The Auto-Generate Photocell Position button will generate optimal locations automatically for the selected photocell type. This button will initiate correlation calculations for each photosensor selected with the check boxes. Note, a Radiance engine limitation limits this optimization calculation to a grid with 315 points or less. This tool is not necessary, but if it is desired then the space size in combination with grid spacing must be entered on the Geometry Input and Advanced Options pages so that a workplane/ceiling grid of no more than 315 points will be created.

The auto-generated default photosensor locations represent the points on the ceiling that provide the best correlation with the average workplane illuminance for each zone. Once calculated, the correlation plots are shown in the lower right corner of the screen, see example plots for zones 1, 2 and 3 from our demo model to the right. These plots represent a grid of ceiling points and each value is the correlation between the illuminance at that point and the workplane illuminance under the annual daylight conditions and different electric lighting conditions. The correlation values in this plot are color-coded so that a lower correlation is represented by darker colors and a better correlation is represented by lighter colors.

Correlation values for other zones can be displayed by choosing the zone of interest and sensor type in the pull-down menus and pressing the View Correlation button. Correlation plots will become unavailable for viewing if values in the table for the corresponding sensor change. This is because the correlation is only accurate for a given photocell type, mounting and rotation. Note that the default rotation is with the prominent photocell housing feature facing north. If the known rotation will be otherwise, enter the rotation vector before pressing the Auto-Generate Photocell Position button and the correlation will be performed for the entered rotation.

Currently, the optimal photocell position feature works for photocells mounted on the ceiling plane and aimed downward. Photocells located at, or just below, the plane of the fixtures can also perform well and can still be analyzed in SPOT by manually placing the photosensor on the underside of a fixture using the location coordinates. Photocells aimed to look at typical walls can also perfrom well.

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