2008

was developed to measure light, taking into account the psychophysical aspects
of the human eye/brain system. Four photometric quantities are used in the
lighting simulation system:
■ 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), which
is used in both the International System (SI) of Units and in the American
System (AS) of Units. If we 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 dependent on the size of the surface itself.
The SI unit of illuminance is the lux (lx), which is equal to 1 lumen per square
meter. The corresponding AS unit is the footcandle (fc), equivalent to 1 lumen
per square foot.
Part of the light incident on a surface is reflected back into the environment.
The light reflected off a surface in a particular direction is called luminance,
the quantity that is converted to display colors to generate a realistic rendering
of the scene. Luminance is measured in candelas per square meter or candelas
per square inch. The candela was originally defined as the luminous intensity
emitted by a single wax candle.
Finally, luminous intensity is the light energy per unit time emitted by a point
source in a particular direction. The unit of measure of luminous intensity is
the candela. Luminous intensity is used to describe the directional distribution
of a light source, that is, to specify how the luminous intensity of a light source
varies as a function of the outgoing direction.
Because 3ds Max works with these physically based photometric values, it can
accurately simulate real-world lighting and materials.
Glossary | 7893