L02 – Lighting Sources1. Overview - Qualitative aspects of daylighting [scb]2. Daylight I – Overcast Sky, prediction technique [scb]3. Daylight II – Clear sky, prediction techniques[scb]4. Daylight III – Integration of daylight techniques[scb]5. Light Sources 1 – Physical generation of light [rf]Incandescent sources 6. Light Sources II – Low pressure discharge lamps [rf]7. Light Sources III – High pressure discharge lamps [rf]9. Lighting Design I – Zumtobel Visit [gd]VPI/Brightness management 10. Lighting Design II – Controls & Gear [km/pr]Lighting Controls(strategic issues) Control Gear Staff: [scb] – Stephen Cannon-Brookes [rf] – Richard Forster [gd] – Grant Daniels L02 (1). Daylight – Overcast Sky CalculationsTraditionally levels of daylight inside a building have been characterized by the Daylight Factor, the ratio of indoor to outdoor illuminance under an overcast sky. Although the daylight factor cannot account for the orientation and direct sunlight that occurs on bright days, it can be used in daylight design for worse case situations. It is, therefore, most appropriate for locations where cloudy conditions predominate. Daylight qualityThe quality and intensity of daylight varies with latitude, season and local weather conditions. In contrast with the tropics, direct sunlight in temperate zones cannot by relied on for the lighting of the interiors of buildings, and therefore we must rely on lighting from the sky. We have measurements that show the average outdoor illuminance on a horizontal surface due to the whole diffuse sky. They show a peak value of 35Klux at noon in July and August. We are also interested in the luminance distribution of the sky. Traditionally the overcast sky has been used for daylight design in the UK. This has a dark horizon and brighter zenith (with ratio of about 1:3). Luminance does not vary with azimuth, which simplifies daylighting calculations, but means we cannot take account of lighting conditions that do change with azimuth on sunny days. We should consider the overcast sky model as a worse case scenario. We assume 5000 or 10000lux for the horizontal illuminance due to the sky dome. Simple prediction techniquesNo-sky lineThis involves drawing a line into the space, at working-plane height, which shows the point at which the sky is no longer visible. Beyond this line, any light within the interior is from either external or internal reflections. Daylighting in the space will appear uneven if part of the interior is beyond the no-sky line. Room Depth (Index)This is a measure due to Lynes/ BS 8206 Pt 2 1991. It states that a room will appear unevenly lit if
where, · L – depth of room · W – width of room · H – height of window opening · Rb – area weighted reflectance for rear half of room (0.5 is typical for an office) Daylight factorThe daylight factor is a ratio of the indoor illuminance at a point divided by the simultaneous horizontal unobstructed outdoor illuminance. For a particular point in a building, the daylight factor is a constant, provided that sky luminance distribution follows the CIE Overcast sky definition. Daylight factors can be measured in a scale model using an artificial sky, or by a number of calculation techniques. Point daylight factor calculation – Sky ComponentBRS simplified daylight tableThis table allows the daylight factors to be calculated when scale diagrams are not available, and is appropriate for the early stages of a design. The tables give data for an overcast sky, with rectangular vertical windows fitted with clean clear glazing. Allowance can be made for, · some simple obstructions · the use of other glass type · the presence of dirt on the glass The following information is needed · H, the effective height of the window head above the working plane · HW, height of working plane above floor · W1 & W2, the effective widths of the window on either side of a line drawn from the reference point normal to the plane of the window. · D, the distance from the reference point to the plane of the window The ratios H/D, W1/D and W2/D are worked out, and the sky component can be read directly from the table. Waldrom diagramGrid methods of calculating sky components can be applied with accuracy to a wide range of circumstances, but are inclined to be tedious. They are particularly used when window and obstruction are complex in outline. The most commonly used grids are based on the Waldrom Diagram. It comprises a grid representing 50 percent sky component (i.e. half the sky hemisphere) and is so constructed so that equal areas of grid represent equal sky component. The area of the sky visible through the window from the reference point is plotted on the grid in lines of angular co-ordinates from information gained from a scaled plan and section of the building. The area of the patch of the sky plotted on the diagram is then proportion to the sky component at the reference point, where one unit on the diagram represents one per cent daylight factor. The superimposed curved lines on the grid, known as droop lines correspond to the horizontal edges of obstructions parallel to and at right angles to the plane of the window. BRS daylight factor protractorsThe Daylight Factor Protractors, originally produced by the Building Research Station, are the most widely used by designers in this country. The protractors are quicker to use than grid methods, but may not be so accurate when the obstructions and windows are of complicated shape. For the majority of cases, it is possible to assume an average simple outline for external obstructions without serious loss of accuracy. The protractors are circular, with two semi-circular scales, one for sky component and an auxiliary scale to correct for windows of finite length. An ordinary angle protractor is included to help in determining the required angle of elevation. The full set of protractors cover uniform and overcast sky for a wide range of glazing conditions. Point daylight factor calculation – External ComponentThe externally reflected component can be calculated by considering the external obstructions as a patch of ‘sky’ whose luminance is some fraction of the sky obscured. In other words, the equivalent sky component is calculated by one of the methods already described and is then converted to the ERC by allowing for the reduced luminance of the obstructing surfaces compared to the luminance of the sky. In practice, unless the actual luminance is known, it is assumed to be uniform, with a luminance one-tenth of the average luminance of the sky. Point daylight factor calculation – Internal ComponentThe internal component is the light reaching the reference point after reflection and inter-reflection off the surfaces of the room. This depends on the reflectances of the surfaces, the amount of light reaching them from outside, and the obstructions and ground outside. Computers can attempt to calculate accurately the reflections, but often a simpler method will suffice. BRE inter-reflection formulaThe formula is applicable when relatively high accuracy is required for estimating the internally reflected component for side-lit rooms. It is given in the form,
where · W – Area of window · A – Total area of ceiling, floor and all walls, including area of window · R – Average reflectance of ceiling, floor and all walls, expressed as a fraction · Rlw – Average reflectance of the floor and those parts of the wall below the plane of the mid-part of the window (excluding the window wall) · Ruw – Average reflectance of the ceiling and those parts of the wall above the plane of the mid-part of the window (excluding the window wall) · A constant, having values depending on obstruction outside the window (Value of C for no obstruction – 39; value of C for 800 obstruction – 5) Point daylight factor calculation – Summation of componentsWhen the three components of daylight factor have been estimated as above, their separate values are simply added together. However, the resultant DF will usually need additional correction factors to allow for · Deterioration of surface reflectance · Dust or dirt on the glazing · Types of glazing other than clear glass used · Obstructions caused by the window framing Average daylight factorIn the early stages of daylight design, when window shapes and positions have not been decided, it is often not convenient, or even possible to calculate the detailed distribution of the Daylight Factors. What is required is a single parameter that can give a rough indication of interior daylight availability for different window areas; average daylight factor is used for this purpose. It can be thought of as the arithmetic average of all the daylight factors for the reference plane. For side-lit interiors, the Average daylight factor can be calculated by the following,
where, · W – Total glazed area of windows · A – Total area of ceiling, floor, walls and windows · R – Average reflectance of ceiling, floor, walls and windows · q - The vertical angle in degrees subtended at the centre of the window by unobstructed sky As in the case of point daylight factors, average daylight factors are also subject to correction factors. Scale model studiesThis is a direct method for measuring DF since a model can be exposed to an overcast sky allowing internal and external illuminance measurements to be compared. Either a simulated artificial sky or real overcast skies may be used. · Illuminance cells are used to measure illuminance levels in the model and on the unobstructed external horizontal surface · Illuminance can either be read directly or calculated as daylight factors · Models are particularly good for complex forms · Scale modelling makes the distribution of daylight easily legible – it is a good ‘hands-on’ technique · Performance accuracy is dependant on the representational accuracy of the model · Models can represent a major investment of time – the type, scale and complexity of a model must reflect the task to which it is being put ‘Daylight’ and other softwareSoftware is available for providing a similar, if not larger, scope of daylight prediction techniques as from graphical ad modelling techniques · Simple software packages rely on estimation and assumptions and tend to be only applicable for orthogonal spaces · Complex software relies on extensive input of data on the space, often in the form of a 3D CAD model · Accuracy is difficult to ascertain, but similar problems occur with the choice of materials and their photometric properties. Some materials are difficult to describe · Choose a package appropriate for the information required and time available for its use · Visualization is a feature of more sophisticated packages – it may be more easily achieved using another medium L02 (2). Daylight – Clear Sky CalculationsClear sky is the term used for sky without clouds. The following assumptions are usually made, · The sun is a point source · Sunlight reaches the ground as parallel rays · The sky dome is clear, and the effects of atmospheric pollution is negligible · Sun brightness varies with altitude – lower brightness at dawn and dusk · The sky dome is assumed to be of constant brightness for all azimuth, but three times brighter at the horizon than at the zenith · Clear skies are brighter than overcast skies so are less important for lighting adequacy analysis in temperate climates Sunlight is primarily a control issue, and much of clear sky prediction is directed at its management or exclusion from interiors. No statutory requirements are placed on daylight, although this is a requirement in some other countries, although the BS daylight code does suggest that “interiors in which occupants have a reasonable expectation of direct sunlight should receive at least 25% of probable sunlight hours. At least 5% of probable sunlight hours should be received during the winter moths, between 23rd September and 21st March. Sunlight is taken to enter the interior when it reaches one or more window reference points”. Sun and Earth relationshipThe variation in the sun and earth’s relationship is made apparent to the observer by changes in the sky brightness and sun position with respect to, · The time of year · The time of day · Seasons · Sky clarity For most daylighting work, it is easier to assume the sun moves in relation to the earth. This allows the sun earth relationship to be seen in terms of · A celestial sphere · A predicted path for the sun through the sky · Variable declination of the sun relative to the earth · Variable speed of the sun’s declination – fastest at equinox and slowest at solstices · Solar time – sun appears to travel 10 of azimuth every four minutes Apparent sun pathMost clear sky analysis starts with the description of a reference point on a unobstructed horizontal surface under a hemispherical sky-dome. The horizontal plane is delineated by orientation in degrees and is symmetrical about a north-south axis. The sun rises in the eastern half, reaches to a peak in the centre line and descends symmetrically in the western half. In latitudes away from the equator the peak of this apparent sun path varies by season, the lowest being at the winter solstice, and the highest at the summer solstice. At latitude 520 (UK), the highest noon altitude is 61.50 at the summer solstice, and the lowest is 14.50 in the winter. Normally we use a stereographic projection of the sun path, both because it is the easier to construct, as well as it is available for many latitudes. · Sun path projected onto a circular azimuth chart · Altitude represented by concentric rings · Reference point at centre of chart · Sun path varies for different latitudes Sunlight availabilityIn the UK, sunlight availability graphs are readily available, and other can usually be estimated. An average hour probability protractor is used to predict probable hours of insolation for all orientations. This is useful if the requirements of the BS Daylight code are to be met. The protractor is simply placed over the plan of the building or window, and the sunlight availability figures are unobstructed views are summed and converted to the number of days of insolation. The protractor readily demonstrates the need to avoid windows of due north ±450 if sunlight adequacy is to be achieved from that direction alone. Shading - protractor analysisThe principal causes of shading are · Neighboring buildings · Vegetation · Overhangs and other features of the building envelope Prediction of shading is needed to determine whether desirable sunlight is admitted or undesirable sunlight is excluded. Shadow angle protractorThis hemisphere protractor can be used to calculate, using stereographic projection the location of shading relative to a point or opening in the vertical plane, and is used in conjunction with a sunpath diagram. It requires the use of drawing to allow the relative positions of obstructions to be determined and to be described as angles (Horizontal and Vertical shading angle) The shadow angle protractor can also be used with the sunlight availability protractor to determine insolation probability and determine exposure to winter or summer sunlight. L02 (3). Integration of daylighting techniquesObjectives for daylightingComfortable visual environment· Avoidance of glare · Appropriate levels of illumination · Appropriate type of illumination for visual tasks undertaken · Integration of task and spatial illuminance Maximize use of available daylight· Minimize use of electrical energy Statutory requirements· Rights to light · BS Daylight code · “interiors in which occupants have a reasonable expectation of direct sunlight should receive at least 25% of probable sunlight hours. At least 5% of probable sunlight hours should be received during the winter moths, between 23rd September and 21st March. Sunlight is taken to enter the interior when it reaches one or more window reference points”. · Min Average Daylight Factors · Electric lighting not normally used during the day – 5% · Electric lighting used during the day – 2% · Average working plane illuminance in poorly daylit areas from electric lighting to be not less than 300 lux · Energy saving – envelope performance (thermal transmission of materials) Integrating daylight into the design process· Working in the design team · Plan of works · Choice of techniques appropriate to the design stage Site planning· Climate analysis – sunpaths, sunlight probability, sky brightness · Exposure to sunlight of major planes · Penetration of sunlight to ground floor/ external areas · Perdiction tools · Small scale models –heliodon · Graphical tools shading masks, sunpath · Computer models – sun shadows Sketch scheme· Identification of performance criteria · Relationship of building type to daylighting – a general strategy · Optimization of the building footprint · Identification of areas for more detailed design · Identification of worse case scenarios · General strategy for materials finish and appearance · General strategy for introducing electric lighting · Prediction tools · Basic energy calculations – LT method · Average daylight factors for typical spaces · No-sky lines · Sunlighting requirements Major spaces· Performance specifications e.g. DFs, limited sun introduction · Evaluation of performance · Appearance –finishes: colours and textures, Brightness control · Surrounding spaces – is adaptation considered? · Integration of electric lighting design · Prediction tools · Point daylight factor · Scale models – qualitative and quantitative analysis · Computer simulation · Sunpath analysis · Insolation prediction Detailed Design· Refine performance criteria – often as a result of earlier analysis · Adequacy – usually a check by this stage! · Envelope design – size and form of openings · Solar control – light shelves, blinds etc… · Glare control – brightness of window wall in comparison to window · Switching strategy co-ordinated with daylight availability and control: localized switching, time switching, occupancy detection and photocell control · Prediction tools · Scale models – large, even full size · Point by point prediction · Computer analysis of component form – assessment of effect on performance · Visual representation - renderings Sequential Experience· The user’s experience · Control of brightness during movement from space to space · Adaptation · Visual focus · Prediction tools · Visualization – hand renderings, computers, scale models Aspects of a daylighting brief (Hopkins building, Nottingham campus)1. Maximize the use of daylight 2. Minimize glare 3. Maximize solar gains in winter 4. Minimize solar gains in summer 5. Maximize exposed thermally massive radiant surfaces 6. Seamless integration of daylight and electric lighting 7. Minimization of electrical energy consumption 8. Environmentally benign sourcing of materials 9. Optimized colour rendering 10. Durability/ low maintenance 11. Low embodied energy contents of materials 12. Integration of renewable energy sources e.g. Photovoltaic cells for electricity generation 13. Buildability / integration with other building components e.g. façade/ structure 14. Building integration – value adding for individual components L02 (5-7). LampsIncandescent (GLS)· Efficacy – 12 lumens/Watt · Colour Rendering Index – 1A · Colour Temperature - Warm (2,500K – 2,700K) · Lamp Life – 1-2,000 hours An incandescent lamp acts as a ‘grey body’, selectively emitting radiation, with most of it occurring in the visible region. The bulb contains a vacuum or gas filling. Although this stops oxidation of the tungsten filament, it will not stop evaporation. The darkening of bulbs is due to evaporated tungsten condensing on the relatively cool bulb surface. With an inert gas filling, the evaporation will be suppressed, and the heavier the molecular weight, the more successful it will be. For normal lamps an argon: nitrogen mixture of ratio 9/1 is used because of its low cost. Krypton or Xenon is only used in specialized applications such as cycle lamps where the small bulb size helps to offset the increased cost, and where performance is critical. Gas filling can conduct heat away from the filament, so low conductivity is important. Gas filled lamps normally incorporate fuses in the lead wires. A small break can cause an electrical discharge, which can draw very high currents. As filament fracture is the normal end of lamp life it would not be convenient for sub circuits fuses to fail. Tungsten-Halogen (TH (K), TH (M))· Efficacy – 18 lumens/Watt · Colour Rendering Index – 1A · Colour Temperature – Warm (3,000K-3,200K) · Lamp Life – 2-4,000 hours · Advantages · More compact · Longer life · More light · Whiter light (higher colour temp.) · Disadvantages · Cost more · Increased IR · Increased UV · Handling problem These were first used in situations where the extra performance is necessary. They employ a filament in the same way as GLS, and thus rely on thermal radiation. The addition of the halogen gas enables the filament to operate satisfactorily at higher temperatures and thus increase the visible radiation. The Halogen Cycle requires a minimum wall temperature of 2500C. This affects the bulb shape, which is much smaller and the material used, quartz. Quartz has almost zero thermal expansion, and therefore at the seal where the lead wires enter this is a design problem. The solution is to use a very thin molybdenum foil, but this is only effective to 3500C. This is thus a very narrow margin between the minimum and maximum wall temperatures. The filament centre will be at about 28000C, with the central wall temperature bout 7500C. Halogen CycleThe main purpose of the halogen gas is so that the filament temperature can be increased without excessive evaporation. The ‘halogen cycle’ uses the thermochemical reactions within the tungsten vapour. · Tungsten vapour moves away from filament · At 10000C, it combines with iodine vapour to form tungsten iodide. · This is still a vapour at 2500C (bulb wall temp.) · As it circulates within the lamp, it will separate into distinct elements when it is above 10000C · The tendency is therefore to constrain the tungsten evaporation to the space immediately surrounding the filament Fluorescent (MCF)· Halophosphate · Efficacy – 80 lumens/Watt (HF gear increases this by 10%) · Colour Rendering Index –2-3 · Colour Temperature – Any · Lamp Life – 7-15,000 hours · Tri-phosphor · Efficacy – 90 lumens/Watt · Colour Rendering Index –1A-1B · Colour Temperature – Any · Lamp Life – 7-15,000 hours Passing electricity through a gas or metallic vapour will cause electromagnetic radiation at specific wavelengths according to the chemical constitution and the gas pressure. The fluorescent tube has a low pressure of mercury vapour, and will emit a small amount of blue/green radiation, but the majority will be in the UV at 253.7nm and 185nm. The inside of the glass wall has a thin phosphor coating, selected to absorb the UV radiation and transmit it in the visible region. This process is approx. 50% efficient. Fluorescent tubes are ‘hot cathode’ lamps, since the cathodes are heated as part of the starting process. The cathodes are tungsten filaments with a layer of barium carbonate. When heated, this coating will provide additional electrons to help start the discharge. This emissive coating must not be over-heated, as lamp life will be reduced. The lamps use a soda lime glass, which is a poor transmitter of UV. The amount of mercury is small, typically 12mg. The latest lamps are using a mercury amalgam, which enables doses closer to 5mg. This enables the optimum mercury pressure to be sustained over a wider temperature range. This is useful for exterior lighting as well as compact recessed fittings. Induction lampsThe induction process is like a cordless telephone – the energy required to operate the lamp is transmitted via an antenna at radio frequencies. This e-m radiation will set up eddy currents in the lamp, which can be used to create light. There is no hard wire connection to the lamp, and this dramatically changes bulb design. · Induction can be used for incandescent and discharge lamps, but the only commercial induction lamps are based on the fluorescent discharge · It contains a low pressure mercury and is phosphor coated · Production of light is exactly the same process as the conventional fluorescent tube · The only change is the method of transfer of electrical energy within the lamp · There are no cathodes or filaments, and no wire have to penetrate the glass wall · They are capable of very long lives independent of switching frequency Compact Fluorescent (SL, PL, 2D, 2L)· Efficacy – 60 lumens/Watt · Colour Rendering Index – 1B · Colour Temperature – Warm, Intermediate · Lamp Life – 7-10,000 hours Discharge Lamps· As for fluorescent lamps the electrical energy is transformed into radiated energy by the discharge through a gas/metal vapour · The spectral distribution is dependant on the chemical and the pressure/temperature of the discharge · Most of the light is generated from the discharge, although a phosphor coating is used for the mercury types to improve colour and efficacy · Control gear is needed to initiate the discharge and to stabilize the current flowing through the lamp Low Pressure Sodium (SOX)· Efficacy – 100 – 200 lumens/Watt · Colour Rendering Index – 3 · Colour Temperature – Yellow (2,200K) · Lamp Life – 16,000 hours · Warm up – 10 minutes, hot re-strike – up to 3 minutes This is a low pressure lamp which has a similar geometry to the fluorescent tube. SOX lamps have a glass U-tube to reduce overall length. It contains about 400 mg sodium and neon gas. Starting is by applying a high voltage that initially creates a discharge in the neon gas. This is a characteristic red. The heat from this discharge gradually vaporizes the sodium, and the lamps colour changes to yellow as the light output increases. These lamps remain popular due to their high efficacy, although the yellow colour is a problem. The U-tube is contained in a outer glass envelope, which has a special I-R reflecting film as well as a high vacuum. Both help keep the inner tube at a stable temperature even when the exterior ambient conditions may change. High Pressure Sodium (SON)· Efficacy – 50 - 90 lumens/Watt ( better CRI, lower Efficacy) · Colour Rendering Index – 1 – 2 · Colour Temperature – Warm · Lamp Life – 24,000 hours, excellent lumen maintenance · Warm up – 10 minutes, hot re-strike – within 60 seconds · These are much more compact than SOX · Operating sodium at higher pressures and temperatures makes it highly reactive. PCA outer layer used · Contains 1-6 mg sodium and 20mg mercury · The gas filling is Xenon. Increasing the amount of gas allows the mercury to be reduced, but makes the lamp harder to start · The arc tube is contained in an outer bulb that has a diffusing layer to reduce glare. This is not a phosphor, since SON produces no significant UV component. · The higher the pressure, the broader the wavelength band, and the better CRI, lower efficacy. High Pressure Mercury (MBF)· Efficacy – 50 - 60 lumens/Watt ( excluded from part L) · Colour Rendering Index – 3 · Colour Temperature –Intermediate · Lamp Life – 16,000 - 24,000 hours, poor lumen maintenance · Third electrode means control gear is simpler and cheaper to make. Some countries has used MBF for road lighting where the yellow SOX lamp was considered inappropriate · Arc tube contains 100 mg mercury and argon gas. Envelope is quartz · No cathode pre-heating; third electrode with shorter gap to initiate discharge · Outer phosphor coated bulb. It provides additional red light using UV, to correct the blue/green bias of the mercury discharge · The outer glass envelope prevents UV radiation escaping Metal Halide (MBI)· Efficacy – 80 lumens/Watt · Colour Rendering Index – 1A –2 depends on halide mix · Colour Temperature – 3,000K – 6,000K · Lamp Life – 6,000 - 20,000 hours, poor lumen maintenance · Warm-up – 2-3 minutes, hot re-strike 10-20 minutes · The choice of colour, size and rating is greater for MBI than any other lamp type · They are a developed version of the two other high intensity discharge lamps, as they tend to have a better efficacy · By adding other metals to the mercury different spectrum can be emitted · Some MBI lamps use a third electrode for starting, but other, especially the smaller display lamps, require a high voltage ignition pulse · The halides act in a similar manner to the tungsten halogen cycle. As the temperature increases there is disassociation of the halide compound releasing the metal into the arc. The halides prevent the quartz wall getting attacked by the alkali metals. L02(10). Lighting Design II – Controls & GearLighting Controls(strategic issues)Control systems have many benefits for an installation · Energy saving · User satisfaction · Ability to modify scheme for new situations – flexibility · Provision of management information There are two main types of control; switching and dimming. There are three main types of activation methods; timeswitches, occupancy detectors and photocells. Note: the recommended maximum distance for a switch to be placed from a luminaire is 8m or 3 times the mounting height, from the furthest luminaire. Absence detection – In this system, manual switch on is combined with turn off by a presence detector. PhotocellsPhotocell control is used in mainly daylit spaces. If it measures the combined electric and daylight level, it should be set at two to three times the working level for when it switches on the electric contribution. On the whole, people do not like ‘active spaces’. Also, a delay should be built in to stop frequent changes when daylight levels change rapidly. There are two possible sensor positions · Look down – measures combined electric and daylight · Look out – measures only daylight falling outside the building There is a need to calibrate sensors. Centralized controlThis can either have a dedicated PC for lighting control, or be part of a larger building management system. The benefits are that · You can have a mixture of many different lighting policies · Dimming is possible · You can group luminaires together into logical units · Excellent management information is gathered Scene-set controlThis allows for a number of preset scenes to be programmed, which can then be accessed by the user. This has the advantage of transparency- and ease-of-use. Intelligent luminairesThese allow the luminaire to do occupancy and illuminance monitoring. They can also create a constant maintained illuminance throughout the maintenance cycle. Daylight IntegrationDaylight use in open-plan offices (Bordass, Heassman & Leaman; NLC ’94). Problems with daylight use in buildings are · Poor window design · Poor control design · Incorrect occupant perception of the lighting – people use horizontal surfaces to judge lighting levels (Hunt, BRE, early80s) · Changes in building use · Manual switch off seldom happened until last person left office · Insufficient number of photocells · No delay in switching · Light breaks - changes in amount of foliage · Glare · Ribbon windows caused VDT problems – secondary and reflective glare · Automatic blinds need manual override · Circulation and ancilary offices need separate photocells · HID lamps · need to take into account the run-up and re-strike times · Tend to be left on in daylight, since they are usually uplighters and have similar color temp. to daylight · Tinted glass – problems with color shift |
