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

where hC = height of cavityW = width of cavityL = length of cavity

+

=WLWLhRCR rc5

+

=WLWLhFCR fc5

+

=WLWLhCR C5

RCR (room cavity ratio)

CCR (ceiling cavity ratio)

FCR (floor cavity ratio)

+

=WLWLhCCR cc5

3) Find the effective ceiling and floor cavity reflectances (from the reference #1)- Use the actual ceiling reflectance value for when fixtures are surface mounted or recessed- Use the actual floor reflectance for if the floor is the work plane

4) Select the CU from fixture manufacturers data (from the reference #2)- Adjust the CU if is other than 20%, using factor from reference #3

fc

fccc

5) Determine the required total lumens (F), number of luminaires (N), and area per luminaire(AL)

F = EA / (CULLF)N = F / FlAl = LW / N

- Light loss factorwhere LLD = Lamp Lumen Depreciation

LDD = Luminaire Dirt Depreciation

LDDLLDLLF =

The LLD factor depends on the type of lamp and the replacement schedule

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

fc

1. Fill out Worksheet- Room Identification: Class room

- 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

hfc(floor cavity height): 0.75 m

(ceiling reflectance): 80 %

(wall reflectance): 50 %

(floor reflectance): 20 %w

f

c

Referenc #3

Referenc #1

Referenc #1

2. Find the required lumens

=

3. Determine number of luminaires

Number of luminaires =

=

4. Determine area per luminaire ALAL= =

5. Check S:MH ratio

6. Arrange luminaires- Draw the luminaires position with dimensions on the rightclass room plan.

LLFCUAEF

=

= LAS

FLumens per luminaires

8 m

12 m

L WNumber of luminaires

7 m

9 m

1 m 1 m2.5 m 2.5 m

2.2 m

2.2 m

2.2 m

1.2 m

1.2 m example

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

Dt

light source

Dt

L

light source

P P

I

EhEn

Ev

D

P

1) Illuminance from a point source

- Normal plan illuminace:

- Horizontal plan illuminance:

- Vertical plan illuminance:

2) Illuminance from a line-shape source

3) Illuminance from a band-shape source

I

Eh

D

PH

L

I

Eh

D

PH

L

E I Pn = 2

E Eh n= cos

E Ev n= sin cos

E IP

HP

LP

HLL D H

h = ++ +

12

12 2 2tan

E I HP

HP

LP

HLL D H

h = ++ +

2 21

2 2 2tan

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

Over 1/10Daylighting

Over 1/7Artificial Lighting + Daylighting

Over 1/3Artificial Lighting

Recommended Uniformity of Illuminance

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, furniture-integrated 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 27C.

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.