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Note: The Hold mode will not function until at least 3 measurements
have been stored to memory.
3.10 Measurement Considerations
Keep the plastic domed cosine corrector clean and free of scratches.
It may be cleaned with a soft cloth and isopropyl alcohol.
The light level measurement should be taken at the application
height. In an offi ce the LM82 should be positioned at desk height and
in a corridor, it should be placed on the fl oor.
It is important that the light source completely fi lls the sensor with
minimal movement of the meter during measurement.
It is also advisable where possible to hold the LM82 away from your
body when taking measurements to avoid refl ections or creating a
shadow on the sensor.
For best accuracy, repeat the measurement several times to ensure
that the light source has remained stable.
The Inverse-square Law
The illuminance E at a point on a surface varies directly with the
intensity I of a point source, and inversely as the square of the
distance d between the source and the point. If the surface at
the point is normal to the direction of the incident light, the law is
expressed by E=I/d
2
.
Cosine Law
The illuminance on any surface varies as the cosine of the angle of
incidence. The angle of incidence θ is the angle between the normal
to the surface and the direction of the incident light. The inverse-
square law and the cosine law can be combined as E=(I cos θ)/d
3.12 CIE Photopic Curve
Wavelengh
CIE photopic
(nm)
luminous
effi ciency
coeffi cient
V
380
0.00004
390
0.00012
400
0.00040
410
0.00121
420
0.00400
430
0.01160
440
0.02300
450
0.03800
460
0.06000
470
0.09098
480
0.13902
490
0.20802
500
0.32300
510
0.50300
520
0.71000
530
0.86200
540
0.95400
550
0.99495
555
1.00000
560
0.99500
570
0.95200
580
0.87000
590
0.75700
600
0.63100
610
0.50300
620
0.38100
630
0.26500
640
0.17500
650
0.10700
660
0.06100
670
0.03200
680
0.01700
690
0.00821
700
0.00410
710
0.00209
720
0.00105
730
0.00052
740
0.00025
750
0.00012
760
0.00006
2
.
03/09/2014 09:51:04
Photopic
lumen/watt
conversion
factor
0.027
0.082
0.270
0.826
2.73
7.92
15.71
25.95
40.98
62.14
94.95
142.1
220.6
343.5
484.9
588.7
651.6
679.6
683.0
679.6
650.2
594.2
517.0
431.0
343.5
260.2
181.0
119.5
73.08
41.66
21.86
11.61
5.61
2.80
1.43
0.715
0.355
0.170
0.082
0.041
3.11 Spectral Mismatch Correction Factor
Spectral mismatch is the result of the differences between the
relative spectral responsivity of a light meter's sensor and the CIE
photopic curve it is designed to simulate, when the light meter is
used to measure a light source with a different relative spectral
power distribution to that of the light source used to calibrate the
light meter.
The LM82 has been calibrated against a standard tungsten light
source at a correlated colour temperature of 2856K.
Where the best possible accuracy is required when measuring
another light source type, it is suggested the LM82 should be
calibrated for that light source type so as to determine the spectral
mismatch correction factor (previously known as colour correction
factor) for that light source type.
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8
LM82 instructions rev1.indd 13
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