Measurement of LUX Level and Uniformity at Indoor and Outdoor Lighting (Part-1)

Introduction:

• Working plane illuminance (Lux Level) need to be measured in the field for cross check of whether the existing installation meets a design requirement or not.
• Field surveys may also be useful to identifying the causes of complaints about lighting, hence the results of field surveys may be useful for the designer, installers and end users.
• There are various methods are developed for field measurement of Interior Lighting and External Lighting.
• The Measurement Methods recommended by the various national lighting bodies are generally similar or slightly derivatives to each other. The most common method / Standard is BEE, CIBSE, IES and DIN code
• The most of methods require to measurement of illuminance at points on a grid at working-plane height or at Floor, but the grid size and position of the measuring points may be differed from various standard to standard.
• The IES method and its derivatives use the position of the grid according to the luminaire locations.
• The CIBSE and DIN methods use a position of grid according to the room size.
• The techniques of analysis of the field measurement results also differ

Basic Requirements for Exterior & Interior Light Level Measurement

•  The following Points should be considered for accurate measurement of interior and exterior lighting Lux level.
• Where possible, use the same calibrated illuminance measurement meter (LUX Meter) If the same meter is not available, use the same make and model of calibrated meter to minimize error.
• When taking measurements, verify that any objects/materials are not blocking any light to the meter head. The use of a remote meter head cabled to the meter body is recommended to prevent the operator from blocking the meter’s “view” of the lighting system being measured.
• In Outdoor Lighting it is essential to measure of illuminance should be done in night (proper dark).
• For indoor lighting, measurements with lights ON and Lights OFF technique can be followed and the daylight variation is not too much and the survey time is not too long.
• In an installation of fluorescent discharge lamps, the lamps must be switched on at least 30 minutes before the measurement to allow for the lamps to be completely warmed up.
• In many situations, the measuring plane may not be specified or even non-existent. Hence it is necessary to define measurement height, typically 0.8 to 1 meter from the ground or floor level.
• The lux measurement procedure simply requires positioning a meter’s sensor on the surface or location where you wish to measure the incident light.
• The sensor should face the light source at a right angle. If the sensor is not perpendicular to the light, the measurement will be incorrect, though some lux meters have a cosine correction to account for the angle.
• Meters that require a colour correction factor may have a means of inputting the CCF to adjust the result for LEDs or fluorescent lights; otherwise, you will have to manually multiply the measured lux by the CCF.

 INDOOR ILLUMINATION (LUX LEVEL) MEASUREMENT.

 (1) As per Room Index Method (as per BEE Code / CIBSE Code):

•  This methos is more suitable where measuring Plan / Points for an interior is more rectangular than square. First, we need to be found Room Index.
• Based on the room index, the minimum number of illuminance measurement points is decided by Room Index Number
• Room Index (RI) = (L x W) / H x (L+ W)
• Where L = Length of Room
• W = Width of Room
• H= Height of the luminaires above the plane of measurement 

 Sample calculation

• Measure Illumination Level of an office room have length, L = 7.5 m and width W = 5 m,
• Solution:
• Suppose Height of Illumination from Floor is 2 Meter
• Room Index RI = (L x W) / H x (L+ W)
• Room Index RI = (7.5 x 5) / 2 x (7.5+ 5)
• Room Index RI = 1.5
• From Table 4.2 minimum Illumination Measure Points should be 18 No’s
• The illuminance measurements Points with Measured Value in Lux are marked on the grid.  

 (2) As per Point Layout Method

• For office and other task areas, identify a set of measurements points on desktops and other work surfaces that best represents lighting conditions in the space.
• It may not be possible to develop a uniform spacing grid, but points should be chosen that represent the various lighting conditions across the space.
• For each separate horizontal grid, identify a vertical plane representative of the lighting in the area (typically the gridline directly between two light fixtures).
• On this vertical plane, set a grid (line) of points at 1.5 Meter above the site surface at each of the corresponding horizontal measurement points.
• The following figures provide sample layouts for selecting horizontal measurement points for typical areas where lighting measurements are taken