1. Evaluating Roadway Lighting Systems Using Unit Power DensityBy
David M. Keith
Street and Area Lighting Committee Conference 2001
Marshall Design, Inc Marshall Road Boulder, CO

2. What is Unit Power Density?
Unit Power Density (UPD) is the energy for lighting divided by the area of the roadway
units: Watts / square foot or Watts / square meter (W/ft2) (W/m2)
Marshall Design, Inc Marshall Road Boulder, CO

3. UPD Calculation
UPD = #Luminaire * (1.15*Watts/luminaire) (LumCycle * #Lanes * Width of each lane)
#Luminaire = 2 for staggered arrangement
1.15 factor to match previous work in
IESNA publication LEM
LumCycle is twice the “spacing” for staggered
Marshall Design, Inc Marshall Road Boulder, CO

4. Why use Unit Power Density?
more appropriate than using spacing
spacing definition differs with layout
spacing is inversely proportional while UPD is directly proportional
includes ballast losses, reflects technological opportunities
more universal, useful for different comparisons
Marshall Design, Inc Marshall Road Boulder, CO

5. Why use Unit Power Density?
more appropriate than using money
UPD is a less complex, more stable evaluation
focuses on the lighting system
does not reflect specific utility costs
does not reflect “the cost of money”
UPD is less specific, more generally useful to public
Marshall Design, Inc Marshall Road Boulder, CO

6. Why use Unit Power Density?
applies to roadway lighting systems (not luminaires!)
corresponds to relative costs in
energy & pollution
installation & equipment
operation & maintenance
evaluation of relative performance and savings through comparisons
less valid comparing different wattages or sources
does not address: aesthetics, light trespass, and many other important lighting issues
Marshall Design, Inc Marshall Road Boulder, CO

7. What are the components of a Roadway Lighting System?
“Roadway Lighting System” includes:
luminaire and lamp (source and wattage)
roadway dimensions and surface
geometry of layout, height & overhang (setback)
operation and maintenance characteristics
design criteria
Marshall Design, Inc Marshall Road Boulder, CO

8. Why Roadway Lighting Systems?
overall performance reflects trade-offs
wattage, mounting height, overhang & spacing
maintenance program
cutoff classifications
source types
design methods
value in comparisons of related cases
directly proportional changes
some comparisons are more valid than others
Marshall Design, Inc Marshall Road Boulder, CO

9. Why Roadway Lighting Systems?
NOT limited to evaluating a luminaire's physical or photometric characteristics
shape or materials
photometric distributions or cutoff categories
allows comparative evaluation of
luminaire and lamp
geometry of layout, height & overhang (setback)
operation and maintenance characteristics
design criteria
Marshall Design, Inc Marshall Road Boulder, CO

10. Design Criteria
ANSI/IESNA RP-8-00 American National Standard Practice for Roadway Lighting
Revised in 2000
three separate design methods
Illuminance
Luminance
Small Target Visibility (STV)
Marshall Design, Inc Marshall Road Boulder, CO

11. Design Methods: Illuminance
Illuminance method
classical
lighting system alone
lamp, luminaire and photometry
system geometry
one uniformity criterion: average to minimum
no constraint on Emax
now includes veiling luminance criterion
constrains Lvmax, from luminance calculation
Marshall Design, Inc Marshall Road Boulder, CO

12. Design Methods: Luminance
Luminance method
recent
roadway and lighting system interaction
lamp, luminaire and photometry
system geometry
roadway surface
two uniformity criteria
average to minimum, maximum to minimum
”moving observer” & glare calculations
Marshall Design, Inc Marshall Road Boulder, CO

13. Design Methods: STV Small Target Visibility method (STV)
brand new in 2000 document
unfamiliar and complex metric VL
uses luminance, both horizontal and vertical
contrast weighted over entire roadway
veiling luminance included
extension of luminance calculations
radically different design techniques
not suitable for optimization
Marshall Design, Inc Marshall Road Boulder, CO

14. Roadway Lighting Criteria
Classifications Eavg Eavg / Lavg Lavg / Lmax / Lveil /
Roadway Area R3 Emin Lmin Lmin Lavg
(lux) (cd/m2)
Major Med
Collector Med
Local Med
Source: ANSI/IESNA RP-8-00
All system calculations meet entire set(s) of criteria – averages, uniformities & glare
Marshall Design, Inc Marshall Road Boulder, CO

15. Sources high pressure sodium (HPS) or metal halide (MH)
difference in Light Loss Factor (LLF) - at end of life!
probably have one or two luminaires contributing to point
0.7 for HPS
0.5 for MH
“both should be even lower”
difference in lamp life and in maintenance
does represent pulse start MH (vertical lamps)
better rated lumens but same LLF 40% of life)
Marshall Design, Inc Marshall Road Boulder, CO

16. Lamp and Luminaire Data
Lamp Wattage Rated Lumens Input Watts LLF
HPS , , ,
MHP , , ,
Marshall Design, Inc Marshall Road Boulder, CO

17. Cutoff Classifications
relative to lamp lumen rating
combination of intensity limits in two separate zones
both just below and anywhere above horizontal FC CO SC
Marshall Design, Inc Marshall Road Boulder, CO

18. Photometric Files Lamp Wattage All FC CO SC NC HPS 150 67 17 21 14 15
All % % % % MH
All % % % %
All % % % %
Marshall Design, Inc Marshall Road Boulder, CO

19. Roadway and Lamp Wattage
Roadway Class
Width Lanes Local Collector Major
(m)
/175 & /175, 250 & & 400
/175 & /175, 250 & & 400
/175, 250 & & 400
& 400
& 400
& 400
Marshall Design, Inc Marshall Road Boulder, CO

20. Optimization For each combination of “photometry & roadway”
Find the geometry with max. luminaire cycle
Over a range of mounting heights and over a range of setbacks/overhangs
Meeting entire set of appropriate criteria
Result is “best” for combination – but may be impractical (too high or out over road)
Marshall Design, Inc Marshall Road Boulder, CO

21. UPD vs Avg Luminance: Collector 2 Lanes 250W HPS
Marshall Design, Inc Marshall Road Boulder, CO

22. Comparing Cutoff Classifications
for 250W HPS, photometric file distribution is
Lamp Wattage All FC CO SC NC
HPS % 27% 20% 10%
Best (lowest) UPD values mostly SC or NC
in “Best 5”, all SC or NC
in “Best 10”, one is CO and all others SC or NC
best FC is tied for 13th best UPD value
Marshall Design, Inc Marshall Road Boulder, CO

23. UPD vs Avg Luminance: Collector 2 Lanes 250W MHP
Marshall Design, Inc Marshall Road Boulder, CO

24. Comparing Cutoff Classifications
for 250W MH, photometric file distribution is
Lamp Wattage All FC CO SC NC
MH % % 19% 15%
Best (lowest) UPD values mostly SC or NC
in “Best 5”, all SC or NC
in “Best 10”, one is FC, others all SC or NC
best FC is 9th best UPD value
Marshall Design, Inc Marshall Road Boulder, CO

25. UPD vs Avg Luminance: Collector 2 Lanes 250W HPS
Marshall Design, Inc Marshall Road Boulder, CO

26. UPD vs Avg Luminance: Collector 2 Lanes 250W MHP
Marshall Design, Inc Marshall Road Boulder, CO

27. Comparing Sources: HPS vs MHP
UPD (W/m2)
#Averaged HPS MHP %Incr to MHP Best % Best % Best % Best %
Marshall Design, Inc Marshall Road Boulder, CO

28. Comparing Design Methods
Base calculations meet criteria for BOTH illuminance and luminance methods
this is the most conservative approach
recalculate for meeting criteria of either illuminance method OR luminance method
different criteria, same optimization procedure
compare each luminaire’s performance under each single method to Base (BOTH methods)
Marshall Design, Inc Marshall Road Boulder, CO

29. UPD by Design Method: Collector 2 Lanes 250W HPS
Marshall Design, Inc Marshall Road Boulder, CO

30. Comparing Design Methods: HPS
UPD (W/m2)
#Avgd Base Illum Lum %Decr to Lum
Best %
Best %
Best %
Best %
Marshall Design, Inc Marshall Road Boulder, CO

31. UPD by Design Method: Collector 2 Lanes 250W MH
Marshall Design, Inc Marshall Road Boulder, CO

32. Comparing Design Methods: MH
UPD (W/m2)
#Avgd Base Illum Lum %Decr to Lum
Best %
Best %
Best %
Best %
Marshall Design, Inc Marshall Road Boulder, CO

33. Comparing Lamp Output corresponds to the product of:
Light Loss Factor (dependent on maintenance)
Rated Lumens (dependent on lamp technology)
for one lane roads with HPS luminaires, make separate calculations for LLF of 0.50 or 0.70
40% LLF increase ~ 16% UPD decrease (3:1)
for all MH, replace standard with Pulse-Start
each 2% lumen increase ~ 1% UPD decrease (2:1)
Marshall Design, Inc Marshall Road Boulder, CO

34. Comparing Overhang Limits
overhang may be restricted by utility or jurisdiction (it’s a maintenance safety issue)
recalculate all 250W HPS and MHP for overhang <= zero (Oh<=0)
allow setbacks, but no luminaires over roadway
typically no effect or increase UPD up to 15%
may change which files have lowest UPD values
increase greater for wider roadways
Marshall Design, Inc Marshall Road Boulder, CO

35. What are the characteristics of a Roadway Lighting System?
“Roadway Lighting System” includes:
luminaire and lamp (source and wattage)
roadway dimensions and surface
geometry of layout, height & overhang (setback)
operation and maintenance characteristics
design criteria
uplight (like skyglow but more specific to lighting)
Marshall Design, Inc Marshall Road Boulder, CO

36. What is Unit Uplight Density?
Unit Uplight Density (UUD) is the uplight from lighting divided by the area of the roadway
units: lumens / square foot or lumens / square meter (lms/ft2) (lms/m2)
Marshall Design, Inc Marshall Road Boulder, CO

37. UUD Calculation
UUD = Uplight + ReflfromRoad + ReflfromOffRoad (LumCycle * #Lanes * Width of each lane)
Uplight: all “up lumens” (2 luminaires for staggered)
ReflfromRoad: 0.07 * lumens onto the roadway
ReflfromOffRoad: 0.18 * “down lumens” not on road
Marshall Design, Inc Marshall Road Boulder, CO

38. Comparing Uplight with the “best six” luminaires
from 400W MHP on 4 lane Major road
two FC, two CO and two NC
all have full spherical photometric data
evaluate Unit Power Density (UPD) and Unit Uplight Density (UUD) for each luminaire
does more cutoff correspond to less uplight?
does system efficiency (UPD) correspond to uplight?
Marshall Design, Inc Marshall Road Boulder, CO

39. UPD vs Avg Luminance: Major 4 Lanes 400W MHP
Marshall Design, Inc Marshall Road Boulder, CO

40. Comparing UPD and UUD Base UPD Base UUD (W/m2) (lms/m2) FC 0.78 3.8NC NC
Marshall Design, Inc Marshall Road Boulder, CO

41. Comparing Uplight with the “best six” luminaires
evaluate UPD and UUD for each luminaire
revise conditions
for overhang <= 0 (luminaire not over roadway)
for Luminance design method
for Small Target Visibility (STV) design method
compare trends and UPD-UUD relationship across different conditions
Marshall Design, Inc Marshall Road Boulder, CO

42. Comparing Uplight: UPD
Marshall Design, Inc Marshall Road Boulder, CO

43. Comparing Uplight: UUD
Marshall Design, Inc Marshall Road Boulder, CO

44. Comparing Uplight
more stringent cutoff (FC or CO) does not necessarily correspond to less uplight
as UPD increases, UUD increases
as UPD decreases, UUD decreases
changes in UUD are nearly (but not always) proportional to changes in UPD
more efficient lighting system (lower UPD) does correspond to less uplight
Marshall Design, Inc Marshall Road Boulder, CO

45. Summary of UPD Comparisons
deltaUPD corresponds strongly to deltaUUD
deltaUPD is ~ 1/2 to 1/3 of deltaLampOutput
UPD drops up to 25% for Luminance method
UPD drops up to 35% for STV method
systems with lowest UPD values typically have distributions with less stringent cutoff
When full cutoff distributions are required, what is the increase in UPD?
Marshall Design, Inc Marshall Road Boulder, CO

46. % Increase (any Wattage) in Base UPD for “Full Cutoff Required”
Road Local Collector Major
#Lanes 1L L L L L L L L
HPS
Best % 17% 15% 17% 18% 12% 14% 15%
Best % 28% 27% 28% 18% 17% 18% 16% MH
Best % 19% 18% 22% 20% 13% 14% 12%
Best % 22% 20% 22% 24% 16% 15% 13%
Marshall Design, Inc Marshall Road Boulder, CO

47. % Increase in UPD for Full Cutoff
Marshall Design, Inc Marshall Road Boulder, CO

48. Conclusions of UPD Evaluations
There is a substantial potential for reductions in equipment, costs, energy use & uplight which correspond to lower Unit Power Density values for roadway lighting systems.
Comparing systems can lead to results which may be counterintuitive (FC ~ less efficiency).
The best use of this work may be for comparisons with specific UPD values developed from proposed roadway lighting systems with similar characteristics.
Marshall Design, Inc Marshall Road Boulder, CO

49. Evaluating Roadway Lighting Systems Using Unit Power Density
By David M. Keith
for a copy of this presentation:
resodance.com/mdi/SALC2001.html
for questions or more information:
Marshall Design, Inc Marshall Road Boulder, CO