D ESCRIPCIÓN

When a lighting scheme has been designed and installed it is normally necessary to perform a survey as part of the commissioning process. A survey would also be necessary in the case of any dispute over the performance of an installation.

When performing a survey a grid of points is generally placed over the area to be surveyed. These grid points are the measurement points at which a reading of light will be taken.

To perform a survey adequate equipment is required. This is generally either an illuminance meter or a luminance meter, dependant upon the criteria used during the design of the installation. It is essential that the equipment used is suitable for the task. It therefore needs to be calibrated, with a current calibration certificate from a competent company with traceability to national standards. It also needs to have a suitable range of sensitivity to be able to measure the light levels present in the installation. So to measure emergency light levels a more sensitive meter would be necessary that could measure low light levels.

When making a scheme survey it is essential to keep a complete and accurate record of the state of the whole installation at the time of the survey, which is the lighting equipment and the space the lighting is in. (Photographs are a valuable addition to a written record.) Examples of information of note are:

With regard to the measuring equipment

− Type of meter, manufacturer, model, serial number and calibration date

− Details of any additional equipment, such as tripods, tape measures, etc. should be noted

With regard to the luminaires

− The luminaire manufacturer and manufacturers’ code

− Details of the lamps (number, type and age)

− The supply voltage (value and stability)

− The state of maintenance of the installation (lamps and luminaires)

− Details of luminaire control systems being used

− Geometric details of the luminaire positioning

− The presence/absence of daylight, including a background reading of luminance/illuminance with daylight only (luminaires turned off). Note that the quantity of daylight may vary significantly over time so ideally daylight should be excluded from measurements of electric light unless the aim is to measure a constant illuminance installation (daylight control)

− The ambient temperature in the space

− Any other factors which could influence the measurement Before taking any measurements it is important that the output of the luminaires is stable. Therefore the lighting should ideally be operated for one hour before taking any measurements, and at least 30 minutes. Additionally to ensure the stability of the meter photocell it should be exposed to the stable light levels for approximately five minutes before taking any measurements.

When defining a measurement grid this is dependant upon the application being surveyed.

Interior measurement grids

Frequently for sports lighting the grid definition is defined by the sports governing body, so for an indoor sports facility any requirements specific to a particular sport should be used.

However, if no specific requirements exist, or the installation is not a sports facility, the measurement points for verification of the design should be in the same location and plane as the calculation points used during the design. Therefore, if a measurement plane was calculated which was tilted to mimic the orientation of the task, the same measurement plane should be used for verification.

Note that during design it should be ensured that the grid spacing does not coincide with the spacing of the luminaires in the installation as this can distort the calculated results, and therefore the scheme performance.

Exterior measurement grids – sports and area

Frequently for sports lighting the grid definition is defined by the sports governing body, so any requirements specific to a particular sport should be used. However, if no specific requirements exist, or the installation is not a sports facility, the measurement points for verification of the design should be in the same location and plane as the calculation points used during the design. Therefore, if a measurement plane was calculated which was tilted to mimic the orientation of the task, the same measurement plane should be used for verification.

Exterior measurement grids – road

For road lighting the grid is normally defined in the relevant standard and is generally related to the spacing of the road lighting lanterns. Therefore the relevant standard should be referenced for the grid definition which should be the same as the grid used for calculation during design.

When marking the measurement grid in the area to be measured the method of marking is dependant upon the measurements to be taken. When measuring illuminance small markers (such as sticky dots) may be placed upon the surface to show the measurement point. However when measuring luminance this would invalidate the reading and so for luminance readings markings should be used to sight the luminance meter, and then moved before the reading is taken.

When taking luminance readings in a road lighting installation the position of the meter will be a significant distance from the measurement point. This has two implications:

− The luminance meter must be able to restrict the angle of measurement to allow only the relevant grid position to be measured, typically to two minutes of arc in the vertical plane and 20 minutes of arc in the horizontal plane.

− The grid markings must be visible from a large distance.

Therefore three-dimensional objects should be used to mark the grid points and removed individually as each grid point is measured.

The method of marking out the grid should be recorded with details of equipment used and fixed reference points used to locate the grid. To record the measured values a diagram should be used to assign reference numbers to each grid point.

A table of values may then be completed containing the grid reference number and the measured value.

− When taking measurements it is advisable to wear dark matt clothing to prevent light reflecting from clothing onto the photocell, giving abnormally high readings. However, if safety requirements require high visibility clothing, care should be taken to minimise light reflection onto the photocell.

− The use of a tripod is advisable, especially for luminance readings or readings using heavy equipment.

− For measurement grids that are not at ground level the use of a stand, at the correct height and orientation for the task plane, can help ensure a photocell is correctly positioned at a measurement position.

− It is good practice to measure the background light levels without the lighting installation turned on. Even moonlight can have a noticeable effect on light levels. Also to take these measurements after measuring the installation with the lights turned on, as the background light levels may vary considerably during the warm-up time for the lighting.

− When measuring horizontal illuminance it cannot be assumed that the ground is horizontal, especially in outdoor applications. Care must be taken to ensure the photocell is horizontal, even if this is not a true representation of the ground.

− Correction factors should be applied to readings to compensate for the lamp type used in the schemes.

However, highly coloured or monochromatic light sources will give erroneous readings using conventional light meters.