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1172 Chapter 15: Lights and Cameras
the photometric center, measured along a line
leaving the center in the specified direction.
Example 1: Isotropic Distribution
Example of isotropic distribution
A sphere centered around the origin is a
representation of an isotropic distribution. All the
points in the diagram are equidistant f rom the
center and therefore lig ht is emitted equally in all
direct ions.
Example 2: Ellipsoida l Distribution
Example of ellip soidal distribution
In this example, the points in the negative Z
directionarethesamedistancefromtheorigin
as the corresponding points in the positive Z
direction,sothesameamountoflightshines
upward and downward. No point has a very large
X or Y component, either positive or negative, so
less light is cast laterally from the light source.
IES Standard File Format
You can create a photometric data file in the IES
format using the IES LM-63-1991 standard file
format for photometric data. (IES stands for
Illuminating Engineering Society.) However,
only the information relevant to 3ds Max is
described here. For a complete description of the
IES standard fi le format, see IES Standard File
Format for Electronic Transfer of Photometric
Data and Related Information, prepared by the IES
Computer Committee (h
ttp://www.iesna.org).
The luminous intensity distribution (LID)
of a lumina ire is measured at the nodes of a
photometric web for a fixed set of horizontal and
vertical angles. The poles of the web lie along the
vertical axis, with the nadir corresponding to a
vertical angle of zero degrees. T he horizontal
axis corresponds to a horizontal angle of zero
degrees and is oriented para llel to the length of
the luminaire. This type of photometric web is
generated by a Type C goniometer and is the
most popular in North America; other types of
goniometry are supported by the IES standard file
format but are not discussed here.
The photometric data is stored in an ASCII
file. Each line in the fi le must be less than 132
characters long and must b e terminated by a
carriage return/line-feed character sequence.
Longer lines can be continued by inserting a
carriage return/line-feed character sequence. Each
field in the file must begin on a new line and must
appear exactly in the following sequence: