions between those of photopic and scotopic Between 3.4 and cd/m 2 Important region because many outdoor applications fall within this range Page 12 of 19

13 Issues: No single function describes the visual response differs with luminance level No official mesopic system agreed upon (%) Wavelength (nm) 74 Issues: What is the most salient visual response for mesopic photometry? Brightness (Kinney, Palmer, Kokoschka, Ikeda, Sagawa) Reaction Time (He, et. al.) They give similar functions (Berman & Clear) Visual Response Detection threshold Brightness Spatial resolution (visual acuity) Temporal resolution (flicker detection) Spatial contrast Reaction time Visual search Attention Color & form recognition Viewing Condition Light level Adaptation level ( cd/m 2 ) Eccentricity (foveal, parafoveal) Field size (point, 2, 10, large-field) Duration (steady, flash) Observer age (young, adult, aged) Surround conditions Photopic Luminance Multiplying Factor Light Source (relative to HPS) Issues: Multiplying Factors Use a lookup table with multipliers for equivalent visual effectiveness at different luminance levels. 77 Lookup Method (Bierman, Rea) 10 cd/m 2 1 cd/m cd/m cd/m 2 HPS 3000K FL Incandescent MH HPS 3000K FL Incandescent MH HPS 3000K FL Incandescent MH HPS 3000K FL Incandescent MH Page 13 of 19

14 Tri-Chromacy of Human Vision 100 z Tri-Chromacy & Brightness (%) 50 y Wavelength (nm) 80 Opponent Colors Model Tri-Chromacy of Human Vision Opponent Colors Model Tri-Chromacy of Human Vision The three inputs act upon our neural system in a comple way z y B z y G z y R - - Blue/Yellow Red/Green Opponent Opponent (R G) - B (R B) - G Achromatic R G Opponent Colors Model as the Basis for a System of Photometry Opponent Colors Model as the Basis for a System of Photometry A comple spectral weighing function based on the opponent colors model General form may look like... β ( (ar bg cb) p achromatic term R/G term β ( (ar bg cb) p (dr eg fb) p (dr eg fb) p (gr hg ib) p ) 1/p (gr hg ib) p ) 1/p 83 B/Y term 84 Page 14 of 19

15 Lamp Description Photopic Lumens Thornton Guth Howett Mean Rank Rank Rank Rank MH 100W Metal Halide (Na/Sc) D65 CIE D65 Illuminant DPLUS Daylight Delue Fluorescent C K Colortone Fluorescent CW Cool White Fluorescent TL K RE Fluorescent CW-EW Cool White EW Fluorescent TL K RE Fluorescent C K Colortone Fluorescent CDM4K Metal Halide (Na/Tl/Dy/Li) TL K RE Fluorescent Endorsed by CIE CDM3K Metal Halide (Na/Tl/Dy/Li) WW Warm White Fluorescent TL K RE Fluorescent Halogen PAR38 Halogen White HPS White SON HPS Incan. 100W A HPS 70W High Pressure Sodium CIE Spectral Weighting Functions AKA: Luminous Efficiency Functions Other Lightfair Presentations Function Usage V(,) photopic 2 V M (,) photopic 2 physiologically meaningful V 10 (,) photopic 10 physiologically meaningful V b,p (,) photopic point efficiency for brightness I Ian Ashdown Photoreceptive Systems and Elderly & Others with Low/Poor Vision Mark Rea, Alan Lewis V b,2 (,) photopic 2 efficiency for brightness V b,10 (,) photopic 10 efficiency for brightness V (,) scotopic > Thank You 89 Page 15 of 19