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The field size of any lighting fixture can be calculated by multiplying
the throw by the multiplication factor:
[Throw] x [mf] = [Field Size]
In the example below, the throw is 18' and the fixture's mf is .68,
giving a field size of 12.24':
18 x 0.68 = 12.24
Remember that in most cases, we are basing our calculations on the
distance between the fixture and the performer's face, rather than on
the distance from the fixture to the floor.
A fixture's peak candela is its intensity, in lumens, as
measured from right in front of the instrument, directly on its axis.
Intensity is measured in footcandles, using the following formula:
[Peak Candela] / [Throw2] = [Footcandles]
Assuming that the fixture in the above example has a peak candela of
88,000, its intensity can be calculated thusly:
88,000 / 324 = 271.60
A fixture's peak candela and multiplication factor usually can be found
in the data sheets provided by the manufacturer. Information for some commonly
used fixtures is in the table below:
|
|
Fixture
|
Lamp
|
PC
|
MF
|
Color Frame
|
|
|
19° Source 4
|
HPL575
|
288,000
|
0.31
|
6.25"x6.25"
|
|
26° Source 4
|
HPL575
|
159,000
|
0.46
|
6.25"x6.25"
|
|
36° Source 4
|
HPL575
|
82,000
|
0.65
|
6.25"x6.25"
|
|
50° Source 4
|
HPL575
|
35,900
|
0.93
|
6.25"x6.25"
|
|
|
26° Source 4 Junior
|
HPL575
|
92,000
|
0.46
|
6.25"x6.25"
|
|
36° Source 4 Junior
|
HPL575
|
38,000
|
0.65
|
6.25"x6.25"
|
|
50° Source 4 Junior
|
HPL575
|
22,900
|
0.93
|
6.25"x6.25"
|
|
|
Altman 360Q 6"x16
|
GLC
|
231,750
|
0.18
|
7.5"x7.5"
|
|
Altman 360Q 6"x12"
|
GLC
|
164,925
|
0.48
|
7.5"x7.5"
|
|
Altman 360Q 6"x9"
|
GLC
|
38,800
|
0.68
|
7.5"x7.5"
|
|
Altman 360Q 4.5"x6.5"
|
GLC
|
58,425
|
1.03
|
7.5"x7.5"
|
|
|
Altman 65Q 6" Fresnel (Spot)
|
BTN
|
92,500
|
0.28
|
7.5"x7.5"
|
|
Altman 65Q 6" Fresnel (Flood)
|
BTN
|
8,800
|
1.4
|
7.5"x7.5"
|
|
|
Source 4 PARNel (Spot)
|
HPL750
|
174,000
|
0.41
|
7.5"x7.5"
|
|
Source 4 PARNel (Flood)
|
HPL750
|
43,100
|
0.78
|
7.5"x7.5"
|
|
|
Source 4 PAR (VNSP)
|
HPL575
|
287,100
|
0.31
|
7.5"x7.5"
|
|
Source 4 PAR (NSP)
|
HPL575
|
272,310
|
0.33
|
7.5"x7.5"
|
|
Source 4 PAR (MFL)
|
HPL575
|
136,590
|
0.39/0.55
|
7.5"x7.5"
|
|
Source 4 PAR (WFL)
|
HPL575
|
55,071
|
0.57/0.87
|
7.5"x7.5"
|
|
Source 4 PAR (XWFL)
|
HPL575
|
26,686
|
1.1
|
7.5"x7.5"
|
|
|
PAR 64 (VNSP)
|
FFN
|
400,000
|
0.17
|
10"x10"
|
|
PAR 64 (NSP)
|
FFP
|
330,000
|
0.25/0.46
|
10"x10"
|
|
PAR 64 (MFL)
|
FFR
|
125,000
|
0.37/0.81
|
10"x10"
|
|
PAR 64 (WFL)
|
FFS
|
40,000
|
0.83/1.43
|
10"x10"
|
|
Notice that the larger a fixture's field angle
(for example, 50°, as opposed to 26°) the larger its
multiplication factor and therefore the field of light.
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|