Light distribution curves,
illumination and isolux diagrams
77 986
77 972
cd/klm
77 831
β = 25°/72°
1000
500
360
cd/klm
C 180 - C 0
C 270 - C 90
C 195 - C 15
Fig. 1
30°
60°
30°
0°
30° 60°
Fig. 2
Light distribution curves
Careful planning work is required to solve
lighting tasks. The necessary information about
the properties of luminaires is given in light
distribution curves, illumination and isolux
diagrams.
We have explained the knowledge required
on this page.
specify in what direction and with what intensity a luminaire
emits light. The luminous intensity value in candela [cd] for
a given luminaire is obtained through the multiplication
of the value read off the diagram in cd/klm with the total
luminous flux in kilolumen [klm] of the lamps used in the
luminaire.
Light distribution curves are usually presented in a polar
coordinate system (Fig. 1).
With floodlights, we specify the light distribution using
Cartesian coordinates (Fig. 2). In the diagrams, the half
beam angles are also shown:
• continuous line, C 180 - C 0 β = 25°
• dotted line,
C 270 - C 90 β = 72°
The luminous intensity values are given on the vertical axis,
and the illumination angle on the horizontal axis.
In a polar representation, the diagram specifies what
sections through the luminaire (C-levels) the curves refer to.
126
2.06 Information about light and illumination · Light distribution curves, illumination and isolux diagrams
77 584
lx
77 584
LED
500
400
2
300
0
200
2
100
4
1
Fig. 3
3
5
7
9
cd
8
8000
6
4000
2
β = 28°/92°
Ē
77 910
LED
H = 4,0 m
4
8
0
m
60°
30°
0°
30° 60°
Fig. 4
m
0
2
4
6
8
4
10
2
12
1 0,5
14
16
0,2
18
lx
20
Fig. 5
Illumination diagrams
Isolux diagrams
for floodlights (Fig. 3) specify the dimensions of the area
limited by the two half beam angles (Fig. 4) and the average
degree of illuminance depending on the distance from this
area.
Using the parameters “floodlights – area – horizontal axis”
it is possible to read off:
(Fig. 5) specify the distribution of the illuminance on
a visible surface.
Points with the same illuminance are connected to each
other by means of curves (isolux lines). The luminaire is
located vertically above the drawing layer at the mounting
height (h) above the coordinate origin.
Depending on the luminaire type, the mounting height (h)
or the appropriate pole height (H) is specified.
The light point interval is approximately twice the distance
at which the isolux line which belongs to half of the value
of the desired minimum luminance runs.
Example: In the isolux diagram shown here, the 0.5 lx line
runs laterally at a distance of 15 m from the coordinate
origin. If a path is to be illuminated – Emin ≥1 lx – then light
point spacing of approx. 30 m must be selected.
With luminaires with rotationally symmetrical light
distribution, the resulting isolux lines are concentric
circles.
• the height (continuous line, C 0 - C 180 β = 28°)
and the width (dotted line, C 270 - C 90 β = 92°)
of the limited area on the vertical axis on the right
• the average degree of illuminance (parabola) in lx on the
vertical axis on the left
Example: At a distance of 8 m, the half beam angles of the
floodlight 77 584 limit an area of 3 m in height and 10 m wide.
The average degree of illuminance on this area is 100 lx.
Uniform illumination can be obtained if the distances
between the floodlights correspond to the width of the
area limited by the half beam angles. When a façade is
illuminated from the ground, only about one third of the
specified degree of illuminance can be achieved.
127
In practice
Determination of the distance of a floodlight from the illuminated surface
on the basis of a light distribution curve
77 584
lx
LED
500
Ē
10 m
400
2
300
0 Height Width
200
2
•
100
1
3
5
7
Half beam angle
E = 40°/97°
4
9
3m
8m
m
'LVWDQFHRIPRIƅRRGOLJKWWRVXUIDFH
The floodlight with flat beam light distribution has a half
beam angle of 40° to 97°. The advertising sign / company
logo to be illuminated has a height of 3 m (blue line) and a
width of 10 m (red line).
At a distance of 8 m, the half beam angle corresponds
exactly to the dimensions of the advertising sign.
The parabola specifies the average degree of illuminance
in lux; these values can be read off on the y-axis on the left.
128
The average degree of illuminance of the advertising sign is
100 lx. Changes in the distance of the floodlight influence not
only the average degree of illuminance but also the size of
the illuminated surface.
The following must be noted: the larger the half beam angle,
the more stray light is created.
The sign will still be well illuminated with a size of 4 x 12 m.
2.06 Information about light and illumination · Light distribution curves, illumination and isolux diagrams
Determination of the luminaire spacing “a” for pathway illumination
of Emin = 1 lx on the basis of the isolux diagram
99 853
LED
H = 0,8 m
0,2
lx
4
3
2
20
1
5
2
1
0,5
3m
0
m
0
1
2
3
½a
4
5
6
7
8
9
10
11
12
13
14
½a
a
The bollards line the pathway and cast their light asymmetrically onto the ground. For planning purposes, the 1lx
line and the 0.5 lx line are decisive. At 3 m, degrees of
illuminance of 0.5 lx add up to 1 lx on the opposite side of
the path. The luminaires should be positioned with a light
point spacing of a maximum of 6 m. The specified maximum
degree of illuminance can be taken as a planning reference
for good uniformity. The average degree of illuminance
results from the arithmetic average of all degrees of
illuminance on a surface, calculated from the number
of measuring points on the x axis and on the y axis.
129
In practice
Determination of the luminaire spacing “a” for square illumination
with BEGA light building element 88 165 on the basis of the
isolux diagram
88 165
15
LED
12
0,15
lx
0,3
9
0,8
6
1,5 lx
2,5
3
10
m
3
6
9
12
15
21
24
a = 18 m
a = 18 m
For luminaires with rotationally symmetrical light
distribution, the isolux diagram shows one quarter
of the total luminous intensity.
The average degree of illuminance for a square
situation should be 10 lx.
About 20 lx is reached directly at the luminaire.
In order to obtain an average of 10 lx, the minimum
illuminance between two luminaires should be about
2 to 3 lx. In the diagram, the red circles indicate
these values. Consequently, luminaire spacing is 18 m.
130
a = 18 m
88 165
2.06 Information about light and illumination · Light distribution curves, illumination and isolux diagrams
Determination of the luminaire spacing “a” for square illumination
with BEGA pole-top luminaires 99 515 on the basis of the isolux diagram
1 lx
99 515
1
16
2
12
5
8
15
0,2 0,5
a = 34 m
LED
H = 5,0 m
1 lx
lx
0,5 0,2
4
0
m
20
16
12
8
4
0
4
8
12
a = 20 m
16
20
a = 20 m
99 515
The same principle applies for asymmetrical light
distribution. In this case, the position of the luminaires
varies. The luminaires are arranged singly along the
edge of the square and in double versions on the
surface of the square.
131