Photometry

When you use photometric lights, 3ds Max provides physically based simulation of the propagation of light through an environment. The results are not only highly realistic renderings, but also accurate measurements of the distribution of light within the scene. The measurement of light is known as photometry. This topic introduces the quantities used for defining and measuring light.

There are several theories that describe the nature of light. For this discussion, we define light as radiant energy capable of producing a visual sensation in a human observer. When we design a lighting system, we’re interested in evaluating its effect on the human visual response system. Thus photometry 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.