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SECTION 3 - DESIGN OF A LIGHTING SCHEME
The flowsheet below at Figure A3.1 summarises the steps in a lighting design whilst Figure A3.2 includes a more detailed checklist of the key information required. This approach can be used to evaluate complex cases.
Figure A3.1 - Scheme Design
Figure A3.2 - Design Checklist
The Environmental Zone
Nationally recognised environmental zones are defined as follows, together with typical topographical areas:
E1: Intrinsically dark Areas - National Scenic Areas
E2: Areas of low district brightness - Rural or small village locations
E3: Areas ofmediumdistrict brightness - Urban or small town locations
E4: Areas of high district brightness - Large town or city centre with high levels of night time activity
The lighting designer should clearly indicate which zone has been selected together with the justification for selecting that zone.
This zone is used for evaluating obtrusive light calculations.
Obtrusive Light Calculations
The ILE Guidance notes and the CIE Report 150 provide technical limitations on obtrusive light levels. The design should therefore be carried out in conjunction with the monitoring of these calculated limits for all, or at least the first three, of the following items.
A Direct line of sight of the light emitted fromluminaires is probably the principal source of obtrusive lighting complaints as it can produce the offensive glare effect that is referred to in the Introduction. This is light radiated directly from the luminaire where the limits quoted by the ILE and CIE relate to intensity values from individual luminaires when viewed fromexternal view points into the site. It is normal practice to calculate that the design mitigates the recommended limiting values, shown in the table below, fromthe critical viewpoints.
B Light intruding in through property windows can be predicted by calculating values on a vertical grid representing a window, or series of windows. The recommended limits are additive to what is already being experienced, pre development. If the pre development limits, shown in the table below, are already exceeded, the new design will require to show, by calculations, that zero light intrusion will be provided by the new development.
Both A and B above are essential elements in proving that the new development proposal willmitigate themajority of residents' concerns and in the case where the development requires the use of all night lighting themore onerous "post curfew" limiting values should be chosen as themaximum limit.
C The upward light ratio will vary between individual luminaires depending on the respective tilt angles and light distribution in their intended installed arrangement. Since the upward light ratio calculation is done for the complete installation the proposal application should state the individual luminaire elevations against which the calculation is based. Many quality luminaires produce a 0% upward light ratio at zero degrees of tilt but will produce an upward light ratio of 2.5% with 10º of tilt. Some luminaires can produce asmuch as 50% upward light ratio at tilt angles greater than 40º and if this was the case then the lighting installation could rightly be classed as being an obtrusive and inefficient lighting solution.
It should be remembered that the purpose of any lighting installation is to illuminate an area and will normally be projected downwards on to the desired area, be it road or area lighting. It is how this downward light is managed and its effectsmitigated and controlled that are important: there will always be a degree of reflected upward light. Although the evaluation covers the direct upward component of light from the complete installation it does not include the light reflected upwards from the ground. Neither the ILE nor the CIE have defined or quantified this reflective element since no two developments have the same ground cover. As a general rule the darker the building or ground cover surfaces are, the lower the upward reflected component and conversely the lighter and wetter the building or ground cover then the higher the upward reflected component will be.
D The effect of glare, as viewed by an external observer, can be controlled by
limiting the viewed intensity as described in A, however, sometimes it is necessary to carry out a second glare assessment with sports lighting to protect the interests of spectators and players. This calculation process is additional to that already carried out in A and is not a substitute evaluation.
E An additional glare evaluation may be required to protect the interest of all road users, including pedestrians and cyclists. The termThreshold Increment ( TI) has been used in street lighting quality assessment calculations for at least 15 years but its use is now being extended to ensure that vehicle drivers are not subjected to a threshold increment level greater than 15% fromnon-street lighting installations situated adjacent to a public highway. Again this is an additional assessment and is not a substitute for the requirements described in A and should be carried out as a cumulative process with the existing street lighting provision included in the calculation.
F Building luminance is normally only carried out for structures, which are architecturally transformed at night by the application of illuminating techniques. Different surface textures and colours reflect light in different proportions and any luminance calculation should include for a schedule of surface colours and reflection factor characteristics to be assumed in the calculation process.
G The calculation of the combined effects of direct and upward reflected light is generally unnecessary, and as stated in C, there are currently no national or international recommended limits onmeasuring success or failure with a proposed lighting design. It can, however, be used to good effect to demonstrate a visual comparison between the old and new lighting installation techniques. It can also be used to demonstrate the difference in upward reflections where new lighting has been designed in conjunction with landscaping techniques that soften their effect by reducing the area allocated to hard landscaping.
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