Instruments & Methods of Measurement

Posted by MRB
1Measurement/estimation of parameters
The measurement of following parameters is required.
2Power input
3Length & width of room, Mounting height
2Illuminance measurements
Lux meters corrected for V-lambda should be used for measurement of illuminance. The accuracy of 5% and suitable range up to 10000 lux should be used.
Usually lux meters are calibrated under the “standard light tungsten source of 2856 K” precisely. If these are used under different type of light source, the following correction factor is used on the measured value of lux.
Table 4-1: Correction factors for lux meters
Light sourceCorrection factor
Mercury LampX 1.14
Fluorescent LampX 1.08
Sodium LampX 1.22
DaylightX 1.00
Metal halideX 1.00
The above corrections factors are dependent on the type of lux meter used. Actual figures for the type of instrument used for measurement will be available in the calibration certificate.
Accuracies of readings should be ensured by
1Using accurate illuminance meters for measurements
2Sufficient number and arrangement of measurement points within the interior
3Proper positioning of illuminance mete
4Ensuring that no obstructions /reflections from surfaces affect measurement.
2.2Determination of Illuminance measurement points
Based on the room index, the minimum number of illuminance measurement points is decided as per the following table 4.2.
Room index, RI = (L x W) /(Hm x (L + W))
L = Length
W = Width
Hm = Height of the luminaires above the plane of measurement
Table 4-2: Number of points for measuring illuminance
Room indexMinimum number of measurement points
For ± 5% accuracyFor ± 10% accuracy
RI < 184
1 < RI < 2189
2 < RI < 33216
RI > 35025
For a space having length L= 5m, width W = 5 m and lamp mounting height of 2.6 m, RI = 0.96 i.e. there must be at least 8 measurement points. In figure above, 9 points are selected in view of the symmetry of the space.
The measurement grid should be positioned to cover a representative area of the working plane. To accurately determine the illuminance on the working plane, the greater the number of measurement points the better. This will account for any wide variations of illuminance in calculation of the average.
3Power measurements
Portable power analysers of preferably ±1.0 % error or less can be used for power measurements.
If power to all the lamps for an area is supplied from a single source/panel, measurement can be done at the panel. If power to the lamps is supplied from many points, we recommend measuring the power consumption of as many fittings as possible and then estimating the total power consumption by extrapolation.
4Summary of instrument accuracies
The table given below summarises accuracy requirements of various instruments.
For calibrating various instruments, visit for a detailed list of accredited laboratories. Calibration interval suggested for instruments is 6 months.
Table 4-3: Summary of instrument accuracy
Instrument and rangeAccuracy