Lighting levels 

 

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Composition with light

Calculating the Daylight Factor

 

Terminology:

 

Luminance should not be confused with illuminance

Illuminance is the measure of light falling on a surface (lumens per square metre or lux)

 

whereas

Luminance refers to light reflected from it or emitted by it(candela per square metre or alternatively a possible illuminance x reflection factor).

 

Units of measurement

Proper illumination depends on the location and design of windows, light sources, the type of light fixtures, the selection of the light source, the intensity and distribution of the source and fixture combination, and the room environment.

 

Illuminance or illumination is measured by the units fc (lx) ; fc being the symbol for foot candles and (lx) the symbol for lux in the International System (SI).

 

Luminance or brightness is measured by the units fL(cd/m2); fL being the symbol for foot Lamberts and (cd/m2) the symbol for candela per square meter.

 

Luminaire is the name given to a complete light source and the light fixture.

Issues of Room Lighting

 

The following factors are involved: room proportions, ceiling height, reflectance from various walls and ceiling, color of surfaces, type of reflection from surfaces, type of work or how the room is used, type of fixtures, pattern of fixture installation, output of fixtures, efficiency of fixtures, light losses due to lamp aging, and allowance for dirt conditions.

 

Illumination Values for Interior Lighting

New research has indicated the desirability of modifying recommended levels of artificial illumination by use of weighting factors. It is possible to design illumination accurately to include the age of the user, to include the light reflecting value of walls, floors and ceilings,

and to allow for the legibility of reading material and various kinds of visual tasks. The previous method for determining artificial illumination values consisted of tables of single-value recommendations.

 

Both systems are included. The new way is described first as simply as possible. Fractional modifiers have been chosen to provide a means for increase or decrease ratios in lieu of tabulating three values of many digits. The earlier standards are included as a generalized guide to give a relative feeling for some common applications.

 

Levels of Artificial Illumination

An accurate method for determining the amount of artificial illumination needed is described here.

 

Three lighting conditions are included:

 

general lighting throughout the room

illumination on the task

illumination on task, obtained by combining general and local (supplement) lighting

 

Values are in lux (lx) which are the units used in the SI metric system.

 

Divide values in lux by 10 for approximate footcandles.

 

General Lighting throughout Room

Three categories are included for various types of activities:

 

A   Use 30 lux for public areas with dark surroundings.

B   Use 75 lux for simple orientation for short temporary visits.

C  Use 1 50 lux for working spaces where visual tasks are only occasionally performed.

 

Adjustments to these values are made by selecting two weighting factors

 

          Age of user is important (e.g. +40 years of age require more light)

 

          Wall, floor and ceiling reflectance will vary, influencing the illumination

 

Select one of the following conditions and obtain factor.

 

User age: under 40 (factor -1)

40-55 (factor 0)

over 55 (factor +1)

 

Select one of the following conditions and obtain factor.

 

Average room reflectance is:             over 70% (factor 1)

30% to 70% (factor 0)

under 3Q% (factor+ 1)

 

Add the 2 factors algebraically for weight of influence.

 

If total weight is (-1), 0, or (+1) use a, b and c values.

 

If total weight is (-2) decrease a, b and c values by 1/3

 

If total weight is (+2) increase a value by 2/3 and b and c values by 1/3

 

Illumination on the Task

Three categories are included for various types of tasks:

 

D Use 300 lx for performance of visual tasks of high contrast or large size such as reading printed matter, typed originals, handwriting in ink and good xerography. Use also for rough bench and machine work, ordinary inspection and rough assembly.

 

E Use 750 Ix for performance of visual tasks of medium contrast or small size such as reading medium-pencil handwriting, and poorly printed or reproduced matter. Use also for medium

  bench and machine work, difficult inspection

  and medium assembly.

 

F Use 1500 lx for performance of visual tasks of low contrast or very small size such as reading handwriting in hard pencil on poor quality paper and very poorly reproduced matter. Use also for very difficult inspection.

 

Illumination on Task, Obtained by Combining General and Local (Supplement) Lighting

 

G Use 3000 lx for performance of visual tasks of low contrast and very small size over a prolonged period such as fine assembly, very difficult inspection, fine bench and machine work.

 

H  Use 7500 Ix for performance of very prolonged and exacting visual tasks such as the most difficult inspection, extra fine assembly, bench and machine work.

 

I  Use 15000 Ix for performance of very special visual tasks of extremely low contrast and small size.

 

Adjustments to D, E, F, G, H and I values are made by selecting 3 weighting factors.

 

Select one of the following conditions and obtain factor.

 

User age: under 40 (factor -1), 40-55 (factor 0), over 55 (factor +1)

Select one of the following conditions and obtain factor.

 

Task background reflectance (e.g. a page)

 

17 over 75% (factor-1), 30% to 70% (factor 0) under 30% (factor +1)

 

Select one of the following conditions and obtain factor

 

Speed and/or accuracy of task performance:

 

not important (factor -1), important (factor 0) critical (factor +1).

 

Add the 3 factors algebraically for weight of influence.

 

If total weight is (-1), 0, or (+1) use D, E, F, G, H and I values.

 

If total weight is (-3) or (-2) decrease D, E, F, G, H and I values 1

 

If total weight is (+2) or (+3) increase E, F, hand I values 1/3 and D, G, values 2/3

 

 

An example of illuminance categories for various activities or areas is listed.

 

01             Dining / C

02             Grooming, shaving, make-up / D

03             Food preparation / E

04             Serving and other non-critical tasks / D

05              Laundry / D

06             Ironing / D

07             Reading books, magazines, newspapers / D

08              Reading advanced piano scores / E

09          Reading advanced piano scores (substandard size) / F

10             Reading simple piano scores / D

11             Sewing Dark Fabrics (hand machine) / F

12             Sewing medium fabrics (hand & machine) / E

13             Sewing high contrast / D

14             Study (evaluate effect of veiling reflections) / E

15             Table games  / D

16             General lighting for conversation, relaxation and entertainment / B

17             General lighting for passage areas and for safety / B

 

 

Additional Illumination Adjustments

 

A maximum lighting ratio of 5:1 is recommended when passing from one area to another for adaptation and psychological reasons.

 

For example: a corridor next to an office with 750 Ix should not have less than 150 Ix. This recommendation however cannot hold with the presence of daylight.

A minimum of 200 lx is recommended for horizontal illumination in interiors used for continuous work.

 

Veiling reflection or reflected glare is caused by regular reflections near or over diffuse reflections of displays. Veiling brightness on the retina reduces image contrast, making the reading of displays difficult. Evaluation is determined best by testing real situations.

 

 

Illumination levels and limiting glare indices for various functions

 

The table following lists illumination levels suitable for a range of situations: the quality of these levels could be influenced by glare and an acceptable limiting index is also shown. The glare index is calculated by considering the light source location, the luminances of the source, the effect of surroundings and the size of the source.

Glare indices for artificial light range from about 10 for a shaded light fitting having low output to about 30 for an un shaded lamp.

 

As seen from this illustration, various basic decisions have to be made concerning lighting objectives and whether the system involves daylight, electric light or a combined system. With electric or combined systems, further decisions must be taken concerning the way light is distributed by particular fittings, and upon their positions relative to each other as well as in relation to the surface to be illuminated. As with day lighting, light-coloured and highly reflective room surfaces help to provide more illumination from the same amount of  energy source it preserves the luminance effect of the light source.

 

Location                                  Illuminance                                        Limiting Glare Index

(lux or lrn/m')

Entrance hall                              150                                                       22

Stairs                                        150                                                       22

Corridors                                   100                                                       22

Outdoor entrances                      30                                                         22

Casual assembly work               200                                                       25

Rough/heavy work                 300                                                       28

Medium assembly work               500                                                       25

Fine assembly work                 1000                                                     22

Precision work                           1500                                                     16

General office work                 500                                                       19

Computer room                         750                                                       16

Drawing office                            750                                                       16

Filing room                                300                                                       22

Shop counter                             500                                                       22

Supermarket                             500                                                       22

Classroom                                 300                                                       16

Laboratory                                 500                                                       16

Public house bar                         150                                                       22

Restaurant                                 100                                                       22

Kitchen                                      500                                                       22

Dwellings

Living room                 50                                                         N/A

Reading room                 150                                                       N/A

Study                            300                                                       N/A

Kitchen                          300                                                       N/A

Bedroom                     50                                                         N/A

Hall/landing                  150                                                       N/A

 

Library

Reading area                  200                                                       19

Tables                           600                                                       16

Counter                         600                                                       16

 

N/A = not applicable