This option will set the automatic control function to PHOTOCELL CONTROL and will be based on the GAIN calculation method.
To use this lighting function you will need to have calculated a DAYLIGHT FACTOR within the zone for which the lighting function will be referencing (the 'control zone').
To use the function, under the Control Logic area on the dialog, set the Control Type to "Photocell Control" on the drop-down list. Then choose "Gain Method" from the Calculation Method drop-down options. Under the Parameters area, choose whether the applied zone will act as the control zone. The applied zone will be the zone to which the internal condition is applied. Otherwise you will need to specify which zone in the model will act as the control zone. If you select another zone from the drop-down list to be the control zone, its illuminance levels will determine when and how the lights will be operated within the applied zone.
The calculated lighting gains will be a function of the minimum and target room illuminance levels. You will need to input the illuminance level at which the lighting gain will be a maximum, and specify at what illuminance level the lighting gain will be at its minimum value. You can do this by entering the maximum and minimum gains in W/m^2, or you can enter the lighting efficiency in units of W/m^2/100lux and then input a minimum gain as a percentage, this will be the percentage of the maximum lighting gain that will be used when the natural room illuminance exceeds the target room illuminance. The max gain will then be derived from the efficiency value.
The lights will operate as follows, in the first occupied hour the function will look to see what the illuminance level is, and if it is at or below the target room illuminance. If the illuminance level is at (or above) the target illuminance then the lighting gain will be set to the minimum value. So the lighting gain during the first occupied hour will be the minimum gain, as determined by the Lighting Control Function dialog box. In the second occupied hour, the function will look to see if the illuminance level has changed. If the illuminance level is still at or above the target illuminance then the lighting gain will not be changed from the value that was used in the first hour, but the function will always look to see if the lighting gain needs to be updated at each hour. If the illuminance level has dropped below the target room illuminance, meaning that the room will have less access to daylight than it did during the previous hour, the lighting gain will be increased. The lighting gain will be increased linearly as illuminance levels decrease linearly, until the minimum illuminance level is eventually reached, at which point the lighting gain will be set to the maximum value. Similarly, if the illuminance level increases, then the lighting gain will decrease. The lighting gain will decrease linearly as the illuminance level increases linearly.
The defining characteristic of the photocell control function is this, during the occupied period, if the illuminance increases at any hour, the lighting gain will be reduced. So the lighting gain will decrease linearly as illuminance level increases linearly, until the target room illuminance level is reached, at which point the lighting gain will be at its lowest value. Note that this is not how the manual control function operates.
Example - plclg,0,10,500,18.75,1.25,2.5
The function string begins with a '0', which indicates that the control zone is the applied zone.
This is followed by the number '10', which represents the minimum illuminance level.
The next number is '500', giving the target room illuminance.
The '18.75' value is the maximum lighting gain, which applies when the illuminance level is at its minimum.
The '1.25' is the minimum lighting gain, and this will be the heat output at the target room illuminance level.
Lastly, '2.5' is the unoccupied gain. During hours with no sensible occupancy gain, the lighting gain will be set to this value.
Let's look at a worked example, based on this function string. Assume the zone area is smaller than the area cut-off value. Suppose the illuminance level in the first hour is 500lx, which is the target room illuminance. This means that the lighting gain will be at its minimum value. The minimum value will be determined by the minimum percentage gain value. So the lighting gain at hour 1 will be 6.67% of the maximum lighting gain. At hour 2, the illuminance then falls to 300lx, so the lighting gain will be increased linearly. At hour 3, the illuminance level increases to 400lx, and the lighting gain will not change from the value is was at during hour 2. At hour 4 the illuminance level has fallen to 200lx and so the lighting gain will increase to again to compensate. When the illuminance level has fallen to the minimum lux level, as determined by the efficacy value, the lighting gain will be increased to its maximum value. The lighting gain will never be reduced during the occupied period, it will only ever be increased at the next hour or remain at the same value. The PIR factor of 0.95 will be used as a multiplication factor to reduce the calculated gain to model automatic presence detection behaviour. Finally, the display lighting gain will be added.