Incorporate manual overrides on both occupancy and daylight sensors into your lighting system design only when necessary. Manual overrides must be used with caution, and should only be temporary. Otherwise, the sensors may be overridden and forgotten, negating potential financial and performance benefits.
Before the sensor system is designed, identify occupant lighting needs that can be met with daylight. Typical light levels in offices are 30–50 footcandles for most tasks. The Illuminating Engineering Society of North America (IES) prescribes footcandle levels for various project types, occupant types, and tasks. (See Resources.)
Daylight and occupancy controls may be standalone or integrated into a central, intelligent lighting control system. It might take additional wiring and commissioning, but this type of integrated system is usually the most efficient.
A dimming system reads the amount of daylight in the space and automatically adjusts light levels to meet a certain threshold for light that works for the activities in the space. Getting this type of system to perform properly requires experienced contractors, as well as guidance from the manufacturer on fixture and sensor placement. Daylight dimming response should be slow and imperceptible so it isn’t distracting to occupants.
Stepping is a common approach to controlling lighting systems with daylight sensors. This systems works by turning off a group of fixtures (or ballasts within a group of fixtures) when the daylight in the space reaches a certain level. This system is usually cheaper than dimming, but the sudden switching of lights on and off can be disruptive for occupants.
ASHRAE-90.1 requires the installation of an automatic shut-off for spaces larger than 5,000 ft2. Both timers and occupancy sensors fulfill this requirement.
Consider various strategies for harvesting available daylight—like adding light-shelves, reflective surfaces and light-colored finishes. Keep partitions low, especially those parallel to the windows. Alternatively, provide clear or frosted glazing in higher partitions to provide acoustic benefit while still allowing for daylight penetration.
Look to create a progressive lighting design that controls the fixtures closest to the windows separately from another set of fixtures that are deeper into the space.
