Finding the ‘Sweet SPOT’ in Photosensor Placement

By Jim Crockett, Editor-in-Chief March 1, 2006

With all the discussion of daylighting in the pages of this magazine, a critical component for a successful design scheme that can’t be overlooked is proper photosensor placement. For those not as up-to-date on this subject as they’d like to be, there’s good news from out West. The California Energy Commission’s PIER—Public Interest Energy Research—program has developed a new tool that’s “spot-on.” The Sensor Placement and Orientation Tool, according to the organization, is usable by anyone capable of using Microsoft Excel. In fact, the user interface is an Excel worksheet. Behind it is a sophisticated daylighting software program known as Radiance, created for PIER by Architectural Energy Corp., that is invisible to the user. Based on user input, the tool calculates nighttime illumination levels and the range of daylight contribution throughout the year.

SPOT accounts for many variables such as room geometry, surface reflectances, solar orientation, electric lighting layout and window design. For example, the tool allows the user to define room geometry and electric lighting layout for the space to be analyzed. The program also lets the user define reflectances of all the surfaces in the room or building, and of the ground outside as well, and specify any overhangs and light shelves to be considered for shading purposes. For electric lighting, the user can also define the type of luminaire to be used.

The software also accounts for building orientation and usage, allowing schedules to be created for occupancy and establishing desired control strategies for shading devices.

The design portion of SPOT uses the geometry and site information provided and reports three main sets of information back to the user: electric lighting performance under nighttime conditions, annual daylighting performance under a sampling of conditions and photosensor placement recommendations for each luminaire zone.

SPOT then allows the user to apply what-if analyses to various photosensor placements in the luminaire zones, adjust the photosensor system settings and run annual performance calculations. A Photosensor Analyzer screen allows users to mix and match the various photosensor scenarios defined previously and analyze how they will perform under a larger set of representative days and sky conditions.

Keep in mind SPOT was developed with classrooms in mind, according to PIER, but it also may be applied to other spaces such as offices. The software handles top and side daylight sources and can model any electric lighting source from Illuminating Engineering Society files. More importantly, say its creators, SPOT will help designers effectively comply with the daylighting requirements in tougher state energy codes, specifically, California’s Title 24 where separate controls are required for daylit areas.

To install SPOT, users’ computers must meet the following specifications: Windows 2000, NT, or XP; Excel 97 (or higher); a 400-MHz processor and 128 MB of available disk space.

For more information visit where reports documenting this project are available. The software itself may be downloaded from the web at .

SPOT’s perks

Easy-to-use Excel style program

Automatically calculates photosensor locations based on building geometry, orientation and scheduled usage

Helps designers meet today’s tougher energy codes

Free of charge