Basics of Indoor Lighting CalculationsDEA 3500 Human Factors: Ambient Environment

Determining Light Levelslight levels can be too low or too high for a taskappropriate light levels depend on the visual task to be performedthe IESNA has 9 categories (A thru I) of illumination values (A thru C don’t involve direct

visual performance), each divided into 3 values (low, middle, high)For A thru’ C a speed and accuracy correction is not needed (visual task performance isn’t

relevant)For A thru’ C the average reflectance of the room (ceiling, walls, floor) is used rather than

the reflectance of the task background

Task and Worker CharacteristicsSelection of a value in each illuminance range depends on various weighting factors (WF’s)

for the visual task(s) and worker characteristicsage <40 (-1), 40-55 (0), >55 (+1)speed or accuracy not important (-1), important (0), critical (+1)task background reflectance >70% (-1), 30-70% (0), <30% (+1) the weighting factors are added algebraically to obtain a Total Weighting Factor (TWF)

(e.g. -1, 0 +1 = 0)The TWF is used to determine the appropriate low, middle or high illuminance value in a

category

Lumen MethodBy definition:Illuminance (fc) = Luminous flux (lumens)/Workplane area (ft2)

e.g. 6000 lumens source for a 100 ft2 areaIlluminance = 6000/100 = 60 fc

e.g. assume office space 40’ x 50’, illuminance 100fc, 2 lamps/fixture, 12000 lumens/fixture. How many fixtures?

Illuminance (fc) = total lumens/((# lamps/luminaire) (area/fixture))100 = 12000/(2 x area/fixture)area/fixture = 12000/(2 x 100)= 60 ft2

# fixtures = total area/area per fixture = 40 x 50/60 = 33.3# fixtures = 34

Luminaire DesignUsually not all lumens from a lamp in a luminaire reach the work plane, and the % of light

that does is termed the coefficient of utilization (CU) of the luminaire.Polar curves (graphs)

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– polar curves plot the light output from a luminaire as a function of the angle of incidence from the lamp

Light Loss Factors (LLF)Initial lumens from all lamps decrease with age and with the effects of several factors.Recoverable light loss factors:

– Area atmosphere (how dirty the space is)– Room surface dirt depreciation (RSDD) - affected by area atmosphere, cleaning

frequency, room cavity ratio– Lamp burnout factor (# on/total lamps)– Lamp lumen depreciation (LLD) - (maintained lumens/initial lumens)– Luminaire dirt depreciation (LDD) - extent to which dirt accumulation on luminaire

decreases its performance.Total light loss factor (recoverable)

LLF = (RSDD)(LBO)(LLD)(LDD)

Light Loss Factors (LLF)Unrecoverable light loss factors:

– Luminaire ambient temperature (~1% light output change with every 1°F change)– Voltage to luminaire (~3% light output change with every 1% voltage change)– Ballast factor (commercial ballasts are less efficient than reference test ballast)– Luminaire surface depreciation - luminaire surface aging degrades reflective performance

IESNA Zonal Cavity Method

IESNA Zonal Cavity MethodDetermine cavity ratios:

»Ceiling (CCR) = ((5hCC)(L+W))/(L)(W)»Room (RCR) = ((5hRC)(L+W))/(L)(W)»Floor (FCR) = ((5hFC)(L+W))/(L)(W)

Determine cavity reflectances for ceiling and floor cavities.Select coefficient of utilization (CU - usually given by manufacturer)Determine light loss factors (LLF).Compute average illuminance:fc = (# luminaires)(# lamps/luminaire)(initial lumens/lamp)(CU)(LLF)

total room area (ft2)

# luminaires = (fc)(total room area)(# lamps/luminaire)(initial lumens/lamp)(CU)(LLF)

Lighting SurveyDetermine # survey points by calculating the room index (RI):L = length (meters); W = width (meters);Hm = height of light fitting above workplane

e.g. for a room 6 m long, 3 m wide, fixtures 1.5 m above workplane

RI = (L x W)/ (Hm x (L+W)) = (6x3)/(1.5x(6+3)) = 18/13.5 = 1.33

Minimum # survey points: RI #points<1 4

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Organize survey points on even gridbut not directly under fixtures

Standard Classification of Office ServiceabilityASTM E1662-95a (2005) presents a standard classification of the serviceability of an office

facility in terms of Sound and Visual EnvironmentServiceability is rated in terms of:

– Privacy and Speech Intelligibility– Distraction and Disturbance– Lighting and Glare– Adjustment of Lighting by Occupants– Distant and Outside Views

Standard Classification of Office ServiceabilityASTM E1662-95a (2005) – Lighting and glare

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