About Weblights

    About Weblights

    A weblight (web) is a 3D representation of the light intensity distribution of a
    light source. Weblights can be used to represent anisotropic (non-uniform) light distributions
    derived from data provided by manufacturers of real-world lights. This gives a far
    more precise representation of the rendered light than either a spotlight or point
    light is capable of.

    This directional light distribution information is stored in a photometric data file
    in the IES format using the IES LM-63-1991 standard file format for photometric data.

    To describe the directional distribution of the light emitted by a source, the source
    is approximated by a point light placed at its photometric center. With this approximation,
    the distribution is characterized as a function of the outgoing direction only. The
    luminous intensity of the source for a predetermined set of horizontal and vertical
    angles is provided, and the system can compute the luminous intensity along an arbitrary
    direction by interpolation.

    NOTE:Web distribution is used only in rendered images. Weblights are approximated as point
    lights in the viewport.

    Goniometric Diagrams

    Photometric data is often depicted using a goniometric diagram.

    Goniometric diagram of a web distribution

    This type of diagram visually represents how the luminous intensity of a source varies
    with the vertical angle. However, the horizontal angle is fixed and, unless the distribution
    is axially symmetric, more than one goniometric diagram may be needed to describe
    the complete distribution.

    Photometric Webs

    The photometric web is a three dimensional representation of the light distribution.
    It extends the goniometric diagram to three dimensions, so that the dependencies of
    the luminous intensity on both the vertical and horizontal angles can be examined
    simultaneously. The center of the photometric web represents the center of the light

    The luminous intensity in any given direction is proportional to the distance between
    this web and the photometric center, measured along a line leaving the center in the
    specified direction.

    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 from the center and therefore light
    is emitted equally in all directions.

    Example of Ellipsoidal distribution

    In this example, the points in the negative Z direction are the same distance from
    the origin as the corresponding points in the positive Z direction, so the same amount
    of light shines 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.

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