Photometric Conventions

Having the correct Photometric Settings is critical to obtaining accurate results from Photopia. When you send a product for testing, the laboratory often determines the correct photometric angles, but when you are running Photopia it is something you must always be aware of. The angle sets are often broken down to vertical and horizontal angles, described below.

Vertical Angles

Vertical photometric angles go from directly below the fixture at 0 degrees to directly above at 180 degrees. The vertical angles that you choose are governed by the vertical distribution of your product. See the table below for information on the correct choice of vertical angles.

	Direct	Indirect	Direct/Indirect
Distribution	Light Directed Downward	Light Directed Upward	Light Directed Upward and Downward
Orientation	Beam in -z direction	Beam in +z direction	Beam in -z direction
Vertical Angles	0-90 degrees	90-180 degrees	0-180 degrees

Horizontal Angles

The horizontal photometric angles that you will choose depend on the horizontal symmetric of your product. Fixtures can have four types of symmetry, as outlined in the table below. In Photopia it is important to choose the correct angle set based on the fixture symmetry because otherwise the output can become incorrect. Photopia always has data for the full 0-360 degrees, but averages down to the angle set that you choose. In the extreme case all 360 degrees are averaged together when you choose a horizontal angle of "0" only.

	Axially Symmetric	Quadrilaterally Symmetric	Bilaterally Symmetric	Completely Asymmetric
Examples	Revolved Downlights	Louvered Fluorescent	"Asymmetric" Fluorescent Wallwash, Roadway	Directional Tunnel Lighting
Lamp Orientation		Along y axis
Outside U.S.		Along x axis
Beam Direction			Along +y axis
Horizontal Angles	0 degrees only	0-90 degrees	0-180 degrees	0-360 degrees