The use of luxmeters
Illuminance is readily measured by a portable instrument called a lightmeter or luxmeter. The instrument is held in the plane of measurement and the illuminance is displayed. A typical range of measurement for a luxmeter is from 1 lux to 100,000 lux, specialised meters are available for reading levels above and below that range.
For accurate measurements, the detectors should be cosine and colour corrected. The cosine correction is to allow for the effects of light falling on the cell at oblique angles. Colour correction is necessary in order to match the spectral sensitivity of the human eye, and is defined by the spectral response of the CIE Standard Observer, known as the CIE Vl (“V-lambda”) curve. There are a wide range of lightmeters in the marketplace. Prior to performing measurements consider the accuracy and range of measurement required.
For accurate measurements, the detectors should be cosine and colour corrected. The cosine correction is to allow for the effects of light falling on the cell at oblique angles. Colour correction is necessary in order to match the spectral sensitivity of the human eye, and is defined by the spectral response of the CIE Standard Observer, known as the CIE Vl (“V-lambda”) curve. There are a wide range of lightmeters in the marketplace. Prior to performing measurements consider the accuracy and range of measurement required.
Field measurements
Measurements of illuminance in a lighting system should be made after dark. With outdoor installations this is essential. For interior installation it is often possible to exclude the daylight by blinds or curtains. The established “lights ON, lights OFF” technique of measurement can be reasonably accurate, provided the daylight is not varying too widely, or the survey time is not too long.
An installation of discharge or fluorescent lamps should be switched on at least half an hour before the measurements are taken to allow for the lamps to be completely warmed up. If the luminaire is of the fully enclosed type, a longer stabilization time may be needed. Similarly, new lamps of either HID or fluorescent type should have a “Burning In” time of at least 100 hours before measurements are taken. GLS or Tungsten Halogen lamps need a 10 hour pre-measurement period.
The procedure to be adopted for the measurement process will depend on whether the space is furnished or unfurnished, occupied or unoccupied. In some instances the effects of body shadow should be taken into account. In some cases the purpose of the measurements may be to ascertain compliance with Tender Documents or a Lighting Standard. Measurements may also be for comparison with a previous photometric survey, or to ascertain if the maintenance or replacement point has been reached.
In many situations the measuring plane is either not specified or is non-existent. Therefore some measurement height must be predetermined so that consistent readings can be taken. A height of one metre above ground or floor level is generally considered to be an acceptable height. A monopole with a small platform to support the meter is helpful to achieve a consistent measurement height. It is important to remember that the field measurements are only valid for the conditions which exist at the time of the survey. If the measurements are to be a part of an ongoing survey process they should be recorded in a log book or spreadsheet. It is important to record the following information :
• details of the luminaire, including installation age
• meter make and serial number
• a detailed description of the surveyed area with a diagram indicating measurement points & photographs
• the height of the measuring plane
• time of day
• ambient temperature
• condition of interior surfaces, clean or dirty
An installation of discharge or fluorescent lamps should be switched on at least half an hour before the measurements are taken to allow for the lamps to be completely warmed up. If the luminaire is of the fully enclosed type, a longer stabilization time may be needed. Similarly, new lamps of either HID or fluorescent type should have a “Burning In” time of at least 100 hours before measurements are taken. GLS or Tungsten Halogen lamps need a 10 hour pre-measurement period.
The procedure to be adopted for the measurement process will depend on whether the space is furnished or unfurnished, occupied or unoccupied. In some instances the effects of body shadow should be taken into account. In some cases the purpose of the measurements may be to ascertain compliance with Tender Documents or a Lighting Standard. Measurements may also be for comparison with a previous photometric survey, or to ascertain if the maintenance or replacement point has been reached.
In many situations the measuring plane is either not specified or is non-existent. Therefore some measurement height must be predetermined so that consistent readings can be taken. A height of one metre above ground or floor level is generally considered to be an acceptable height. A monopole with a small platform to support the meter is helpful to achieve a consistent measurement height. It is important to remember that the field measurements are only valid for the conditions which exist at the time of the survey. If the measurements are to be a part of an ongoing survey process they should be recorded in a log book or spreadsheet. It is important to record the following information :
• details of the luminaire, including installation age
• meter make and serial number
• a detailed description of the surveyed area with a diagram indicating measurement points & photographs
• the height of the measuring plane
• time of day
• ambient temperature
• condition of interior surfaces, clean or dirty
Measuring the average illuminance
A lightmeter indicates the illuminance at the point of measurement only, not the average in the space. To find the average illuminance in an area at the time, it is necessary to divide the area into a number of equal areas which should be as nearly square as possible. The illuminance at the centre of each square is then measured, and the results averaged. The minimum number of equal areas required for accuracy can be determined by first calculating the room index (k) :
k= (Length . Width) / (Hm . (Length + Width))
where Hm is the height of the luminaires above the plane of measurement. The working plane is usually taken to be 0.85m for work benches or 0.72m for desk top height unless the main plane of the work is known to be some other height above floor level. If the work is performed down to floor level, then the floor is taken as the working plane and plane of measurement. If the proposed points coincide with luminaire positions, or are in constant relationship with the luminaire positions, increase the number of measurement points.
k= (Length . Width) / (Hm . (Length + Width))
where Hm is the height of the luminaires above the plane of measurement. The working plane is usually taken to be 0.85m for work benches or 0.72m for desk top height unless the main plane of the work is known to be some other height above floor level. If the work is performed down to floor level, then the floor is taken as the working plane and plane of measurement. If the proposed points coincide with luminaire positions, or are in constant relationship with the luminaire positions, increase the number of measurement points.
Room index (k) | Min points for accuracy +/- 5% | Min points for accuracy +/- 10% |
---|---|---|
k < 1 | 8 | 4 |
<1 <= k < 2/td> | 18 | 9 |
2 <= k < 3 | 32 | 16 |
3 < k | 50 | 25 |
Table 1 : Room index vs measurement count for given accuracy.
Luxmeter maintenance
As a general rule meters should be re-calibrated every year. If a meter is to be used in a legal case it should have been re-calibrated within the preceding 12 months. It is good practice to have the meter’s calibration checked either just before or just after the photometric survey if the results are likely to be subject to litigation.
Although the present day luxmeter is a fairly robust device, all meters need to be handled carefully. Similarly they should not be left in places where they are subject to excessive heating. For example, they should NEVER be left in the glove box of a car for long periods in the summer months. Not only will this upset the calibration, but the cell can also be destroyed by the heat.
Although the present day luxmeter is a fairly robust device, all meters need to be handled carefully. Similarly they should not be left in places where they are subject to excessive heating. For example, they should NEVER be left in the glove box of a car for long periods in the summer months. Not only will this upset the calibration, but the cell can also be destroyed by the heat.
Luxmeter accuracy
Lightmeters, like all other measuring devices, have an inherent accuracy which must be considered when using the meter to perform measurements. Manufacturers usually quote illuminance accuracy as a percentage of the reading, typically in the range 2% to 5%. This is exclusive of errors introduced by cosine response and spectral response of the meter.
Calibration of the meter can improve the accuracy of measurements. A calibration history of the meter can show how stable the meter is over time. Following a calibration, it is important to check the previous calibration to see if there has been a shift in the accuracy. If significant, it could have a bearing on the accuracy of readings since the last calibration, it may also indicate that the meter has suffered damage since the last calibration. Calibration reports should be filed for future reference.
Calibration of the meter can improve the accuracy of measurements. A calibration history of the meter can show how stable the meter is over time. Following a calibration, it is important to check the previous calibration to see if there has been a shift in the accuracy. If significant, it could have a bearing on the accuracy of readings since the last calibration, it may also indicate that the meter has suffered damage since the last calibration. Calibration reports should be filed for future reference.
Types of illuminance
Typically illuminance is measured with the face of the detector horizontal. Horizontal illuminance is the common measurement considered in Lighting Standards, however vertical illuminance has its place. For example, the illuminance on a blackboard or a wall chart is measured with the face of the detector vertical.
Semi cylindrical illuminance may assist in the lighting design process, such measurements need the application of special illuminance detectors. Half cylindrical measurements can be useful in the modelling of the human figure or on a row of columns in the colonnade of an historic building. It has been found to be more reliable than horizontal illuminance at a distance in outdoor areas, such as in car parks or public places.
Cubic illuminance is defined as the incident illuminance at a point, in terms of three pairs of mutually perpendicular planar illuminances, think of a cube and placing the illuminance meter in turn on each face and performing a measurement. These three pairs of illuminance values, either measured or calculated, can be used as indicators of the spatial distribution of illuminance in terms of light, shade and modelling. This is particularly relevant in areas such as public spaces, seminar and conference rooms, and especially video conferencing rooms where recognition of a person’s facial expression is an important part of the communication process.
Semi cylindrical illuminance may assist in the lighting design process, such measurements need the application of special illuminance detectors. Half cylindrical measurements can be useful in the modelling of the human figure or on a row of columns in the colonnade of an historic building. It has been found to be more reliable than horizontal illuminance at a distance in outdoor areas, such as in car parks or public places.
Cubic illuminance is defined as the incident illuminance at a point, in terms of three pairs of mutually perpendicular planar illuminances, think of a cube and placing the illuminance meter in turn on each face and performing a measurement. These three pairs of illuminance values, either measured or calculated, can be used as indicators of the spatial distribution of illuminance in terms of light, shade and modelling. This is particularly relevant in areas such as public spaces, seminar and conference rooms, and especially video conferencing rooms where recognition of a person’s facial expression is an important part of the communication process.