7 Glare a - University of Hong Kongwork6002/7 glare a.pdfsurface mounted prismatic or opal diffuser...

UNIVERSITY OF HONG KONG Faculty of Engineering

M.Sc.(Eng) in Building Services Engineering MEBS6002 LIGHTING ENGINEERING

K.F. Chan (Mr.) of 12 Jan., 2008 All rights reserved

GLARE CONTROL

GLARE

According to definition 45-25-295 in the International Lighting Vocabulary (CIE

publication 17), glare is the condition of vision in which there is discomfort or a

reduction in the ability to see significant objects, or both, due to an unsuitable

distribution or range of luminances, or due to extreme contrasts in space or time.

There are 2 forms of glare:-

- discomfort glare, which causes discomfort without necessarily impairing the

vision of objects (definition 45-25-315)

- disability glare, which impairs the vision of objects without necessarily

causing discomfort (definition 45-25-320)

These 2 forms of glare are quite different phenomenon. Disability glare depends

mainly on the quantity of light falling on the eye and is largely independent of the

luminance of the source. Whereas for discomfort glare, source luminances is a

major factor. Furthermore, disability glare is little affected by time, while discomfort

glare can build up considerably in an interior where people are exposed to high

luminance sources for long periods.




Disability glare is most likely to occur when there is an area close to the line of sight,

which has a much higher luminance than the object of regard. Then, scattering of

light in the eye and changes in local adaptation can cause a reduction in the contrast

of the object. This reduction in contrast may be sufficient to make important details

invisible and hence may influence task performance. Alternatively, if the source of

high luminance is viewed directly, noticeable after-images may be created. The most

common sources of disability glare indoors are the sun and sky seen through windows

and electric light sources seen directly or by reflection. Care should be taken to avoid

disability glare in interiors by providing some method of screening windows and

avoiding the use of highly specular surfaces.

Freedom from discomfort caused by glare is an important criterion of lighting quality.

The sensation of glare in a lighting installation experienced by people in the form of

discomfort, annoyance, or irritation is a complex function of the imbalance within the

luminance pattern experienced by the visual mechanism, certain parts (glare sources)

lying above the range to which the eye is adapted at the time.

Experimental work has shown that the main factors influencing discomfort glare are

the luminance of the sources and their apparent size, their position in the field of view

and the luminance of the general environment. However the actual perception of glare

varies from person to person. Lighting which is considered to be uncomfortable in one

environment may be acceptable in another, and it is possible to determine values

which represent from experience and practice a suitable comfort criterion which is

acceptable to most people for a given occupation in a given location. If the calculated

value is greater than the recommended limit, modifications to the lighting system or

the interior will be required.




A high source luminance, large source area, low background luminance and a

position close to the line of sight all increase discomfort glare. Unfortunately most of

the variables available to the designer alter more than one factor. For example,

changing the luminaire to reduce the source luminance may also reduce the

background luminance. These factors could counteract each other, resulting in no

reduction of discomfort glare. However, as a general rule, discomfort glare can be

avoided by the choice of luminaire layout and orientation, and the use of high

reflectance surfaces for the ceiling and upper walls.

It is found that with the exception of visual display screen areas if discomfort glare is

adequately controlled, disability glare will be kept within bounds as well.




It has been generally agreed that the source luminance must be greater than 500 to

700 cd/m2 for discomfort glare to exist.

FACTORS AFFECTING GLARE

In general, the lower the mounting height, the higher the glare.

Furthermore, the longer the room, or the more non-cut off luminaires visible, the

higher the glare.

The combined effect of mounting height and number of non cut-off luminaires is

shown in the following diagram.




It is important to note that bright-sided luminaries are more glaring when viewed

crosswise rather than endwise. Because of this, it is very common to install

surface mounted prismatic or opal diffuser enclosed fluorescent fittings parallel

to line of sight in office, schools, library etc. when there is no false ceiling.

Exposed battens are commonly used in schools in Hong Kong. In general, no false

ceiling is used in the classrooms. To reduce glare, reflectors are supplied together

with the batten to impose a cut-off angle to the luminaire.

It is also interesting to note that for such type of fluorescent fixtures, the apparent area

of the horizontal base panel remains unchanged, therefore, for fixtures without

brighted sides, such as recessed fixtures in false ceiling application, glare is relatively

unaffected by orientation.




Glare control with non cut-off luminaries. In this case, the room size is important

because this affects both θ and total value of ω .

Glare control with cut-off luminaires. In this case, the actual size of room is

unimportant. Instead, glare depends on effective room size, controlled by the

shielding angle of the luminaries. Increasing the shielding angle reduces both the

effective size of room and the degree of glare.

GLARE CONTROL SYSTEMS

In practice means used for luminance control of lamps in luminaires or installations

are based on the application of one or a combination of the following principles :-

• Shielding

• Deflection

• Translucence

• Indirect lighting




There are generally 4 widely used methods to ensure that the glare index, G, of a

lighting installation does not exceed an appropriate value. These 4 methods are, viz.,

- glare index system;

- luminance limiting system;

- shielding angle system;

- visual comfort probability system.

1) Glare index systems

Glare index calculated with the aid of an appropriate glare formula is the most flexible

method in that it can be tailored exactly to meet the situation prevailing in a given

installation. The formula can indicate which of the components need to be

manipulated in order that glare, in certain installations, can be controlled.

Basic glare formula calculation has been converted into tables or graphs. This requires

some simplification and restriction of the problem, but lends itself more readily to use

in lighting codes and design handbooks.

2) Luminance limiting systems

These are particularly applicable in the case of luminaires with an approximately

constant luminance distribution.

It has been found that it is possible to calculate Ls values which will restrict glare to

any desired value of G, which can then be tabulated in terms of other parameters such

as room size, mounting height, etc. If desired, these luminance limits can be adjusted

by a suitable factor to alter the value of G.

For luminaires with approximately constant luminance distribution, the luminance

limit system is potentially as flexible and precise as the Glare Index method.

For luminaires with a non-constant luminance distribution, limiting luminances over a

range of γ angles must be prescribed, in order to restrict glare to the desired value of G.

(γ = angle between the light beam and the downward vertical)

The system becomes in effect an acceptability gauge, by comparing luminaires

luminance distribution with that of the prescribed limiting one.

In either case, the luminance limiting system has practical benefits for both the

manufacturer and installation designer. The manufacturer can, with simple photometry,

publish in a conventional catalogue precise luminance data at the prescribed γ angle,

or angles, for each of his luminaires. The designer can select the luminaire appropriate

to the particular installation parameters. Furthermore, a designer can develop a




luminaire with pre-determined requirements for limiting glare.

In general, luminance limiting systems can be used in 2 ways:-

a) To determine the limiting luminance value which, under a given set of conditions,

will ensure that the installation glare number does not exceed a selected figure on

a relevant scale. This type of system works best with luminaires having a

reasonably constant luminance distribution throughout the critical zone. By

choosing a suitably spaced set of luminance limits, the selected glare number can

be readily varied merely by using a limit one or more steps lower (or higher) on

the scale.

b) To determine the limiting luminance distribution which, under a given set of

conditions, will ensure that the glare number does not exceed a selected figure.

Once again, variations in the latter figure can be provided for by employing a set

of alternative luminance distributions. This type of system works best with

luminaires which reduces the luminance progressively throughout the range of

angles down to γ = 450. This is widely used in Australia and Germany, and the

recommendations in Rule 1 of the Interim Glare Limiting System incorporated in

CIE Publication No. 29 (1975) Guide on interior Lighting are also in this category

and are based on the German data.

3) Shielding angle system

This is the most practical and effective basis for rules designed to limit glare from

cut-off type luminaires, especially those incorporating high-wattage discharge lamps

or incandescent filament lamps. The conventional glare formula approach does not

lend itself particularly well to the latter task, one reason being that with such sources,

lumen output may become more important than luminance itself.

Cut off (shielding angle) principle, source hidden when

θ is less than S




Consequently, whilst the framework for such rules can be worked out on theoretical

principles, the actual details are best decided empirically, based on the observations

and opinion of a panel of experienced lighting engineers.

This method is most widely and successfully used for many years in Australia.

Measurement of shielding angle

A recessed fluorescent fitting for use in shops – its strong sideway throw of light is

suitable for illumination of vertical display.

Recessed fluorescent fittings for generally office application. The louvers are for glare

control




Recessed fluorescent fixtures for use in areas where there are a large number of video

display terminals

Typical polar curve of the type of fluorescent fittings used in shops. Note its

”batwing” shaped intensity distribution which gives a strong side throw of light

Typical polar curve of the type of fluorescent fittings for general office application

Typical polar curve of the type of fluorescent fittings for use in areas where there are a

large number of VDT’s. Note that its side throw is strongly restricted to avoid glare,

especially those reflected from the display screens.




In areas where glare control is very important, there are also other options

Mirror attachment with a dark light high-gloss parabolic side and lamellas for high

output and low luminance. Suitable for areas where glare control is necessary such as

offices where VDU are used.

Mirror attachment with highly polished small cells for an ultra-low brightness and

appearance and a low cut-off angle. Suitable for areas with wide spread use of

computer and display screens




4) The visual comfort probability system

This system is used in USA and Canada. The visual comfort probability system,

VCP, provides discomfort glare ratings in terms of the percentage of the population

that considers the glare in a given lighting installation to be on the borderline between

comfort and discomfort (BCD). The system states that glare will not be a problem if

the VCP is 70 or more and if certain maximum luminaire luminances are not

exceeded.

7 Glare a - University of Hong Kongwork6002/7 glare a.pdfsurface mounted prismatic or opal diffuser...

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