##### Document Text Contents

Page 1

SLL logo.eps

The SLL Code for Lighting

222 Balham High Road, London SW12 9BS

+44 (0)20 8675 5211

www.cibse.org

The Society of

Light and Lighting

Page 2

CIBSE colour logo.eps

Page 181

Eqn13.eps

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Thus with T set to 1.625 RI and U set to 2.6 RI, equation 11.3 may be used to calculate the form

factor for the ceiling and the working plane, FF(C,F) and FF(F,C).

The form factors for the fl oor to walls and ceiling to walls are easy to calculate as the fl ux that

leaves the ceiling and does not get to the fl oor must go to the walls. Thus:

FF C W FF C F

FF F W FF F C

( , ) ( , )

( , ) ( , )

= -

= -

1

1

(11.7)

The form factors for the walls to the ceiling and fl oor can be derived by using the reversible

nature of form factors thus:

FF W C FF C W

Area C

Area W

( , ) ( , )

( )

( )

= (11.8)

As Room Index is in fact the area of the ceiling divided by half the area of the walls, then the

above equation may be rewritten:

FF W C FF C W

RI

( , ) ( , )=

2

(11.9)

Likewise:

FF W F FF F W

RI

( , ) ( , )=

2

(11.10)

The wall to wall transfer factor is similarly the fl ux that leaves the wall that does not go to the

ceiling or the fl oor. Thus:

FF W W FF W C FF W F( , ) ( , ) ( , )= - -1 (11.11)

With these equations, it is then possible to calculate all form factors needed for the evaluation of a

standard set of utilisation factors. Table 11.3 gives the calculated form factors.

Table 11.3 Form factors for the three surface cases

RI FF(F,W) FF(C,W) FF(F,C) FF(C,F) FF(W,F) FF(W,C) FF(W,W)

0.60 0.74727 0.25273 0.22418 0.55164

0.80 0.65810 0.34190 0.26324 0.47352

1.00 0.58466 0.41534 0.29233 0.41534

1.25 0.51122 0.48878 0.31951 0.36097

1.50 0.45330 0.54670 0.33998 0.32005

2.00 0.36877 0.63123 0.36877 0.26246

2.50 0.31051 0.68949 0.38814 0.22372

3.00 0.26809 0.73191 0.40213 0.19573

4.00 0.21056 0.78944 0.42113 0.15774

5.00 0.17341 0.82659 0.43352 0.13296

11.3.4 The four surface case

In the case of suspended luminaires, the situation is more complex. To start with, there are three

parallel planes to consider, the working plane, the luminaire plane and the ceiling. Again equation

11.3 may be used to calculate the form factors for ceiling and working plane FF(C,F) and

FF(F,C); luminaire plane and ceiling FF(L,C) and FF(C,L); and fi nally, the working plane and

the luminaire plane FF(L,F) and FF(F,L). The values of T and U used are given Table 11.4.

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Table 11.4 Values of T and U

Form factor calculated T U

FF(C,F) and FF(F,C) RI × 1.625 × 3/4 RI × 2.6 × 3/4

FF(F,L) and FF(L,F) RI × 1.625 RI × 2.6

FF(L,C) and FF(C,L) RI × 1.625 × 3 RI × 2.6 × 3

The other form factors are derived in a similar way to that used in the three surface case. The

factor for the ceiling to the frieze is given by

FF C S FF C L( , ) 1 ( , )= - (11.12)

The form factor for the frieze to the ceiling may be calculated by using the following equation:

FF S C FF C S

Area C

Area S

( , ) ( , )

( )

( )

= (11.13)

Given that the relationship between the area of the frieze and ceiling is a function of room index,

the above equation may be rewritten as:

FF S C FF C S

RI

( , ) ( , )=

3

2

(11.14)

The form factor for the ceiling to the walls is given by:

FF C W FF C L FF C F( , ) ( , ) ( , )= - (11.15)

The reverse form factor for the walls to the ceiling is given by

FF W C FF C W

Area C

Area W

( , ) ( , )

( )

( )

= (11.16)

The above equation may be rewritten in terms of the room index as follows

FF W C FF C W

RI

( , ) ( , )=

2

(11.17)

The frieze to frieze form factor is that fraction of the fl ux from the frieze that does not go to the

ceiling or the luminaire plane. Its calculation is simplifi ed by the fact that the form factors from

the frieze to the luminaire plane and the ceiling are the same

FF S S FF S C( , ) ( , )= -1 2 (11.18)

The form factor for the fl oor to the frieze is the fraction of the fl ux leaving the fl oor that reaches

the luminaire plane but does not reach the ceiling, so the equation for the factor is given by

FF F S FF F L FF F C( , ) ( , ) ( , )= - (11.19)

Page 361

Index Terms Links

This page has been reformatted by Knovel to provide easier navigation.

visual performance 3 3–6 7–9

315

see also visual task performance

visual search 9–10

visual sensitivity 11 147

visual size 4

visual spectrum 9

visual system 3–4 147

eye-brain pathways 23

spectral sensitivities 11 147

visual task, definition 315

visual task performance 6–12

and glare 38–39

improving 12

Landolt C matrices 7 8

mesopic conditions 10–11

Relative Visual Performance (RVP) model 8–9 9

visual search 9–10

W

water and sewage plants 104

well-being 19

Wien’s displacement law 247

window, definition 315

windows

glare control 120–121

minimum glazed area for view 118

size and proportion 118 128

and view 116–118

work place 315

see also indoor workplaces; outdoor workplaces

SLL logo.eps

The SLL Code for Lighting

222 Balham High Road, London SW12 9BS

+44 (0)20 8675 5211

www.cibse.org

The Society of

Light and Lighting

Page 2

CIBSE colour logo.eps

Page 181

Eqn13.eps

173

C

h

a

p

te

r E

le

ve

n

: In

d

ire

ct lig

h

tin

g

Thus with T set to 1.625 RI and U set to 2.6 RI, equation 11.3 may be used to calculate the form

factor for the ceiling and the working plane, FF(C,F) and FF(F,C).

The form factors for the fl oor to walls and ceiling to walls are easy to calculate as the fl ux that

leaves the ceiling and does not get to the fl oor must go to the walls. Thus:

FF C W FF C F

FF F W FF F C

( , ) ( , )

( , ) ( , )

= -

= -

1

1

(11.7)

The form factors for the walls to the ceiling and fl oor can be derived by using the reversible

nature of form factors thus:

FF W C FF C W

Area C

Area W

( , ) ( , )

( )

( )

= (11.8)

As Room Index is in fact the area of the ceiling divided by half the area of the walls, then the

above equation may be rewritten:

FF W C FF C W

RI

( , ) ( , )=

2

(11.9)

Likewise:

FF W F FF F W

RI

( , ) ( , )=

2

(11.10)

The wall to wall transfer factor is similarly the fl ux that leaves the wall that does not go to the

ceiling or the fl oor. Thus:

FF W W FF W C FF W F( , ) ( , ) ( , )= - -1 (11.11)

With these equations, it is then possible to calculate all form factors needed for the evaluation of a

standard set of utilisation factors. Table 11.3 gives the calculated form factors.

Table 11.3 Form factors for the three surface cases

RI FF(F,W) FF(C,W) FF(F,C) FF(C,F) FF(W,F) FF(W,C) FF(W,W)

0.60 0.74727 0.25273 0.22418 0.55164

0.80 0.65810 0.34190 0.26324 0.47352

1.00 0.58466 0.41534 0.29233 0.41534

1.25 0.51122 0.48878 0.31951 0.36097

1.50 0.45330 0.54670 0.33998 0.32005

2.00 0.36877 0.63123 0.36877 0.26246

2.50 0.31051 0.68949 0.38814 0.22372

3.00 0.26809 0.73191 0.40213 0.19573

4.00 0.21056 0.78944 0.42113 0.15774

5.00 0.17341 0.82659 0.43352 0.13296

11.3.4 The four surface case

In the case of suspended luminaires, the situation is more complex. To start with, there are three

parallel planes to consider, the working plane, the luminaire plane and the ceiling. Again equation

11.3 may be used to calculate the form factors for ceiling and working plane FF(C,F) and

FF(F,C); luminaire plane and ceiling FF(L,C) and FF(C,L); and fi nally, the working plane and

the luminaire plane FF(L,F) and FF(F,L). The values of T and U used are given Table 11.4.

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Table 11.4 Values of T and U

Form factor calculated T U

FF(C,F) and FF(F,C) RI × 1.625 × 3/4 RI × 2.6 × 3/4

FF(F,L) and FF(L,F) RI × 1.625 RI × 2.6

FF(L,C) and FF(C,L) RI × 1.625 × 3 RI × 2.6 × 3

The other form factors are derived in a similar way to that used in the three surface case. The

factor for the ceiling to the frieze is given by

FF C S FF C L( , ) 1 ( , )= - (11.12)

The form factor for the frieze to the ceiling may be calculated by using the following equation:

FF S C FF C S

Area C

Area S

( , ) ( , )

( )

( )

= (11.13)

Given that the relationship between the area of the frieze and ceiling is a function of room index,

the above equation may be rewritten as:

FF S C FF C S

RI

( , ) ( , )=

3

2

(11.14)

The form factor for the ceiling to the walls is given by:

FF C W FF C L FF C F( , ) ( , ) ( , )= - (11.15)

The reverse form factor for the walls to the ceiling is given by

FF W C FF C W

Area C

Area W

( , ) ( , )

( )

( )

= (11.16)

The above equation may be rewritten in terms of the room index as follows

FF W C FF C W

RI

( , ) ( , )=

2

(11.17)

The frieze to frieze form factor is that fraction of the fl ux from the frieze that does not go to the

ceiling or the luminaire plane. Its calculation is simplifi ed by the fact that the form factors from

the frieze to the luminaire plane and the ceiling are the same

FF S S FF S C( , ) ( , )= -1 2 (11.18)

The form factor for the fl oor to the frieze is the fraction of the fl ux leaving the fl oor that reaches

the luminaire plane but does not reach the ceiling, so the equation for the factor is given by

FF F S FF F L FF F C( , ) ( , ) ( , )= - (11.19)

Page 361

Index Terms Links

This page has been reformatted by Knovel to provide easier navigation.

visual performance 3 3–6 7–9

315

see also visual task performance

visual search 9–10

visual sensitivity 11 147

visual size 4

visual spectrum 9

visual system 3–4 147

eye-brain pathways 23

spectral sensitivities 11 147

visual task, definition 315

visual task performance 6–12

and glare 38–39

improving 12

Landolt C matrices 7 8

mesopic conditions 10–11

Relative Visual Performance (RVP) model 8–9 9

visual search 9–10

W

water and sewage plants 104

well-being 19

Wien’s displacement law 247

window, definition 315

windows

glare control 120–121

minimum glazed area for view 118

size and proportion 118 128

and view 116–118

work place 315

see also indoor workplaces; outdoor workplaces