4.5.3. Lighting Design: Zonal-Cavity Method (Lumen method)
1) Divide the room into layers or cavities
2) Determine the cavity ratios by formulas
- If fixtures are surface mounted or recessed, CCR will be zero
 L +W 
CR = 5 ×h C ×

 L ×W 
where
hC = height of cavity
W = width of cavity
L = length of cavity
• RCR (room cavity ratio)
 L +W 
RCR = 5 ×h rc ×

 L ×W 
• CCR (ceiling cavity ratio)
 L +W 
CCR = 5 ×h cc ×

 L ×W 
• FCR (floor cavity ratio)
 L +W 
FCR = 5 ×h fc ×

 L ×W 
3) Find the effective ceiling and floor cavity reflectances (from the reference #1)
- Use the actual ceiling reflectance value for ρ cc when fixtures are surface mounted or recessed
- Use the actual floor reflectance for ρ fc if the floor is the work plane
4) Select the CU from fixture manufacturer’s data (from the reference #2)
- Adjust the CU if ρ fc is other than 20%, using factor from reference #3
5) Determine the required total lumens (F), number of luminaires (N), and area per luminaire
(AL)
The LLD factor depends on the type of lamp
and the replacement schedule
F = E∙A / (CU∙LLF)
N = F / Fl
Al = L∙W / N
- Light loss factor LLF = LLD × LDD
where LLD = Lamp Lumen Depreciation
LDD = Luminaire Dirt Depreciation
The LDD factor is determined from the category of luminaire
6) Check S:MH ratio.
- First, estimate spacing by taking the square root of the area per luminaire
MH = mounting height (height
from work surface to light source)
SR (spacing ratio) = S / MH
Reference #1
Reference #2
Reference #3
※ Exercise
1. Fill out Worksheet
- Room Identification: Class room
ρ fc
- Illuminance: 300 lux
- Luminaire Data
Designation: Direct
Lamp per Luminaire: 2
LDD: 0.85
- Lamp Data
Designation: F32 T8 CW
Initial Output: 3150 lm
LLD: 0.9
- Room Data
Height(H): 3 m
Width(W): 8 m
Length(L): 12 m
hcc(ceiling cavity height): 0 m
Referenc #1
hfc(floor cavity height): 0.75 m
Referenc #1
Referenc #3
ρ c (ceiling reflectance): 80 %
ρ w (wall reflectance): 50 %
ρ f (floor reflectance): 20 %
7m
example
1.2 m
2. Find the required lumens
E× A
F=
=
CU × LLF
2.2 m
9m
2.2 m
2.2 m
1.2 m
3. Determine number of luminaires
1m
=
4. Determine area per luminaire AL
LⅹW
AL= Number of luminaires =
S ≅ AL =
6. Arrange luminaires
- Draw the luminaire’s position with dimensions on the right
class room plan.
2.5 m
1m
8m
F
Number of luminaires = Lumens per luminaires
5. Check S:MH ratio
2.5 m
12 m
4.6. Calculation of Illuminance
•
The basic of point source calculations is the inverse square law and the cosine law.
•
Regardless of the luminaire size, it is always possible to choose a distance Dt sufficiently large so
that illuminances produced at distances greater than Dt do vary as the inverse squared distance.
•
Five times rule permits a computational accuracy of at worst 5% of diffuse emitters.
L
L
light source
light source
Dt
Dt
P
P
θ
1) Illuminance from a point source
Iθ
- Normal plan illuminace: En = Iθ
P
Eh
En
D
Ev
θ
β
P2
- Horizontal plan illuminance: Eh = En cosθ
- Vertical plan illuminance: Ev = En sin θ cos β
L
θ
Iθ
2) Illuminance from a line-shape source
H
P
Eh
θ
D
L
θ
HL
 1 H

−1 L
Eh = Iθ 
+ 2
 tan

P L + D2 + H 2 
 2P  P
3) Illuminance from a band-shape source
Iθ
H
P
Eh
D
θ
L
HL
 H H

Eh = Iθ  2  tan − 1 + 2
2
2 
P
P
P
L
+
D
+
H
2



4.7. Uniformity of Illuminance
•
Uniformity of Illumination is a ratio of minimum to maximum illuminance on the working
plane
•
It is necessary to establish a fixture spacing that will give acceptable uniformity of illumination
Recommended Uniformity of
Illuminance
Artificial Lighting
Over 1/3
Daylighting
Over 1/10
Artificial Lighting + Daylighting
Over 1/7
indirect lighting
direct lighting
· Directional down lighting
· General diffuse lighting
· Luminous ceiling
4.8. Checklists for Energy-Conscious Design of Lighting
Systems
General Strategies
1.
Avoid excessively high uniform illuminance levels. After overall levels of 200 to 300 lx are achieved,
provide selective lighting on tasks using portable fixtures, table lamps or floor-mounted lamps, furnitureintegrated fixtures. To minimize veiling reflections, position local lighting so that light on visual tasks will
come from the side, rather than from the front, of the task. Use other supplementary lighting to accent
plants, paintings, entrances, etc.
2.
Group together visual tasks which require similar lighting.
3.
Where illuminance level requirements are not constant, use state-of-the-art dimmers or multilevel ballasts
for fluorescent lamps. Avoid block or group switching of fixtures, which may provide unused light.
4.
Open-plan areas with partial-height barriers can reduce the amount of light that otherwise would be
absorbed by walls in small, enclosed rooms. In open plans, provide separate circulation lighting to avoid
the use of unnecessary ceiling fixtures.
5.
Use a transparent material for the upper part of partitions in order to transmit interior light to corridors,
where low illuminance levels will be sufficient (e.g., about 100 lx)
Light Sources
1.
For constant illuminance level requirements, use single-wattage lamps at the wattage required to produce
the necessary illuminance rather than multilevel lamps which have lower light per watt output.
2.
Because light output per watt increases with lamp wattage, use one incandescent lamp at the wattage
required to produce the necessary illuminance level rather than two or more lower-wattage lamps that
equal or exceed the wattage of the single lamp.
3.
Avoid using extremely long-life lamps which produce less light per watt than short-life lamps, unless
short-life lamps present maintenance problems (e.g., where access for cleaning and relamping is
complicated by hart-to-reach locations).
4.
Use efficient lamps with high lm/W output and efficient, low-wattage-loss ballasts with discharge lamps.
For outdoor lighting (e.g., parking, security), use HID lamps with timers or photocell devices to turn off
lamps when they are not needed.
5.
When fluorescent lamps are used outdoors in cold weather, use high-output or 1500 mA lamps in tightly
enclosed, gasketed luminaires.
Note: Be sure lamps are compatible with color rendition needs.
Luminaires
1.
Use lower mounting heights for semi-direct luminaires and downlights to provide more light on tasks with
less light absorbed by wall surfaces.
2.
Specify luminaires with high CUs and high light loss factors (LLFs). LLF is decimal percent of actual to
rated output.
3.
Use heat-of-light systems, which use water or return air to transfer lamp and ballast heat away from
luminaires (e.g., to building locations requiring heat), since they increase lamp efficacy by keeping
surrounding air temperature below about 27°C.
4.
Use pendant-supported or chain-suspended open fluorescent fixtures rather than enclosed luminaires,
which reduce lm/W output due to buildup of heat from ballast and lamps.
5.
To improve lm/W output or fluorescent lamps, use low-wattage-loss ballasts. Ballasts which operate at
lower power after lamps have started are available.
Coordination with Daylighting
1.
Use lighting systems that are zoned away from daylight sources (e.g., sidelighting at building perimeter,
interior light wells) to allow easy independent switching. Locate tasks requiring highest illuminance levels
at locations where daylighting is best.
2.
Use photocells, timers, or manual switches to reduce illuminance at night or when daylight levels are
sufficient. For fluorescent fixtures, consider using dimming and multilevel ballasts. For incandescent
fixtures, use solid-state dimming devices, as rheostats consume energy continuously.
3.
Use high-reflectance matte wall and ceiling surfaces (e.g., sloped white ceilings can provide useful reflected
daylight). To increase efficiently of reflected light, use sloped surfaces, light shelves, and light-colored
ground cover at base of buildings.
4.
To control glare, use shading devices at window openings (e.g., drapes, fine-mesh screens, narrow-slat
blinds) and at skylights (e.g., prismatic lenses). Window luminance normally should not exceed about 1000
cd/m2 in offices and classrooms.
Note: Be sure that energy consumption due to heat loss and heat gain through openings for daylighting does not
exceed savings in reduced electric lighting energy consumption.