Autodesk LIGHTSCAPE, Manual

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Photometry
7
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Neither radiosity nor ray tracing offers a complete
solution for simulating all global illumination
effects. Radiosity excels at rendering diffuse-to-
diffuse interreflections and ray tracing excels at
rendering specular reflections.
By merging both techniques, Lightscape offers the
best of both. In Lightscape, it is possible to combine
a ray-tracing postprocess with a specific view of a
radiosity solution to add specular reflections and
transparency effects. In this situation, the radiosity
solution replaces the inaccurate ambient constant
used in many programs with accurate indirect illu-
mination values. This leads to a much more realistic
image. In addition, because the direct lighting can
be calculated in the radiosity solution, the ray tracer
does not have to cast any shadow rays, only reflected
or transmitted rays. This greatly reduces the time
required to ray trace an image. By integrating both
techniques, Lightscape offers a full range of visual-
ization possibilities, from fast, interactive lighting
studies to combination radiosity/ray traced images
of exceptional quality and realism.
Photometry
Lightscape is founded on a physically based simula-
tion of the propagation of light through an
environment. The results are not only highly real-
istic renderings, but also accurate measurements of
the distribution of light within the scene. This
section briefly describes the quantities used to char-
acterize these measurements.
You specify the brightness of a luminaire in Light-
scape using the physically based quantities. You can
obtain these values directly from the manufacturers
of various lamps and luminaires. A table of some
common lamp types is provided in Appendix G,
“Common Lamp Values.”
There are several theories that describe the nature of
light. For this discussion,
light is radiant energy
capable of producing a visual sensation in a human
observer.
When designing a lighting system, you want to eval-
uate its performance in terms of the human visual
response. Thus
photometry was developed to
measure light, taking into account the psychophys-
ical aspects of the human eye/brain system.
The lighting simulation system uses four photo-
metric quantities:
• Luminous flux
• Illuminance
•
Luminance
• Luminous intensity.
Luminous flux is the quantity of light energy per unit
time arriving, leaving, or going through a surface.
The unit of luminous flux is the
lumen (lm), used in
both the International System (SI) of units and in the
American System (AS) of units. If you think of light
as particles (photons) moving through space, then
the luminous flux of a light beam arriving at a
surface is proportional to the number of particles
hitting the surface during a time interval of 1 second.
Illuminance is the luminous flux incident on a
surface of unit area. This quantity is useful for
describing the level of illumination incident on a
surface without making the measurement depen-
dent on the size of the surface itself. The SI unit of
illuminance is the
lux (lx), equal to 1 lumen per
Disadvantages 3D mesh requires more memory
than the original surfaces.
Surface-sampling algorithm is
more susceptible to imaging arti-
facts than ray tracing.
Does not account for specular re-
flections or transparency effects.