Cornell University Ergonomics Web

DEA3500: Ambient Environment: Zonal Cavity

BASIC DATA

Reflectances -

• ceiling 80% (0.8)
• walls between luminaire and work plane 50% (0.5)
• walls below work plane 10% (0.1)
• floor 30% (0.3)

Room dimensions -

• length (L) = 30 feet
• width (w) = 20 feet
• height (h) = 10 feet

Working plane - work tables are 3.0 feet above floor

Room is air-conditioned and kept very clean (clean conditions needed for experiments).

Total space including fully recessed luminaires (ceiling) and walls, is cleaned every 19 months.

Lamps are replaced as soon as they burn out. Max. distance between lamp rows center to center = 8'.

Desired lighting level = 100 footcandles

Planning to use 30 luminaires in five continuous rows to provide uniform illumination.

CALCULATING THE COEFFICIENT OF UTILIZATION LUMINAIRES ETC.

Zonal Cavity Coefficient of Utilization Calculation

Room length = 30', breadth = 20', hrc = 7.0', hfc = 3.0'

Steps:

1. Room cavity ratio = 2.9; ceiling cavity ratio = 0.0 (recessed lamps); floor cavity ratio = 1.2 (i.e. for 20 x 30 x 3' depth)

2. Effective ceiling reflectance = .80; effective floor reflectance = .20 (on table for cavity ratio 1.2, % floor reflectance = 30, % wall below work plane = 10%

3. Coefficient of Utilization for this luminaire = .48 (graph - reflectances, room cavity ratio 3). No correction required here.

4. Calculate footcandles. Planning to use 30 luminaires and 2 lamps per luminaire. Check predicted footcandles against recommended levels in table. Alternatively for 100 fc calculate number of luminaires. # luminaires = 100 fc x 600(area = 60,000 = 25 luminaires (5 less than initial 2 x 3150 x .48w x .78 LLF 2358.7 design.

VCP from table VCP crosswise = 71 VCP lengthwise = 70

CALCULATING THE LIGHT LOSS FACTOR

1. Energy requirements

On graph:

• select luminaire distribution [K - direct fluorescent]
• lighting level - x axis (100 fc)
• where these intersect = wiring capacity in watts per square feet required
• Answer = 4.8 watts ft -2

Calculate spacing and fill in fact sheet:

• spacing ratio = ratio of distance between luminaires center = center / height above workplane
• max spacing = 7.7 ft.
• planned spacing = 3.5 ft., therefore no problems.

2. Calculate light loss factor.

• Light loss factor not to be recovered - not known = 1 (value 1 assigned so this doesn't affect computation)

• Area atmosphere - very clean

• Time between cleaning - 18 months. Use graph to find expected dirt depreciation for very clean and 18 months = 10%.

• Calculate room cavity ratio: L = 30, W = 20, hrc = (10 - 3) = 7. Use graph to find room cavity ratio. For 10% dirt depreciation and CR of 3.0 and direct illumination, room surface dirt depreciation = 98% (.98)

• Lamps replaced as soon as burn out, so LBO = 1.0 (100%)

• Lamp lumen depreciation - use table to find LLD = .88.

• Luminaire dirt depreciation - area atmosphere - very clean. Luminaires to have translucent, unapertured bottom enclosure, so they fall into luminaire category V. Use graph to find LDD. LDD = 90% (0.9).

Calculate light loss factor: LLF = 1.0 x .98 x 1.0 x .88 x .90 = .78

CALCULATING VISUAL COMFORT PROBABILITY

Visual Comfort Probability (VCP)

Work surface illumination = 100 fc.

Luminaire mounting heights 8.5, 10, 13, or 16'

Uniform luminaire distribution through space.

Rooms of stated lengths and widths.

Observed seated with eye height 4' and 4' from rear wall.

Observer looking directly forward.

Observer field of view limited to 53o angle above and directly forward.

When VCP 70 or more for both lengthwise and crosswise views, then glare should not be a problem and most people should be comfortable.

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