Lighting Criteria
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Lighting Criteria

In lighting situations it is necessary to consider:

(i) the way the task is lit, and

(ii) the way the interior as a whole is lit and relates to the task.

 

6.1 Task Illuminance

The illuminance needed for the task depends on:

(i) the visual difficulty and complexity of the task,

(ii) the average standard of eyesight, and

(iii) the level of visual performance required.

 

Table 1 Standard Service Illuminance for Various Activities/Interiors

 

Standard

illuminance

(lx)

 

Characteristics of the activity/interior

 

Representative activities/interiors

50

Interiors visited rarely with visual tasks confined to movement and casual seeing without perception of detail.

Cable tunnels, indoor storage tanks, walkways.

100

Interiors visited occasionally with visual tasks confined to movement and casual seeing calling for only limited perception of detail.

Corridors, changing rooms, bulk stores.

150

Interiors visited occasionally with visual tasks requiring some perception of detail or involving some risk to people, plant or product.

Loading bays, medical stores, switchrooms.

200

Continuously occupied interiors, visual tasks not requiring any perception or detail.

Monitoring automatic processes in manufacture, casting concrete, turbine halls.

300

Continuously occupied interiors, visual tasks moderately easy, i.e. large details > 10 min arc and/or high contrast.

Packing goods, rough core making in foundries, rough sawing.

500

Visual tasks moderately difficult, i.e. details to be seen are of moderate size (5-10 min arc) and may be of low contrast.  Also colour judgment may be required.

General offices, engine assembly, painting and spraying.

750

Visual tasks difficult, i.e. details to be seen are small (3-5 min arc) and of low contrast, also good colour judgments may be required.

Drawing offices, ceramic decoration, meat inspection.

1000

Visual tasks very difficult, i.e. details to be seen are very small (2-3 min arc) and can be of very low contrast. Also accurate colour judgments may be required.

Electronic component assembly, gauge and tool rooms, retouching paintwork.

1500

Visual tasks extremely difficult, i.e. details to be seen extremely small (1-2 min arc) and of low contrast. Visual aids may be advantage.

Inspection of graphic reproduction, hand tailoring, fine die sinking.

2000

Visual tasks exceptionally difficult, i.e. details to be seen exceptionally small (< 1 min arc) with very low contrasts.  Visual aids will be of advantage

Assembly of minute mechanisms, finished fabric inspection.

 

6.2 General Brightness

Table 2 Recommended Illuminance Ratios And Surface Reflections

 

Recommendations

Illuminance ratios:

(a) Minimum illuminance/average illuminance (A.I.) on task area

(b) In an interior with general lighting,

A.I. on the ceiling / A.I. on the horizontal working plane

A.I. of any wall / A.I. on the horizontal working plane

(c) In an interior with localised or local lighting, the ratio of illuminance on the task

area to illuminance around the task area

 

0.8 min.

 

0.3 - 0.9

0.5 - 0.8

3 : 1 max.

Surface reflectances:

(a) Ceiling cavity

(b) Principal walls

(c) Window wall surfaces

(d) Floor cavity

(e) Equipment and furnishings in work interiors (e.g. desk top)

(f) Immediate background to a task

(g) Ratio of reflectance of the immediate background to reflectance of the related task

(h) Large surface areas, e.g. on the ceiling or upper walls where indirect lighting is used.

 

0.6 min.

0.3 - 0.7

0.6 min.

0.2 - 0.3

0.2 min.

Matt

0.3 - 0.5

Gloss finished not recommended

 

6.3 Directional Effect

 

Some directional effects of light make it easier to recognize the details of a task; others make recognition more difficult. The directional qualities of lighting may affect:

6.3.1 Modelling

shade01.jpg (37413 bytes)

6.3.2 Surface Texture

 

6.4 Glare

Glare is the discomfort or impairment of vision experienced when parts of the visual field are excessively bright in relation to the general surroundings.

 

6.4.1 Disability Glare

6.4.2 Discomfort Glare

 

Table 3 Limiting Glare Index

 

Limiting glare index

Applications

16

Museums, art galleries, lecture theatres, control rooms, industrial inspection.

19

Classroom, libraries, laboratories, general offices, fine assembly work.

22

Supermarket, circulation areas, medium assembly work.

25

Boiler houses, rough assembly work.

28

Foundries, works store areas.

6.4.3 Reflected Glare

6.4.4 Veiling Reflection

6.4.5 Control of Glare

Glare is controlled by the following means:

(i) to limit the luminance of the lighting sources in the direction of the eye, e.g.             replacing bright source with a number of weak sources,

(ii) to screen the light sources from the view, e.g. introducing downstand screens or light-diffusing fittings to the luminaires,

(iii) to reposition the work station,

(iv) to use working surfaces, paper, office machines etc. that have matt surface,

(v) to raise the background luminance against which the luminance sources are seen,

(vi) to construct external obstructions or low transmittance glass to limit daylight penetration, and

(vii) to construct windows on adjacent or opposite walls to reduce contrast.

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uplight03.jpg (35215 bytes)

6.5 Colour Quality

The quality of colour of a light source are characterized by its colour appearance and its colour rendering capabilities.

6.5.1 Colour Appearance

6.5.2 Colour Rendering

 

Table 4 Colour Rendering Groups

 

Colour

rendering

group

CIE general

colour

rendering index

Typical application

1A

Ra 90

Wherever accurate colour matching is required, e.g. colour printing inspection.

1B

80 Ra < 90

Wherever accurate colour judgments are necessary and/or good colour rendering is

required for reasons of appearance, e.g. shops and other commercial premises.

2

60 Ra < 80

Wherever moderate colour rendering is required.

3

40 Ra < 60

Wherever colour rendering is of little significance but marked distortion of colour is

unacceptable.

4

20 Ra < 40

Wherever colour rendering is of no importance at all and marked distortion of colour is

acceptable.

 

6.5.3 Munsell Colour System

 

colour01.jpg (38065 bytes)

 

6.6 Flicker and Stroboscopic Effects

The light output of all lamps on an a.c. supply has an inherent cyclic oscillation which is small for filament lamps and more marked for discharge lamps including fluorescent lamps. The oscillation may cause flicker, or stroboscopic effects, or both.

The stroboscopic effect is an illusion which makes a moving object appear as stationary, or to be moving in a different manner from that in which it is really moving.

Flicker and stroboscopic effect is reduced by:

(i) shielding the electrodes or screening the ends of fluorescent tubes or using a special circuit,

(ii) mixing tungsten lamps with discharge circuit, and

(iii) dividing lamps across three phases.