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

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

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.

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.

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.

• What is a Lumen?
• Other Types of Lumens and Lumen Limitations
• Lumen Alternatives

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

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.

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.]

 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
Blue LED Yellow Phosphor

 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

 Retina
 Layer of tissue on the back portion of the eye
 contains cells responsive to light (photoreceptors)

ConesRods
(Photopic)
(Scotopic)
Luminous
flux,
Illuminance,
Luminance,
and
Luminous
intensity are
all weighted
by photopic
sensitivity

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.

V I B G Y O RApprox. Color Spectrum
Low light level task visibility
(Mesopic lumens)

Integrating
Sphere
Luminous Flux
(Flow of Light)

 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.

Illuminance meter
Luminance meter Integrating sphere

 A photometer for measuring the directional light
distribution characteristics of sources, luminaires,
media, and surfaces.

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

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

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

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

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

• Use conventional lumens
(candelas, illuminance, etc.) to
design lighting for speed and
accuracy.

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

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.

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)

Spectral Power Distributions – LED

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

Amundadottir, Lockley, Anderson, CIE 2015

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)

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

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:

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.

• 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.

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

• 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

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

• Disadvantage: Doesn’t characterize
any specific response accurately.
MS Rea, Shedding Light on Light and Lighting, 2015

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"

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

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

400 W High
Pressure Sodium
400 W Metal
Halide
50,000 lumens; 24,000 hours

Ceramic Metal HalideHigh Pressure Sodium
78
lpw
115 lpw 95 lpw

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
= (%)

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

Ceramic Metal HalideHigh Pressure Sodium
78
lpw
115 lpw 95 lpw
vis = .38; 95 lpw; CRI = 90vis = .31; 124 lpw; CRI = 21

400 w High
Pressure Sodium
400 w Metal
Halide
vis = .38
vis = .31

Cool White
Warm WhiteSource: Cree data sheet
Blue LED Yellow Phosphor
vis = ?
vis = ?

• 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

• 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

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

This concludes The American Institute of Architects
Continuing Education Systems Course

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

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

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