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ILLUMINATION

Prepared by: Engr. Karen-Christian C. Agno Department of Electrical Engineering, UPLB

IMPORTANCE OF ILLUMINATION CALCULATION

It is where the number of lights is specified and verified for a given room

It where the proper placing of each light is known to achieve maximum lighting

LIGHTING THEORY

It has been said that lighting can be of four stages – source, flux, illuminance and luminance

Source – has lighting intensity (I) and is

measured in candela

Flux – flow of light (φ) and which is measured in

lumens (lm)

Illuminance (E) – the level of illumination in the

working surface from the light source; in lux (lm

per square meter)

Luminance (L) – amount of light leaving the

surface that is being illuminated by the light

source

TWO TYPES OF ILLUMINATION

POINT ILLUMINATION METHOD

This method determines the illumination at a specific point in a room from a light source

Factors to consider – luminous intensity, distance, orientation of the surface

Two Laws

a. Rectilinear propagation of light – light travels in straight lines (300000 km/s) and requires no

medium of propagation

b. Inverse Square Law

c. Cosine Law –

Luminous Intensity (I) - luminous flux in a certain direction; unit in candela

ILLUMINATION


Distance – measured from a source to a given surface; affects illuminance; the closer the surface to the

source, the larger the flux to that portion

LUMEN METHOD

Uniform lighting in an area is considered and the number of lighting fixtures to be installed is

calculated

Aside from direct light from the light source, this method also takes into consideration the

reflected lighting casts from walls, ceilings and floors

The level of reflectivity of the surface varies inversely the number of lighting fixtures to be

installed. For surfaces which are least reflector, higher number of luminaries is required.

Components

a. Total system lamp lumen output – total initial luminous flux which lamps emitted which is

specified by the manufacturer

Based on manufacturers’ catalogue (for lamps and luminaire)

i. Lamps – initial lumen output, lighting design lumen output, correlated color

temperature, color rendering index, physical dimensions, ballast type, power

rating, efficacy

ii. Luminaries – light output ratio (LOR), downward light output ratio (DLOR),

upward light output ratio (ULOR), downward flux fraction, space height ratio,

luminous intensity distribution, physical dimensions, type

Initial lumen output – figure is 10 – 15% higher than the lighting design lumen output

Note that some of the lamp lumens are not released totally; they are trapped within the lighting

fixture and therefore do not reach the area to be illuminated.

Students’ Assignment: Read on the different terms presented on a manufacturer’s catalogue for

lamps and luminaires

b. Coefficient of Utilization (CU) – influenced by the efficiency of the luminaire, luminaire

distribution, geometry of the space and the reflectances of the space surface

- each lighting fixture has a unique CU table for lighting distribution and efficiency

- values are determined based on the room geometry and room surface reflectance

- Based on:

Room Cavity Ratio (RCR)

where hrc = room cavity height

L = length of the room

W = width of the room

ILLUMINATION


Ceiling Cavity Ratio (CCR) – distance between the ceiling and the lighting fixture plane

Floor Cavity Ratio (FCR) – distance between the work plane area and the floor

For irregular shape room,

NOTE:

Actual reflectances must be converted into their effective cavity reflectances (e.g. actual

ceiling reflectance effective ceiling cavity reflectance ρcc, actual floor reflectance

effective floor ceiling cavity reflectance ρfc)

c. Light Loss Factor

Two Types:

Recoverable LLF

o lamp lumen depreciation (LLD) – fraction of initial lumens at a specific time

during the lifespan of the lamp; comes from dirt accumulation on lamps,

reflectors, lenses and room surface and usage through time; most design

calculations were based on “maintained” over “initial” lamp lumens

o lamp burnout factor (LBO) – must be considered in the analysis if lamps were

not immediately replaced after burnout; ratio of the number of lamps which will

burnt out over the total number of lamps in the system

o luminaire dirt depreciation factor (LDD) – amount and type of dirt in the

surrounding, type of luminaire used and expected cleaning cycle for the

equipment

o room surface dirt depreciation factor (RSDD) – amount of surrounding dirt,

proportions of the room and type of lighting equipment used

o area of work plane (Awp) – area of the whole work plane, same with the floor

area; for uniform layout of lighting fixtures, illuminance will be greatest near the

center of the area

Figure 1. Cross-section of a room showing room cavities

ILLUMINATION


Non-Recoverable LLF

o Luminaire Ambient Temperature Factor – temperature variations have a

significant effect on light output of fluorescent lamps though have least effect

on high intensity discharge lamps

o Heat Extraction Thermal Factor – fractional lumen loss or gain due to airflow

(has an effect of temperature on lamps and lamp lumens especially those kinds

of air handling fluorescent lighting fixtures integrated with HVAC system as a

medium for air entrance or exit)

o Voltage to Luminaire Factor – will affect the lumen output of lamps in either

high or low voltage; rate of change in lumen output with voltage variation

differs for every light source but is greatest on incandescent lamps

o Ballast Factor – one of the component to determine the rated lumen output for

a lamp as it maintains the arc within it; ratio of the lamp lumens on commercial

ballasts to the test quality ballasts; for good quality fluorescent ballasts – 0.95

normally while for electronic ballasts – 0.70 to 1.28

o Ballast Lamp Photometer Factor – adjusts the lamp lumen when a different

lamp ballast combination, other than the manufacturer’s set-up, is used

o Equipment Operating Factor – effects of the lamp lumen output of lamps

caused by the ballasts + lamp operating position + the effect of power reflected

back to the lamp from the lighting fixture

o Lamp Position Factor -- lumen output is highly sensible to orientation of the

lamp (a high intensity discharge lamp tilted from a vertical or horizontal

position); ratio of luminous flux in a given operating position to the test position

o Luminaire Surface Depreciation -- adverse changes in metal , paint and other

lighting fixture components resulting in a reduced light output; adjusts light

output to original reflectance

Total LLF = Recoverable X Non-Recoverable

Usually, Recoverable Factors only includes LLD, LBO, LDD and RSDD while Non-Recoverable

Factors only consists of Ballast factor and other non-recoverable factors

Spacing Criteria

- Maximum ratio of spacing to mounting height of the lighting fixture above the working area

- Provides reasonable uniformity of illumination within the space

For uniform illumination, spacing between lighting fixtures must be less than 1.5 times of the

mounting height

ILLUMINATION


a. Point Source Luminaire

Space to Height Ratio (SHR)

Ratio of space (S) to mounting height (Hm) above the working

plane

Expressed in terms of Uniformity Ratio (

Manufacturers provide recommended level of SHR for each

lighting fixture

An allowable variation of illuminance is achieved if the

recommended SHR is implemented and falls within the range

Example: A factory space is 80m long, 20m wide and is 10m high. Point source luminaries will be

used and will be suspended 2m below the ceiling. The working plane is 1m high. The recommended

SHR is 1.5 : 1.

1. Hm = 10 – (1 + 2) = 7m

2. Using SHR = 1.5 : 1, S = 1.5 x 7m = 10.5m

3.

4.

To satisfy the recommended SHR, the minimum number of rows is 2 with 8 luminaires per row.

For additional effects, balance, control or ease of installation, more than the computed value can

be used (based from Interior Lighting Design: Student’s Guide)

5. Actual spacing:

a.

b.

Sw and SL shall be approximately equal.

Space between the last row and the wall < 0.5Sw

Space between the outer luminaries to the adjacent wall < 0.5SL

If work areas are located along side the wall, then Sw and SL is reduced to one-third of the

luminaire spacing

ILLUMINATION


b. Linear Luminaires

Relevant maximum transverse and axial spacing is presented by

manufacturer, where spacing is taken center-to-center

Maximum recommended transverse SHR is usually not the same

with the axial SHR

For high levels of illuminance, it is a common practice to have the

lighting fixtures in a continuous rows with the transverse spacing

Example: A factory space of 80m long, 20m wide and is 10m high is to be illuminated using

continuous rows of twin 1500mm fluorescents. Previous computations resulted that 81 luminaires

are required. Design a reasonable lighting lay-out given a mounting height above the working plane

of 6m and SHR is as follows: transverse (spacing between rows) = 2.00:1; axial (spacing in rows) =

1.75:1

1. Spacing between rows: Hm = 6m Therefore, S = 2 x 6 = 12m

2. Try 3 rows of luminaires, each with 27 luminaires.

80/27 = 2.96m (luminaries will be spaced center-to-center)

2.96/2 = 1.48m (from end walls)

20/3 = 6.67m (transverse spacing)

Finding Average Illumination and Number of Luminaires

1. Determine the general information of the room (average maintained illuminance), lighting fixture

(description, luminaire distribution, lamps per luminaire) and the lamp (type and color, efficacy,

lamp output)

2. Determine the maintenance information – based on category, area atmosphere, operating

characteristics and cleaning cycle

3. Calculate Coefficient of Utilization (CU)

a. Compute for cavity ratios

b. Determine the characteristic of the room based on the approximate surface reflectances

c. Convert actual reflectances to its effective cavity reflectances – for ceiling and floor cavities

(based on IES table)

d. Calculate initial CU and use Figure 9-13 based on the effective ceiling cavity reflectace (ρcc ) and

ρw. Expect the use of linear interpolation

e. If ρfc= 20%, then CU = CU1. Else, CU1 x multiplying factor. To find multiplying factor, use Figure

9-13 based on RCR and other than 20% ρfc

4. Compute for LLF = LLD X LBO X LDD X RSDD

ILLUMINATION


a. LLD – use Figure 7-7

b. LBO – usually assumed to be 90%

c. LDD – refer to Figure 9-5

d. RSDD – use Figure 9-7

References:

Philippine Efficient Lighting Market Transformation Project (PELMATP). Lighting Calculations.

Kelly, Kevin and O’Connell, Kevin. Interior Design Lighting. A Student’s Guide.

Kunarta D.

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