Ray Tracing

FIGURE 5.20 - Raytrace Options Dialog for Standard and Expert editions of TracePro

The Raytrace Options are saved when the Model is written to disk in an OML file. After the data is changed, pressing the Set Defaults button stores the data into the TracePro defaults files to be used the next time TracePro is opened. See “User Defaults” on page 1.9

LC	See “Simulation Options for TracePro LC” on page 5.41 to view the Options
	Dialog for the LC edition of TracePro.

Options

The Options tab of the Raytrace|Raytrace Options dialog box allows you to set parameters that affect how the raytrace is done. They affect the entire model.

•You can select whether importance sampling is done during the raytrace by checking the Importance Sampling check box.

•The Random Rays box allows you to set how many randomly scattered rays are generated whenever a ray strikes a scattering surface.

•Finally, you can set the random number seed and radiometric units.

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Analysis Units

The Analysis Units entry lets you choose between radiometric units or photometric units. The radiometric and photometric units used in TracePro are SI units. The units have the meanings summarized in Table 5.6.

TABLE 5.6. The Choice of Radiometry and Photometry Units

	Radiometry		Photometry
	Name	Unit	Name	Unit
Power	Flux	Watt (W)	Luminous flux	lumen (l)
Power/area	Irradiance*	Watt/meter2	Illuminance	lux (lx)
		(W/m2)
Power/solid	Radiant Inten-	Watt/stera-	Luminous	candela (cd)
angle	sity	dian (W/sr)	intensity

*The Irradiance is on an illuminated surface. The Radiant Exitance is what is emitted (from a source or scattered from an illuminated surface). For the latter, the exitance is (1-a)*E, where a is the absorption and E is the irradiance.

The choice of radiometric or photometric units affects the display of units throughout TracePro. Units are displayed in TracePro as indicated in Table 5.7.

TABLE 5.7. Effect of the choice of Units in TracePro

Place	Radiometric label(s)	Photometric label(s)
Irradiance/Illuminance	Total - Irradiance Map	Total – Illuminance Map
map – top
Irradiance/Illuminance	Unit = W/m2	Unit = lux
map – units
Irradiance/Illuminance	Unit = W	Unit = lumen
map – flux
Irradiance/Illuminance	Irradiance	Illuminance
map options - Map Type
Intensity/Candela plot	Intensity - W/sr	Luminous Intensity - candela
Luminance/Radiance map	Radiance - W/m2-sr	Luminance - cd/m2(nit), foot-
		lambert, or millilambert

Ray Splitting

Placing a check mark in the Ray Splitting check box lets you apply selections for Specular Rays Only, Importance Sampling, Polarization, and Aperture Diffraction to your raytrace.

In “crude” Monte Carlo ray tracing, there is no ray splitting or importance sampling. Each time a ray strikes a surface, the absorption, reflection, and transmission coefficients of the surface are used as probabilities, and one ray component is chosen randomly, weighted by these coefficients. For example, suppose a surface has absorptance equal to 0.1, reflectance equal to 0.2, and transmittance equal to 0.7 (these coefficients must sum to one to satisfy the principle of conservation of energy). When rays strike the surface, 10% of the time they are completely absorbed, 20% of the time they are completely reflected, and

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Ray Tracing

70% of the time they are completely transmitted. That result is achieved by the following method: when a ray strikes the surface, a random number is chosen between zero and one. If the number is between 0 and 0.1, the ray is absorbed. If the number is between 0.1 and 0.3, the ray is reflected, and if the number is between 0.3 and 1.0, the ray is transmitted. The fraction of incident flux reaching the exit surface of the model is determined by simply counting up the number of rays that are absorbed by it and dividing by the number of rays started in the simulation.

If you enable ray splitting, when a ray intersects a surface, several split ray components are generated. The flux from the ray at the intersection is split between the ray components according the their absorption, reflection, and transmission coefficients. Since the number of split rays grows geometrically, a large number of ray segments is created causing TracePro to use a large amount of computer memory. It is important for you to know that If you have a nonimaging system model in which surfaces reflect and transmit light almost equally, you can turn off ray splitting.

In a system model, many split rays are terminated when their flux falls below the flux threshold, keeping the number of ray components to a manageable number. If the use of ray splitting causes the raytrace to proceed slowly, check to be sure that the flux thresholds are not set too low. If the raytrace is slow even with appropriately set flux thresholds, try turning off ray splitting.

At coincident surfaces between two objects, TracePro follows a detailed method for determining the reflectance, transmittance, and absorptance for the rays. See “Coincident Surfaces” on page 7.15 for a complete discussion of this method.

Specular Rays Only Standard Expert

Checking the check box for Specular Rays Only limits the rays to be traced to those segments using specular reflection and transmission. You can use this control to turn off tracing of scattered rays. This is useful for debugging your data or for doing specialized analyses such as ghost image analysis. Any flux that would normally be given to scattered rays is lost, in these cases Conservation of Energy is not ensured.

Importance Sampling Standard Expert

If the Importance Sampling check box contains a check mark, the importance sampling that you have specified is performed during a raytrace. Unchecking the box turns off importance sampling. If you are debugging your model and do not want importance sampled rays to be generated, turning off importance sampling is useful. If importance sampling is turned off, flux that might have been allocated to importance sampled rays is given to random rays.

Aperture Diffraction and Aperture Diffraction Distance

Standard Expert

Checking the Aperture Diffraction check box causes TracePro to bend rays that pass close to a diffracting edge. Unchecking the check box causes any diffracting surfaces to be treated as normal surfaces.

The Aperture Diffraction distance setting controls how diffraction is applied to rays passing by a diffracting edge. If a ray intersects the plane containing the diffracting edge, the ray’s path is altered more if it is closer to the diffracting edge

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and less if it is farther away. In a TracePro model, you can set this distance. If a ray intercept on the aperture plane is within the specified distance from the aperture edge, diffraction occurs. During a raytrace, eliminating diffraction from rays that are far from the edge saves time.

Set the diffracting distance to be small (100x the wavelength) when only rays diffracted through large angles are of interest (especially true for stray light analysis). If all rays are of interest, set the diffracting distance to be large so that all rays are diffracted. Setting the distance equal to half the largest aperture dimension causes all rays to be bent by diffraction. For a discussion of setting up diffraction in TracePro, see “Defining Diffraction in TracePro” on page 4.33.

Random Rays

The Random Rays number specifies how many randomly scattered rays are generated when a ray strikes a surface with diffraction or with a surface property that has scattering. Random rays are those that are scattered in random directions, weighted by the BSDF.

The default value is 1. You can specify any positive number. A large number can cause the ray tree to be very wide, causing the raytrace to proceed slowly and too much memory to be consumed.

Fluorescence Expert

When a model includes Fluorescent properties (See “Fluorescence Properties” on page 4.7 and page 3.12), the effects of fluorescence emission is enabled by setting Fluorescence on in the Raytrace Options dialog. The emission wavebands need to be defined in the Wavelengths tab defined below. See “Fluorescence Properties” on page 3.12

After the initial excitation rays are traced you have the option to:

•stop the raytrace and not trace the emission rays (option: Generate emission source only)

•continue the raytrace and include the emission rays (option: Immediately trace emission wavelengths).

During the raytrace, fluorescent objects or bodies will emit rays simulating fluorescence emission. These rays are stored in File Sources to be used during the emission part of the raytrace. The Insert file source option is used to automatically include the file source(s) into the model. If Insert file source is left unchecked, the model will remain in its “pre-raytrace” state.

Polarization Standard Expert

Checking this box turns on the polarization ray tracing features of TracePro. A Stokes vector will be attached to each ray, and the Stokes vector may be modified when it interacts with a polarizing surface property, when passing through an object that has a Mueller matrix applied to it, or when passing though a birefringent material. You might notice a slowing of the raytrace and more consumption of memory when polarization is enabled.

Discussions and examples of Mueller matrices and Stokes vectors can be found in the Technical Reference section, “Mueller Matrices and Stokes Vectors” on page 7.50 and many textbooks including:

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