Bulk Scatter Properties

Birefringence Expert

When a ray passes through a birefringent media, the ray is split into an ordinary and an extraordinary segment. Birefringent properties are labeled as Uniaxial and require additional information to define two sets of index and absorption data, one for the ordinary and one for the extraordinary ray.

FIGURE 3.4 - Material Editor for a Birefringent Property

For information on birefringent materials see:

•M. Born and E. Wolf, “Principles of Optics”, Pergamon Press, sixth edition, chapter 14 (1980), ISBN: 008026481

•P. Yeh and C. Gu, “Optics of Liquid Crystal Displays”, John Wiley & Sons, chapter 3 (1999), ISBN: 047118201X

•E. Collett, “Polarized Light”, Marcel Dekker, Inc., chapter 23 (1993), ISBN: 0824787293

Bulk Scatter Properties Standard Expert

The Bulk Scattering Property Editor is used to:

•Edit properties that are stored in the bulk scatter property database.

•Create new properties for addition to the bulk scatter database.

One scattering distribution function (SDF) that has been implemented in TracePro

is based on a paper by Jacques and Wang1 that describes scattering in biological tissue and uses the Henyey-Greenstein model. Equation 3.5

1.S. L. Jacques and L.-H. Wang, “Monte Carlo modeling of light transport in tissues,” in Optical Thermal Response of Laser Irradiated Tissue, edited by A. J. Welch and M. J. C. van Gemert (Plenum Press, New York, 1995), pp. 73-100.

TracePro 5.0 User’s Manual

3.9

Defining Properties

	SDF = p(θ)	=	1 – g2	3	⁄ 2	(3.5)
			4-------------------------------------------------------------π(1 + g2 – 2g cosθ)

where g is called the anisotropy factor, and g can take on values between –1 and 1. When g is positive, rays are scattered more in the forward direction, and when g is negative, they are scattered more in the backward direction. When g is zero, the scattering is isotropic, i.e. the same in all directions. See “Bulk Scattering” on page 7.54.

When a ray enters a scattering medium, it propagates a random distance x governed by the probability distribution

P(x) = e–μsxdx

(3.6)

where μs is called the scattering coefficient. The inverse of the scattering

coefficient is the mean free path of the ray in the material. When a ray enters a piece of material that is thin compared to the mean free path, it is likely to pass through the material without being scattered. Conversely, if the material is thick compared to the mean free path, the ray is almost certain to scatter within the material. When a strong scattering coefficient is combined with a strong absorption coefficient, rays will be only weakly transmitted through the material.

A second SDF is available called the Gegenbauer model. The HenyeyGreenstein2 is a special case of the Gegenbauer model setting alpha to 1/2.

	SDF = p(θ) =				K	(3.7)
				– 2g cosθ)α + 1
			(1 + g2
			2	)	2α
	K =	αg	(1 – g			(3.8)
			π[----------------------------------------------------------(1 + g)2α – (1 – g2α)]

TracePro Expert provides a third option for User Defined DLL scattering. See “Using Scatter DLLs” on page 7.56.

Bulk Scatter Property Editor

The bulk scattering property editor defines and modifies scattering profiles stored in the TracePro property database. Select Define|Edit Properties|Bulk Scatter Properties to open the editor window.

2.A. N. Yaroslavsky, I. V. Yaroslavsky, T. Goldbach, and H.-J. Schwarzmaier, “Influence of the scattering phase function approximation on the optical properties of blood determined from the integrating sphere measurements”, Journal of Biomedical Optics 4(1), 47-53 (January 1999)

3.10	TracePro 5.0 User’s Manual

		Bulk Scatter Properties
	TABLE 3.5. Fields used in the Bulk Scattering Information Panel
	Catalog	Property catalog names.
	Name	Scatter Property Name dropdown box. Select the name from the drop down
		list or by typing the name into the text box. If the name is found in the database
		its data is displayed.
	Description	The description contains notes about the Bulk Scatter Property.
	Type	The Type displays the selected scatter model. The scatter model is defined
		when a New Property is added to the database. The three models available
		are:

Henyey-Greenstein

Gegenbauer

User DLL Expert

FIGURE 3.5 - The Bulk Scatter Property Editor

TABLE 3.6. Fields used in the Bulk Scattering Grid Panel

	Anisotropy	Enter the anisotropy factor, g, for the selected wavelength. This measure
		describes the uneven propagation of light from +1 (forward) to –1 (backward).
	Scatter	Enter the scattering coefficient, μs, for the selected wavelength in units of 1/
	Coefficient	mm.
	Alpha	This is displayed for the Gegenbauer scatter model.
	Coeff 0-5	These columns replace the data columns when a DLL scatter model is used.
	Expert	The data entered is passed into the DLL and is used as variable input to alter
		the DLL calculations. In this way one DLL can be programmed and modified
		externally by the user.

Import/Export

The Import/Export format is documented in “Bulk Scatter Property Format” on page 7.95. When the editor is open, the File menu displays Import Property and Export Property menus.

TracePro 5.0 User’s Manual

3.11