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 

Table 4-2: Number of points for measuring illuminance

Room index

Minimum number of measurement points

 

For ± 5% accuracy

For ± 10% accuracy

RI < 1

8

4

1 < RI < 2

18

9

2 < RI < 3

32

16

RI > 3

50

25

 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.  

1

Measurement Reading Details

107 Lux

99 Lux

85 Lux

65 Lux

65 Lux

45 Lux

73 Lux

130 Lux

105 Lux

110 Lux

86 Lux

87 Lux

59 Lux

50 Lux

58 Lux

99 Lux

75 Lux

106 Lux

115 Lux

76 Lux

         

Min

45 Lux

     

Max

130 Lux

     

Average

85 Lux

     
         

U1=MIN/AVG

0.5 Lux

     

U2=MIN/MAX

0.3 Lux

     

 

 (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

2

Measurement Reading Details

107 Lux

80 Lux

100 Lux

75 Lux

100 Lux

65 Lux

73 Lux

70 Lux

75 Lux

60 Lux

99 Lux

87 Lux

95 Lux

58 Lux

98 Lux

60 Lux

65 Lux

63 Lux

66 Lux

78 Lux

75 Lux

78 Lux

62 Lux

99 Lux

87 Lux

95 Lux

58 Lux

98 Lux

         

Min

58 Lux

     

Max

107 Lux

     

Average

80 Lux

     
         

U0 or U1=MIN/AVG

0.7 Lux

     

Ul or U2=MIN/MAX

0.5 Lux

     

About Jignesh.Parmar (B.E,Mtech,MIE,FIE,CEng)
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical). He is member of Institution of Engineers (MIE) and CEng,India. Membership No:M-1473586.He has more than 16 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in “Electrical Mirror”, “Electrical India”, “Lighting India”,”Smart Energy”, “Industrial Electrix”(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

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