mia_photometric

The Problem

The difficulty in using base mental ray shaders to render a physically correct scenes with global illumination (using photons), is in balancing the photon energy and the direct light correctly. This is because the intensity is generally specified as an intensity for some given direction (for example the peak intensity on a spotlight), but the energy is the sum of of all such intensities in all directions.

Given a spotlight, for the same peak intensity, you would need different photon energies depending on the width of the spotlight cone and the shape of the intensity distribution within that cone. And while this can be calculated for a mathematically defined intensity distribution (such as a spotlight cone), it becomes problematic with an arbitrary distribution such as one defined by a light profile.

Furthermore, to ensure optimal convergence of the photon solution for a light which varies its intensity based on the direction of light (such as a spotlight or a light using a light profile), instead of emitting photons of different energy in different directions, photons of similar energy should be emitted in different densities. So the amount of energy-per-area (the density of photon energy) at a given point must match the direct light arriving at that point.

The Solution

The mia_photometric_light solves these issues. It takes all the guesswork out of balancing the photon energy with the light intensity by peforming a numerical integration of the chosen intensity distribution to calculate the proper photon energy automatically. As well, it takes care of adapting the photon density to the distribution.

For information on IES light profiles and how to set them up with the mia_photometric_light shader in the render tree, see Creating Lights With IES Profiles.

Colors

[color]

The color of the light.

Only if this is a normalized color (for example the mi_luminance() of the color is 1.0) will the Intensity settings below be accurate. You can use shaders such as mib_cie_d and mib_blackbody to return normalized colors.

Intensity

Allows you to tweak the intensity like a dimmer. Only when this is 1.0 will the Intensity settings below be accurate.

Intensity

Mode

Specifies which parameter and in what unit the light intensity is defined:

Use the Peak Intensity (cd) mode.
Use the Overall Flux (lm) mode.
Use the Light Profile mode, where the intensity is derived directly from the values stored in the IES profile.

Peak Intensity (cd)

The peak intensity of the light defined in candela. Intensity Mode must be set to Peak Intensity (cd).

Overall Flux (lm)

The overall flux of the light defined in lument. Intensity Mode must be set to Overall Flux (lm).

Distribution

Mode

Defines how the light is distributed in different directions:

Use Isotropic mode, where the light shines exactly the same in all directions.
Use Spotlight mode. The light must have a spread parameter to define the width of the beam. The distribution of light within the cone follows a cosine function, raised to the power of the Bias parameter.
Use IES Profile mode, where the intensity distribution of the light is derived from the values stored in the IES profile.

Note that you can combine the IES profile distribution mode with either the peak or overall flux intensity modes. In these cases, the IES profile is used in relative mode, and tries to accomodate the desired intensity while still using the directional distribution of the IES profile.

Bias

Specifies the distribution of light within a spotlight's cone using a cosine function, raised to the power of this parameter. A bias below 1.0 will push light towards the edges of the cone, and a bias above 1.0 will push light towards the center of the cone. Distribution Mode must be set to Spotlight.

Light Profile

[profile]

Specifies the intensity of the light using the values stored in an IES profile referenced by this shader. Intensity Mode and Distribution Mode must be set to IES Profile.

Units

Units to Meter Scale

Physical lighting in real world units is highly scale dependent, so this parameter defines how many scene units go into one meter. For example, if one scene unit is one millimeter, this would be 1000.

cd/m^2 Factor

The conversion factor between pixel values and candela per square meter luminance values.

This should match the cm2_factor of the mia_exposure_photographic shader to yield a photographically correct render (in terms of intensity).