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Desktop Radiance 1.02 Overview Desktop Radiance Overview
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By Richard G. Mistrick, The Pennsylvania State University
© 2000 Pacific Gas & Electric Introduction
Desktop Radiance is an advanced lighting analysis and visualization tool that can be
used to model simple or complex daylight and electric lighting systems. Radiance
was initially developed as a research tool for a Unix environment, where it utilized a
rather complicated text-based input format.
Radiance is one of the most powerful daylight and electrical lighting analysis tools
available since it can handle virtually any space geometry, as well as non-diffuse
reflectances. The Desktop Radiance version provides the opportunity for more
lighting professionals to easily access this powerful software tool through a graphical
user interface. This document will describe the basic operation of Desktop Radiance
1.0 and provide some simple instructions and tips on how to construct and analyze a
room model using this software. As you read through this document, you should
refer to the flowchart provided in Appendix B of this document, which provides a
general outline of the operations, steps, and possible analysis paths involved in
applying this software to the design of daylight and electric lighting systems.
To learn more about the software, the reader should consult the Desktop Radiance
User Manual, or its help utility, since it is not possible to discuss all of the relevant
features and operations in this document.
The Desktop Radiance version is a more user-friendly derivative of this software that
runs under the Windows operating system from within AutoCAD 14 using pull-down
menus. (It is not yet compatible with AutoCAD 2000.) This user-friendly interface
makes most of the complex Radiance commands transparent to the user. Many, but
not all, of the key operating features in the standard Radiance version are currently
available through this user interface.
For individuals who are familiar with the standard Radiance program, the advanced
features that are not currently part of the Desktop Radiance system can be accessed
through the MS-DOS batch files by modifying the original text-based input.
Desktop Radiance operates by creating the standard text-based input files used in the
UNIX version, then it executes the standard Radiance programs through an MS-DOS
batch file. These aspects of the software are transparent to the user since they are
managed through the pull-down menus in AutoCAD and the Desktop Radiance
simulation manager. Desktop Radiance also contains a number of operational
enhancements, such as the RVIEW program, which has an improved user-interface.
Since Desktop Radiance is still under development, additional features from the Unix
version of Radiance are likely to be implemented in future releases of the Windows
version.
Calculations Permitted
With Desktop Radiance, you can compute horizontal illuminance across an arbitrarily
oriented grid of points, or you can generate a rendered image of a space that can be
queried for the illuminance or luminance of any surface in a rendered image of a
room. Basic operation of the system
Model Construction
Constructing Polygons in AutoCAD
To construct a Radiance model, it is first necessary to create a three-dimensional
model of the architectural space you wish to analyze within AutoCAD. Surfaces can
be constructed using polygons, 3-D faces, three-dimensional objects, lines with
thickness, and extruded or revolved shapes. These include virtually all of the
commands available under the AutoCAD DRAW menu. For example, a rectangular
room can be created using the BOX command. The box must then be
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exploded
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permit different materials to be assigned to the ceiling, walls and floor. To construct a
window on one of these room surfaces, it is necessary to erase and reconstruct the
wall and transparent building elements where the window is located.
A window can generally be constructed of a single polygon, unless mullions are
desired. The wall that surrounds the window then must be constructed of multiple
polygons or trapezoids. It is important when constructing any Desktop Radiance
model to be certain that the corners and edges of each polygon are aligned with the
corners and edges of adjacent surfaces. The snap to feature in AutoCAD should be
applied to ensure that a model is constructed in this manner.
A vertical surface such as a wall or partition can be created by first specifying a
THICKNESS, then applying the LINE command. A thickness is generally an
extension of a line in the z direction to form a polygon. The THICKNESS command
(which can be also entered as TH) is entered on the command line. You are then
prompted to enter the thickness to assign to each line.
You apply any of the commands available under the DRAW menu that create a three-
dimensional object (polygon) to construct your model within AutoCAD. For
example, the 3DFACE command is a common command for creating a single polygon
in space.
Level of Detail
In creating a room, is important to first assess the level of detail that is appropriate to
include in your model. More detail will increase the realism of any image that you
create, but may have little impact on lighting system illuminance calculations, and
will likely slow down the analysis. For proper photometric analysis of your model,
it is important to address all details that impact the amount of light entering a space or
the amount of light reflected within a space. For example, if the exterior wall of a
building is relatively thick, you should enter the thickness of this wall into your model
to accurately describe the view that all points within a space will have of the exterior.
For daylighting situations, it is also important to adequately model the exterior of the
building. However, surfaces that do not impact the amount of light in a space, or
which are not visible in any views that you plan to create, can generally be eliminated
from your model.
Modeling the exterior
It is important to appropriately model the exterior when performing daylighting
calculations. This involves modeling the ground; any other exterior light reflecting or
obstructing objects, such as neighboring buildings or trees; and shading devices such
as overhangs, light shelves, or blinds. At the present time, blinds can only be considered by modifying the radiance text-based files, which is discussed in more
detail in a later section. Exterior elements that impact the amount of daylight striking
a window must be built into your model, to be considered in a Desktop Radiance
analysis.
The ground plane
In a daylight analysis, Desktop Radiance will automatically input a ground reflectance
of 20 percent. This ground material is assumed to be non-Lambertian, such that the
ground luminance looking back toward the sun will be higher than it is looking away
from the sun. This infinite ground plane is also located slightly below the z=0 plane,
so that it does not appear as a luminous floor within your modeled space. If the floor
of your room goes below z=0, it may be necessary to raise the entire room model
above the ground plane. Likewise, if you do not cover the entire floor of your model,
this surface will emit light into your space through this opening.
Exterior Shading and Controls
One significant limitation of the approach used to analyze the ground plane in
Desktop Radiance is that shadows caused by the building or other exterior elements
are not projected onto this surface. If you wish to consider shadows on the ground
plane, you must place a rectangle on the ground where the shadows will be located.
You must also assign a surface material to this polygon so that it is considered as an
obstructing and reflecting object. When doing this, do not to make this ground area
larger than necessary, since it can impact the accuracy of an analysis.
Adjacent Buildings
Adjacent buildings can be modeled as simple three-dimensional objects (boxes or a
single vertical polygon) with appropriate reflectance characteristics. If it is important
to model specular reflections from surfaces on adjacent buildings, such as reflections
from reflective glass, these materials can also be considered, however, depending on
their size and their distance f