II. PHOTOMETRY INFORMATION

The design of site lighting requires an understanding of the unique information used to represent elements of optical performance. Photometry, or the measurement of light intensity and relative illuminating power, is the foundation on which any evaluation of luminaire performance is based. Use of independent testing labs to conduct the measurements and compile the information ensures that the photometry information is accurate and reliable.

Basic Language and Presentation
In order to properly select luminaires appropriate to the specific locations and requirements of a building site, an understanding of some of the basic language and ways that information is presented is required.

Figure 2: Candela Tabulation Data

Candela Tabulation
One of the fundamental units of measurement is the candela, which in 1979 became the international standard to define luminous intensity. Figure 2 shows a typical candela tabulation data sheet prepared by an independent lab with a luminaire orientation diagram for reference.

The Candela Tabulation Data Sheet presents the raw data used for all illuminance calculations and is tabulated with the vertical angles in rows and lateral angles in columns. As the diagram indicates, lateral values from 0° to 90° are in front of the luminaire and referenced as "Street Side." Lateral values from 90° to 180° are behind the luminaire and referenced as "House Side."

Vertical values from 0° to 90° are below the fixture, while values 90° to 180° are at the fixture level and above. As we will see, candela data is also used to define a luminaire's light distribution type and cutoff characteristics.

Footcandle Calculations
The data provided in Candela Tabulation Data Sheets is used to calculate footcandle levels within a proposed lighting design. Generally, this is accomplished by using computers to make calculations, which are, in turn, dependent upon the accuracy of the data. Figure 3 illustrates the relationship of the calculated illumination at a single point to the information provided in the candela tabulation. (See Figure 7 later in this article for the correlating location on an isofootcandle plot.)

Figure 3: Footcandle representation based on Candela Tabulation Data