Conventions and Usage
Footcandle calculations are shown with the luminaire at various mounting heights. Contour lines are drawn through illuminance values. Each contour, from the center out, represents approximately 50% of the value of the previous contour.
The plot is placed over a grid, indicating mounting height divisions, to demonstrate the luminaire's applied performance.
The isofootcandle plots graphically represent the luminaire's lighting pattern, in illuminance, striking a horizontal surface. These plots are scalable as they are represented in mounting height increments.
An approximation of pole spacings required to attain a desired uniformity can easily be determined from the information provided.
These plots also provide a productive tool for the comparison of various luminaires. The easily read visual reference indicates beam patterns graphically, where other information (such as candela tabulations and isocandela curves) may be less clear.
EXAMPLE MAXIMUM CANDELA POINT and lateral angle line is included here to illustrate how illumination levels at a point correlate the Isofootcandle Plot information shown.
The location of the point is 2.12 Mounting Heights lateral to the fixture reference line and .73 Mounting Heights from the Street Side.
The point represents a calculated 2.95fc, which corresponds to its position between the 2fc and 4.1fc Isofootcandle contour lines (at a 14' Mounting Height).
Estimated Spacing and Uniformity
ESTIMATING MAXIMUM SPACING
During the Schematic Design phase of a project, rough luminaire layouts can be created using isofootcandle plots.
This example assumes a desired Minimum Initial Illuminance of 2.0fc, using luminaires mounted on 14' poles.
To estimate a fixture layout - start from the perimeter, where the 2.0fc isofootcandle trace crosses the Reference Line, to establish the maximum single fixture distance to the site perimeter (1.6 MH, figure 11.1).
In order to attain the minimum illuminance (2.0fc) between fixtures, the 1.0fc traces of two fixtures must intersect at the site perimeter and interior. Therefore, lateral spacing is determined by where the 1.0fc trace intersects the Reference Line (2.2 MH), and maximum forward spacing is identified where the lateral spacing line intersects the 1.0fc trace on the street side of the luminaire (1.8 MH). These two dimensions indicate the mid-points between luminaires, in Mounting Heights.
Multiplying these Mounting Height dimensions by the pole height (14') defines the maximum luminaire spacings in both directions. In this example, 60' (4.4 MH x 14') x 50.4' (3.6 NIH x 14).
By overlaying Isofootcandle Plots, a rough idea of illuminances can be accomplished by adding the values of each contour where they intersect (figure 11.2).
More accurate calculations (computer generated evaluations) will generally return levels higher than those achieved using this method, as smaller contributions from every adjacent luminaire would be included.
Through observation of the overlapping of the Isofootcandle Plots, approximate uniformity can also be estimated (figure 11.2).