One of the best ways of improving lighting and at the same time cutting power consumption and costs, is by better lighting control, for instance by dimming or switching off lights as daylight increases and, equally important, increasing the lights or switching them on as daylight fades. To achieve the substantial improvements and savings possible, the first step is to measure the light with a photo-electric cell.

Photocells are available in two basic Types: B and M. Type B photocells are cheaper and suitable in most cases; the response of Type M photocells is more like that of the human eye, and they should be used where more accuracy is needed, for instance in art galleries. Two 25mm photocells are shown above: one in a turned aluminium weatherproof mount for many locations, both inside and outside, which can be fitted with up to 100 metres or more of cable; the other photocell is in a black thermoplastic mount in the lid of the small Amplifier. Further details are given in Selenium Photo-Electric Cells.

However the output from the photocells is fairly low, and non-linear, so amplification is normally needed. Amplified Photocells have an output in volts that is proportional to the light falling on the cell. For instance, when the cell is exposed to 50 lux, the output could be 5.0 volts; this might be used to control the light in a warehouse, or in a car park for example. As another example, the photocell can be mounted on an outside wall some way from the Amplifier, which is calibrated to have an output of 10 volts, when the cell is exposed to 20,000 lux; this combination has been used for controlling the light in an atrium. These Amplified Photocells are often used to interface with a BMS - Building Management System - which in turn controls the lighting. They have also been used to control dimming devices; finally they are used with a data logger for light monitoring by such people as the National Trust, so that they can assess the risk of too much light damaging fabrics or pictures; other light monitoring and data logging examples include architectural model work, and the study of the effects of light on biological activity

Megatron produces two Amplifiers:

• AMP/DC - shown with the photocell in a black thermoplastic mount on the lid; it is more compact than any previous Amplifier from Megatron (see dimensions below), and requires DC power at between 4 and 24 volts, or from a Mains Adaptor for 240v AC (which we can supply); both the output and the power input are from 1 metre flying leads.
  
• AMP/240 - rather larger than the AMP/DC, as can be seen above (see also dimensions below); this Amplifier can be powered directly from 240v AC mains; it is fitted with a terminal block for mains input, photocell and output connections.

On some occasions, rather than a voltage, a switch is required; for instance this could be used to switch on more lighting, or, conversely, to operate a blind or shutter to reduce the light in a gallery with valuable paintings. Megatron offers two Light Sensitive Switches which use the lighting level to operate a change-over relay; they are very versatile in their applications. To avoid the light flickering on and off around the critical point, there is a small difference between the switch on and switch off points, this small difference can be adjusted. The light setting itself can be adjusted between 50 lux and 50,000 lux, or more widely if required. There is also an option of incorporating a time delay of a few seconds, so that the switch does not operate when there is a quick passing cloud, or when a car's headlamps catch it. The standard relay has a 15 amp (non- inductive) rating, so it can control a number of lights or operate a contactor to switch a much larger lighting load.

LSS1 - a light sensitive switch with the same wide operating range as the AMP/DC, more than 50,000 lux or less than 50 lux.

LSS5 - a 5-channel Light Sensitive Switch similar to LSS1 in operation but it has five lighting levels controlled by one photocell. It can be used for instance in a building where the first lot of lights may be wanted when the daylight is just below average, and the last set, perhaps near a window or under a skylight, only when the daylight has really faded, and darkness approaches. This product has been developed from our Light Indicating Displays, used since early 1991 for showing the onset of ''bad light'' at all first class cricket and Test Match grounds.

DIMENSIONS