CHARTERED INSTITUTION OF BUILDING SERVICES ENGINEERS
Factfile No3 August 1996
Working Plane Illuminance In Electrically Lit Spaces
1.0 Introduction This paper offers practical guidance on the measurement of working plane illuminance. This criterion is given prominence in the CIBSE Interior Lighting Code 1994 and in other statutory and institutional recommendations and for this reason assumes importance for designers as the major form of installation appraisal. The range of information to be obtained from an installation is described. In addition guidance on the collection and recording of illuminance measurements is given including the use of standard pro-formas. Finally methods of analysis of the measurements are discussed. Many other aspects of lighting appraisal are not covered here including luminance and vertical illuminance.
2.0 Why Surveys Are Carried Out The contractual conditions associated with the design of artificial lighting systems have focused attention on assessment of their performance. Often designers, their clients or potential tenants of a space, wish to check the accuracy of the calculated lighting level against that actually available in the space. In addition the duty of care imposed on employers, and others, means that the lighting conditions of the workplace must meet minimum standards. A failure to provide the minimum standard can have serious implications for the employer.
Where existing installations are appraised it is important to obtain accurate information concerning the age and maintenance of the installation. Although empty interiors are relatively straightforward, furnished interiors are likely to influence the position at which illuminance measurements can be taken. There may also be shadows on the plane where illuminance measurements are taken. The recent adoption of the maintained illuminance approach to lighting design has altered the earlier UK definition of Maintenance Factor. This change means that the average illuminance on the working plane before maintenance is to be carried out is greater than the maintained illuminance specified by the designer. The measurement of working plane illuminance can be used to assess whether installation performance meets specification. The average measured illuminance should never be lower than the average maintained illuminance.
3.0 Preparation For The Survey It is important to plan in advance all aspects of the survey. This involves liaison with the client or occupier of the space to ensure access, often in the evening, and to ensure that thermal conditions in the test area are stable; checking of measuring equipment and preparation of means of recording the survey.
3.1 llluminance meters to BS 667:1985 There is a wide range of illuminance measurement equipment available for use in lighting surveys. Equipment can be purchased from a reputable company or may be hired for the period of the lighting survey. Light measuring equipment should be regularly calibrated, typically once a year. When the meter is used extensively, or the consequences of the results are critical, 6 monthly calibration is advised. When the meter has been dropped or subjected to extremes of heat or cold it is advisable to re-calibrate. All equipment should be regularly cleaned and batteries checked. Spare batteries should always be taken when carrying out a lighting survey.
There are two types of meter defined in BS 667 of different accuracy. Errors of measurement of ±10% may occur in Type P1, and ±15% may occur in Type P2. Normally a type P1 should be used for field measurement. The manufacturer will provide information concerning the temperature range over which the meter remains accurate. Although this will probably include most building interiors, specialist industrial processes may need extremes of temperature. Manufacturers of photo-voltaic cells can supply correction factors for illuminance measurements when the meter is used in operating temperatures which differ from a normal value. In many cases this is about 25ºC. Selenium photocells are more sensitive to high temperature than silicon photocells, where prolonged exposure to temperatures above 50ºC causes permanent damage. An illuminance meter is shown on Figure 1.
The meters measure the photon impact intensity on the surface of a photocell. The cell is able to use this energy to provide a small current which will vary in proportion to the photon impact intensity. Circuitry in the meter amplifies this signal and corrects it for the spectral sensitivity of the eye (V correction). The output can then be displayed as a reading calibrated in lux, usually on an LED display, but sometimes on a moving coil needle gauge. It is common for meters to be provided with a range of full-scale readings. The scale can be changed by control knobs or switches.
The meters are normally portable, hand-held and the photocell is usually attached to the amplifier by a flexible cable. This allows the photocell to be some distance from the person taking measurements, avoiding the risk of shadowing the photocell.
The photocell must be of the cosine corrected type. This is because when light is incident at less than 90º to the photocell surface, the response of the photocell must be modified by the cosine of the incident angle. This is achieved by a dome or disk of diffusing material being incorporated over the actual measuring cell.
The person making the measurements must be familiar with the use of the equipment. Time should be taken to read and understand the contents of the instruction book. Simple trial measurements are useful.
Figure 1
3.2Other equipment The other equipment needed for the survey may seem self evident but needs to be assembled and checked in advance if an abortive visit or wasted time is to be avoided.
A floor plan of the areas to be surveyed should be obtained from the client beforehand to allow consideration of the grid location. If reflected ceiling plans, showing the luminaire layout, and furniture layouts can be obtained then this will allow even more pre-planning to be carried out before the visit.
A long tape measure, at least 7-10 metres, is needed to lay out the measurement grid. The grid itself can be marked-out by string or chalk marks on the carpet (if allowed); otherwise brightly coloured sticky dots or labels are needed to mark the grid intersection points so as to allow quick and accurate location of the measuring cell. It is possible to buy packs of pre-numbered dots which may help in identifying and recording the readings.
If measurements are to be carried out over empty areas or between desks then a portable stand will be needed to ensure the measuring cell is located at the working plane height. This stand should be light but able to ensure that the measuring cell is horizontal and at the correct height for each reading. An example is shown in Figure 1.
Finally a clipboard or other firm writing surface, pens and paper should be obtained for the recording of the results. If proforma recording sheets, like that shown in Figure 2, are used then time is saved at the measuring site and the readings are more likely to be recorded without error. Where a comparison is to be made between calculated illuminance figures and those measured it will be necessary to estimate the actual room surface reflectances. This can be done by taking a BS colour matching chart and using it to obtain a match against the room surfaces. The chart will then give a reflectance figure that can be used for calculating corrections between the recorded illuminance figures and those calculated.
3.3Preparation of test areas Liaison with the client and/or the occupiers of the space before the visit is vital as all measurements relating to the performance of artificial lighting systems should, whereever possible, be taken at night. This may entail special security clearance or the attendance of the client’s staff at the tests.
As the light output of lamps varies depending on their operating temperature, it is essential that the luminaires will have been operating under normal thermal conditions before measurements start. This may require, for example, both lighting and heating or air conditioning system to be switched on for long enough to achieve steady state conditions.
4.0 THE SURVEY ITSELF Initially the proposed test areas must be checked to ensure that they are representative of the working area and that all lamps and equipment are working correctly. Stray light from surrounding rooms, spaces and through external windows should be minimised by use of blinds, curtains, etc. Any automatic lighting control or daylight linked controls should be set such that the output of the lamps is at full power and will not vary during the tests. All lighting in the area that would normally illuminate the area test grid should be operating. A failure to allow the lamps to reach their normal operating temperature before taking illuminance measurements will mean that initial readings will be low. Where lamps are known to be new they should be run for a minimum of 100 hours under normal operating conditions. A characteristic swirling pattern of light emission from the lamps may indicate that the lamps are new.
Where possible the line voltage supply to the lighting circuit should be monitored. Increases and decreases in lumen output are caused by variations in supply voltage. Record details of any control system and any special measuring or monitoring of the supply voltage or lamp output.
4.1Setting out the test grid The measurement grid should be positioned to cover a representative area of the working plane. This should be where there are no obstructions above the working plane which may reduce the measured levels. Since standard calculations and illuminance prediction are based on the assumption that there are no obstructions above the working plane, ensure that no screens, filing cabinets etc. are present in the measurement area.
The spacing