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Shedding Light on Lumens - Capturing the True Efficiency Of White Light

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Craig Bernecker and Naomi Miller, presenters: Lumens and foot-candles are measures of light so often considered critical to lighting design and the energy efficiency of lighting systems, yet the basis for these units is also often misunderstood. This seminar reviews the foundation for the lumen (and in turn, foot-candles), illustrates why the lumen often misrepresents the perceived quantity of light, and why the lumen is inadequate to describe nighttime visibility, circadian effect, lighting for plant growth, and more. Should we use different measures to evaluate the energy efficiency of lighting systems, especially LEDs?

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Shedding Light on Lumens - Capturing the True Efficiency Of White Light

  1. 1. Craig A. Bernecker, Ph.D., FIES, LC The Lighting Education Institute; Parsons The New School for Design Naomi Johnson Miller, FIES, FIALD, LC Pacific Northwest National Laboratory
  2. 2. Credit(s) earned on completion of this course will be reported to AIA CES for AIA members. Certificates of Completion for both AIA members and non-AIA members are available upon request. This course is registered with AIA CES for continuing professional education. As such, it does not include content that may be deemed or construed to be an approval or endorsement by the AIA of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product. ___________________________________________ Questions related to specific materials, methods, and services will be addressed at the conclusion of this presentation.
  3. 3. Abstract: Lumens and footcandles are measures of light so often considered critical to lighting design and the energy efficiency of lighting systems, yet the basis for these units is also often misunderstood. This seminar reviews the foundation for the lumen, and in turn footcandles, illustrates some of the issues in using these measures to evaluate the performance of lighting systems, and suggests an alternative method of evaluating spectral effectiveness for specific applications.
  4. 4. Learning Objectives:  Understand how the human body processes visible radiation for different needs, weighting the spectrum differently for different tasks  Learn about the different photoreceptors in the eye and their spectral sensitivity  Hear how the lumen was originally derived, subsequently modified, applied to all lighting uses; and is still a unit unsuited for measuring brightness perception, nighttime visibility, circadian physiological effect, etc.  Be able to articulate why the lumen has a narrow use, and why lighting professionals need to be conversant in other ways to evaluate the effectiveness of lighting energy.  See some proposals for modifying the lumen, adding variants on the lumen for specialized applications, and/or evaluating radiance weighted by spectral response curves.
  5. 5. • What is a Lumen? • Other Types of Lumens and Lumen Limitations • Lumen Alternatives
  6. 6. • What is a Lumen? • Other Types of Lumens and Lumen Limitations • Lumen Alternatives
  7. 7. Lumen (anatomy), the cavity or channel within a tubular structure Thylakoid lumen, the inner membrane space of the chloroplast Phenobarbital (trade name) Lumen (website), a database of Digital Millennium Copyright Act takedown requests Lumen (branding agency), a design and branding company headquartered in Milan, Italy Lumens (company), a Sacramento lighting company 141 Lumen, an asteroid Lumen (band), a Russian rock band Lumen Martin Winter (1908–1982), American artist Lumen Pierce, a fictional character in the television series Dexter USS Lumen (AKA-30), a US Navy ship Lumen (unit), the SI unit of luminous flux
  8. 8. Luminous Flux (Flow of Light) “The time rate of flow of light.” Unit = Lumen Symbol =  - typically used to indicate the total amount of light given off by a light source.
  9. 9. Radiant energy that is capable of exciting the retina and producing a visual sensation. The visible portion of the electromagnetic spectrum extends from about 380 to 770 nanometers. - ANSI/IESNA RP-16-1996 [1 Physics a The form of electromagnetic radiation that stimulates the organs of sight, having wavelengths between about 3,900 and 7,700 angstroms. - The New International Webster’s Collegiate Dictionary Of The English Language, 2002.]
  10. 10.  LEDs are narrowband light sources  Many techniques for making white light  Phosphors o Downconvert short wavelength (higher energy) to longer wavelength (lower energy) o Inefficiency (Stokes loss) o Performance degradation over time/temperature Cool White Warm White Source: Cree data sheet Source: Cree data sheet Blue LED Yellow Phosphor
  11. 11.  Cones (~8 Million)  “Photopic” vision  High resolution  Color vision  Good response at 5+ fc  Central vision  Rods (~120 million)  “Scotopic” vision  No color vision  Important <1 fc  Peripheral vision  Low resolution  Sensitivity to motion  Melanopsin-producing ipRGCs
  12. 12.  Retina  Layer of tissue on the back portion of the eye  contains cells responsive to light (photoreceptors)
  13. 13. ConesRods (Photopic) (Scotopic) Luminous flux, Illuminance, Luminance, and Luminous intensity are all weighted by photopic sensitivity
  14. 14. The lumen is the only SI unit based on a human response. It’s watts of radiant energy in the visible range, weighted by V- lambda. The Lumen was defined in 1931 by the CIE based on a 2º visual field. It was redefined in 1978 based on a 10º field, (which effectively adds more blue content to the weighting of the lumen). The 1978 lumen is almost never used.
  15. 15. V I B G Y O RApprox. Color Spectrum Low light level task visibility (Mesopic lumens)
  16. 16. Basic Lighting Measures
  17. 17. Integrating Sphere Luminous Flux (Flow of Light)
  18. 18.  Photometry, a special branch of radiometry, is the measurement of radiation in terms of human visual response  A photometer is any instrument used for measuring specific photometric quantities, including luminance, luminous intensity, luminous flux and illuminance.
  19. 19. Illuminance meter Luminance meter Integrating sphere
  20. 20.  A photometer for measuring the directional light distribution characteristics of sources, luminaires, media, and surfaces.
  21. 21. Efficacy (Luminous Efficacy) “The quotient of the total luminous flux emitted by the total lamp power.” Unit = lumens/watt Used to compare lamp “efficiencies” 150 W incandescent = 18.6 lpw 40 W fluorescent = 69.5 lpw
  22. 22. Standard Incandescent Halogen Halogen Infrared Reflecting Mercury Vapor Compact Fluorescent (5 – 55 watts) Linear Fluorescent Metal Halide High Pressure Sodium Low Pressure Sodium LED (Red, Orange, Green, Blue and White) 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 Lumens/Watt Electrodeless
  23. 23. Illuminance “The areal density of the luminous flux incident at a point on a surface.” Unit = footcandle [lux] Symbol = E - used to describe the quantity (density) of light incident on a surface - E = luminous flux/area Lumens
  24. 24. What is the pathway for circadian effect? • Retinal ganglion cells pick up signals of light and dark • Peak sensitivity around 490 nm (blue) • Rods and cones have some contribution • Yellow wavelengths may counteract blue? • Full effects of polychromatic light not fully understood • Signals sent to the suprachiasmatic nucleus (SCN), the timekeeper of the central brainDr. Mark Rea, Lighting Research Center
  25. 25. 80% of the neural fibers transmit signals to the visual cortex for vision 20% of the neural fibers send their signals to other areas of the body and brain, including those that control body’s timeclock and hormone center • Suprachiasmatic Nucleus (SCN) • Pituitary gland (melatonin signal) • Pineal gland • Adrenal gland • Thyroid gland There are rumors of at least four MORE ipRGCs whose neural fibers do not travel to the visual cortex. IESNA Lighting Handbook, 2000
  26. 26. • What is a Lumen? • Other Types of Lumens and Lumen Limitations • Lumen Alternatives
  27. 27. • Use conventional lumens (candelas, illuminance, etc.) to design lighting for speed and accuracy.
  28. 28. Follow IES TM-12-12 procedure: • Estimate photopic roadway adaptation luminance • Use light source SPD to calculate Scotopic/Photopic (S/P) lumen ratio • Multiply photopic illuminance by effective luminance multiplier to get effective mesopic illuminance
  29. 29. Credit: Theresa Goodman, National Physical Laboratory PhotoCredit:IanAshdown,AGI32 Color starts dropping out at 3 cd/m2 and below. There is no color perception at 10-3 cd/m2 and below.
  30. 30. Where "b" is an average value calculated from measured reference samples for a specific medium. For example, b = 0.012 for watercolors, or b = 0.038 for newspapers. sdf(λ) = exp[b(300−λ)] 0.012 0.038 Museum Materials Damage Function, S(λ)
  31. 31. Damage Function, S(λ)
  32. 32. Spectral Power Distributions – LED
  33. 33. B = 0.012 LED 1 LED 2 LED 3 LED 4 Halogen Filtered Halogen CCT 2740 2756 2771 6437 2863 2854 CRI 81 82 96 75 99 96 CIE Relative Damage 0.91 0.71 0.76 1.35 1.00 0.75 Example Damage Potential Comparison For more information on relative damage go to calculator at http://research.ng-london.org.uk/scientific/spd/?page=home
  34. 34. Amundadottir, Lockley, Anderson, CIE 2015
  35. 35. The Well Building Standard Ratio of melanopic lux to photopic lux (M/P) Use this as a rough guide only. CCT is a very poor way to characterize light sources! How do you evaluate light sources for circadian effect? The results depend on the model you choose. (Example: Lucas et al 2015)
  36. 36. Do the researchers agree on the circadian response function? Nope. Dashed line here is a Lighting Research Center model. “Measuring and using light in the melanopsin age“ by Lucas, Berson, Czeisler, Figueiro, Lockley, Provencio, Skene, Brainard et al. Paper cautions that there is no accepted model of circadian response, and it is highly context-dependent. No clear process for applying this information. January 2014
  37. 37. Figueiro, Bullough, and Rea Relative Effectiveness of Light Sources for Circadian Effect (based on Melatonin suppression) by Figueiro, Bullough, Rea Light Source Circadian LPW 3000K T8 109 4100K T8 67 6500K T8 184 7500K T8 90 Metal Halide 86 White LED 82 Blue LED 295 2700K CFL 38 Incandescent 12 3500K T8 ~109 How do you evaluate light sources for circadian effect? The results depend on the model you choose. Example:
  38. 38. Example: Lighting for Neonatal unit • Design to lux or circadian lux? What model? • Design at multiple times of day? • Measured at eye or workplane? • Who gets control of lighting/programming? • Design for the mom, baby, day nurse or night nurse? • How does light spectrum affect tissue color evaluation? (Cyanosis, jaundice, redness) • How do you know if it’s working? WE DON’T KNOW. NEED STUDY AND DISCUSSION.
  39. 39. • Change illuminance at the eye to get primary effect. (High for day, low for night for diurnal humans) • Change CCT to get secondary effect. (High for day, low for night for diurnal humans) Remember that individual needs for light vary, according to age, health conditions, circadian cycle, light history, work schedule/social schedule. Light “treatment” may vary for different individuals using the same space.
  40. 40. LRC model for scene brightness spectral response • Increased sensitivity to short wavelengths • Different from mesopic response • Seems to be a function of photopic, scotopic, AND ipRGC response. • Varies according to adaptation luminance Besenecker, Bullough and Radetsky 2015
  41. 41. • Scene brightness may contribute to perception of safety • Blue wavelengths will increase scene brightness, and perhaps allow reduction of photopic illuminance compared to HPS? • This may be why LEDs LOOK so much brighter than HPS at equal light levels. 100W HPS (above) 50W 2700 K LED (right) Stanford University www.kenricephotography.com
  42. 42. • What is a Lumen? • Other Types of Lumens and Lumen Limitations • Lumen Alternatives
  43. 43. A proposal by Dr. Mark Rea of the LRC: • Define the “universal lumen” as the area underneath all the photoreceptor sensitivity functions • Define the shoulders as the S-cone and the L-cone curves (everything in grey is included) • Advantage: Doesn’t shortchange short wavelengths for nighttime vision or circadian response or brightness response, for example. MS Rea, Shedding Light on Light and Lighting, 2015
  44. 44. • Disadvantage: Doesn’t characterize any specific response accurately. MS Rea, Shedding Light on Light and Lighting, 2015
  45. 45. 1 the quality or degree of being efficient 2 a: efficient operation b (1): effective operation as measured by a comparison of production with cost (as in energy, time, and money) (2): the ratio of the useful energy delivered by a machine or in a process to the total energy expended or heat taken in. "the boiler has an efficiency of 45 per cent"
  46. 46. Energy Efficiency Ratio (EER) of a particular cooling device is the ratio of output cooling energy (BTU) to input electrical energy (W) EER = ----------------- BTU W
  47. 47. Loudspeaker efficiency is defined as the sound power output divided by the electrical power input.  Acoustic efficiency η (eta) of a loudspeaker is: where Pak = emitted sound power of the speaker Pe = input electrical power
  48. 48. 400 W High Pressure Sodium 400 W Metal Halide 50,000 lumens; 24,000 hours 34,000 lumens; 20,000 hours
  49. 49. Ceramic Metal HalideHigh Pressure Sodium 78 lpw 115 lpw 95 lpw
  50. 50. Visual Efficiency (Visible Radiant Power?) “The quotient of the total radiant flux emitted w/in visible spectrum by the total lamp power.” Symbol = vis Unit = radiant flux (watts)/input (elec.) watts = (%)
  51. 51. Lamp Type Wattage vis lpw Incandescent 100 .09 16 T8 Fluorescent 32 .25-.27 90 Mercury Vapor 400 .15 55 Quartz MH (NaTlln) 400 .24 80 Quartz MH (NaSc) PS 400 .35 110 Ceramic MH, 3000K 100 .35 98 Ceramic MH, 4000K 100 .38 95 HPS 150 .22 90 HPS 400 .31 124 LPS 180 .39 200 Source: Philips Lighting
  52. 52. Ceramic Metal HalideHigh Pressure Sodium 78 lpw 115 lpw 95 lpw vis = .38; 95 lpw; CRI = 90vis = .31; 124 lpw; CRI = 21
  53. 53. 400 w High Pressure Sodium 400 w Metal Halide 50,000 lumens; 24,000 hours 34,000 lumens; 20,000 hours vis = .38 vis = .31
  54. 54. Cool White Warm WhiteSource: Cree data sheet Source: Cree data sheet Blue LED Yellow Phosphor vis = ? vis = ?
  55. 55. • Leave the lumen alone. It’s a metric we all know. • Use the color data from the LM-79 sphere report to sum the radiant power at every wavelength in the visible range. (Visible radiant power) • Use a spreadsheet with different action spectra to evaluate the SPD for the lumens you need for your application (photopic, mesopic, scotopic, melanopic, circadian, blue light hazard, material damage, brightness, whiteness, geranium flowering, and whatever new photoreceptor or material response comes along in the future…..) vis = 0.32
  56. 56. • Additional way to analyze energy efficiency of a light source for a specific application? • Use the color data from the LM-79 sphere report to get full visible radiant power. Multiply by Vλ to get lumens and by alternate sensitivity curve or action spectrum to get alternate lumen count (e.g. mesopic lumens). Specific application efficacy?: Full visible radiant power X sensitivity curve or action spectrum = Electrical Watts
  57. 57. Conclusions • The lumen is a metric that works in narrow conditions • Alternate sensitivity curves or action spectra can be applied to an SPD to determine an alternate type of “lumen.” • CCT is a poor way to characterize an SPD, so use the full spectral data. • Alternate “lumens” or visible radiant power can be used in addition to photopic lumens to evaluate performance of white light
  58. 58. This concludes The American Institute of Architects Continuing Education Systems Course
  59. 59. Thanks! Craig A. Bernecker, Ph.D., FIES, LC The Lighting Education Institute; Parsons The New School for Design Craig.bernecker @ gmail.com Naomi Johnson Miller, FIES, FIALD, LC Pacific Northwest National Laboratory Naomi.Miller @ PNNL.gov

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