Behavioural studies have shown that where only manual controls exist (ie switches at entrance to room), a decision is made by the first entrant to the room as to whether the lights need to be switched on .

Where a room is used continuously throughout the day, lights are invariably left on all day and the status of the lights is not reevaluated until the end of the day .

assessed by measurement or calculation .

Typically 30-60% of office building primary energy consumption .

Whilst the majority of electricity is generated using fossil fuels and transmitted via the grid the efficiency of electrical applications will be affected by the conversion efficiencies (typically 30-40%) and the efficiency of transmission .

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When cooling is necessary then the heat generated by the lighting will contribute to the cooling load. Reducing the energy consumed by lighting will automatically cause the cooling requirements to be similarly reduced .

The luminous efficacy of daylight depends on the condition of the sky with the efficacy of skylight generally higher than that of sunlight. The efficacy of direct light is largely dependent upon the altitude of the sun (typically 60 lm/W at 0^o to 110 lm/W at 90^o. The efficacy of diffuse light is largely determined only by the sky condition, varying from approximately 116 lm/W for an overcast sky to 130 lm/W in a clear sky .

particularly in deep plan offices: those areas close to windows are likely to have a different solution to those deeper parts where daylight hardly penetrates .

Light is a form of electromagnetic radiation and has an energy associated with it. Only that part of the radiation between 350 and 750nm is referred to as light. The luminous efficacy of a light source is a representation of the proportion of visible light in the electromagnetic radiation. Daylight has a relatively high efficacy compared to most forms of artificial light .

Daylight has no direct associated costs either financial or in energy terms though their may be secondary implications .

different tasks have different lighting requirements and hence different control strategies .

daylight contols may be of little use if spaces hardly used in middle of day .

Where a room is used intermittently in a known way (eg according to a timetable), then switching may occur throughout the day .

Windows do 3 things with the solar radiation striking them: Transmit it, absorb it and reflect it and what any particular material does with that radiation will depend on its spectrum. Any radiation which is absorbed by the glazing will release it as heat both inside and outside the space. That released within the space will effectively reduce the efficacy.

When two or more lamps are installed in one fixture the proportion of light emanating from the luminaire will generally be less than that for a similar single lamp luminaire. However, usually only one ballast will be required and thus overall energy consumption will be less for a multi-lamp luminaires .

A proportion of the lamps output will be reflected and absorbed by the luminaire, including the diffuser .

where high frequency electronic DIMMING ballasts are installed in luminaires then the luminaires can 'top-up' the daylight. Otherwise, lamp output cannot be adjusted and will be either on or off .

Luminance is a property of a light SOURCE. Note that any surface which receives light and reflects a proportion of light is a light source. Thus a window, for example, will have a luminance largely determined by the condition of the sky from which it receives its light .

It is probable that the illuminance between the entrance door and switch panel will also be a factor .

For most applications, artificial lighting tries to imitate daylight in the sense of trying to replicate the appearance of surfaces. This is referred to as colour rendering. All lamps are have a Colour Rendering Index .

Shading devices can affect the proportions of sky and sun light entering a space. Where the shading device is internal the efficacy will be reduced as the radiation enters the room but a proportion will be absorbed or reflected (outside the task area) by the shade and will remain as heat in the room .

T'x' refers to the diameter in 'x' eighths of an inch .

The actual efficacy of any light source will depend on the manufacturer of the lamp or the nature of the sky condition depending on the type of light source .

Whilst daylight levels within a space will vary considerably, particularly when comparing levels at a window to that at the back of a sidelit room, this variation will be gradual. Artificial lighting may enable a more uniform level to be achieved from 'front to back' but the variation over a small 'task area' will generally be greater than that for natural light .

Where access to daylight and orientation are similar then it may be possible to zone the building accordingly and identify different solutions for each zone rather than rooms .

The Colour Rendering Index (R_a) of a light source is a measure of how accurately coloured surfaces appear when illuminated by that source. (R_a) takes values between 0 and 100, where 100 represents daylight .

The translucent cover of the luminaire .

The fixture which holds the lamp and its associated equipment .

Efficiency of use of electrical equipment using alternating current depends on the phase difference between voltage and current this is measured in the term Power Factor .

A nanometer is 1 billionth of a metre (10^-9m) or one millionth of a millimetre .

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Crisp and Ure,The energy implications of flexible lighting controls, Proceedings of IEE Conference on Effective Use of Electricity in Buildings, 1980 .

D.R.G. Hunt, The Use of Artificial Lighting in relation to daylight levels and Occupancy, Building and Environment, Vol 14 p21-33,1979 .

Energy Efficiency in Offices Best Practice Energy Consumption Guide 19 BRECSU .

Reference for potential lighting savings in new buildings.

Reference for potential lighting savings in existing buildings.