III. LIGHT DISTRIBUTION PATTERNS

Outdoor luminaires produce lighting patterns that can be identified first by their reach in front of a single fixture location and second by their reach on each side of that location. "Distribution types" describe the reach of the luminaire's light pattern forward of each fixture, while "distribution ranges" define the reach to either side.

Figure 5: Grid and light patterns to determine Distribution Type

Distribution Types
(Refer online for example illustrations of each Distribution Type).
The term "distribution type" defines how far forward of the luminaire (i.e., on the street side) the effective output reaches. The specific classification of distribution types is based on locating the luminaire's effective major output pattern on a grid representing distances in units of Mounting Height (MH) from the luminaire. This pattern is defined by tracing an area representing light distribution at 50% of maximum candela. By measuring where the bulk of this pattern falls on the grid, a luminaire can be classified as follows and as shown in Figure 5. Refer to illustrations of definitions online. (Note that in some cases, minor deviations in a beam pattern may cross the boundary from one type into another. While this has a nominal effect on applied performance, it should not be considered for classification purposes.1):

• Type II defines shallow reaches, when the 50% maximum candela trace lies within1.75 MH on the street side of the reference line.4
• Type III is a mid-range, when the 50% maximum candela trace lies within 2.75 MH on the street side of the reference line.4
• Type IV identifies luminaires with a definite forward-throw distribution, when the 50% maximum candela trace lies beyond 2.75 MH on the street side of the reference line.4
• Distribution is classified as Type V Square for horizontal lamp luminaires when
  the 50% maximum candela trace is symmetric in four quadrants. This distribution is characterized by four candela peaks, diagonal to the reference line.
• Asymmetric5,6 Distribution (similar to Type III): This distribution is for vertical lamp luminaires when the 50% maximum candela trace lies beyond 1.0 MH on thestreet side of the reference line, and inside 1.0 MH on the house side of thereference line. Narrow range distribution is identified when the point of maximum candela falls inside of 2.25 MH; wide range is identified when the point of maximum candela falls beyond 2.25 MH.
• Symmetric5,6 Square Distribution (similar to Type V Square): Distribution is classified as symmetric square for vertical lamp luminaires when the 50% maximum candela trace is symmetric in four quadrants on both street and house side of the reference line. Narrow range distribution is identified when the candela peaks fall inside of 2.25 MH along the reference line; wide range is identified when the candela peaks fall beyond 2.25 MH.

Distribution Range
Distribution range defines how far the distribution pattern reaches laterally, perpendicular to the axis used to identify general type. The ranges used are defined as follows and indicated graphically in Figure 5.

Long Range: A distribution is identified as long range when the point of maximum candela lies from 3.75 to 6.0 MH from the luminaire's centerline, along the reference line.

Medium Range: A distribution is identified as medium range when the point of maximum candela lies from 2.25 to 3.75 MH from the luminaire's centerline, along the reference line.

Short Range: A distribution is identified as short range when the point of maximum candela lies from 1.0 to 2.25 MH from the luminaire's centerline, along the reference line.

Very Short Range²: A distribution is identified as very short range when the point of maximum candela lies from 0 to 1.0 MH along the reference line.

Distribution types only generally describe a distribution pattern. To establish the suitability of a luminaire for a specific application, an evaluation must be completed using actual photometric data for the specific fixture and lamp combination being considered.