Medium Properties

Use the node to specify the refractive index of the medium. This node is created by default and includes all of the selected domains for the physics interface.

Medium Properties

Use the settings in this section to specify how the real part of the refractive index is defined. If the ray intensity or power is computed, then you can also specify the imaginary part of the refractive index, which is used to describe absorption within the medium.

Optical Dispersion Models

It is possible to specify the , n (dimensionless) using one of several optical dispersion models. The options are:

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(the default): use this option to enter a value or expression for the real part of the refractive index n directly. By default the uses values . It is also possible to enter a value or expression. Only ray propagation in isotropic media can be modeled with the Geometrical Optics interface.

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: the dispersion model in each domain is automatically deduced by checking which material properties are defined in each domain. This will often result in different optical dispersion models being used in different domains, especially if the model contains glasses from two or more distinct manufacturers.

For example, suppose that a model contains two lenses consisting of two different glasses, and that the first glass is defined using coefficients whereas the second glass is defined using coefficients. Then selecting will automatically express the refractive index using the equation in the first lens and the (polynomial) equation in the second. An alternative way to use different optical dispersion models in different domains is to use more than one instance of the node and select the dispersion models manually.

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An optical dispersion model where the refractive index is a function of wavelength may be selected. The following built-in dispersion models are available (See Table 3-3 for details):

, or

The coefficients for each of these dispersion models are taken by default. Alternatively, coefficients may be entered.


	For the built-in optical dispersion models, the wavelength is always assumed to be in units of microns (μm). For example, in the Schott (polynomial) model, the coefficients A0, A1, A2, A3, etc. have units of 1, μm, μm2, μm3, and so on. If another source were to provide these coefficients using nanometers instead of microns, then some manual conversion would be required.

For the options and , the refractive index is assumed to be absolute, such that n = 1 is the refractive index of an ideal vacuum. For all other optical dispersion models, however, the refractive index computed by the dispersion equation is a relative index defined with respect to air at a specified reference temperature and pressure. Thus, for each of these other optical dispersion models, the Tref (SI unit: K) and Pref (SI unit: Pa) can either be taken or .

If the reference pressure Pref ≠ 0, then it is assumed that the refractive indices are relative to air at the reference temperature and pressure; that is n = nrel × nair. The conversion from relative to absolute refractive index is made using a model for the refractive index of air. The model always returns absolute indices. If coefficients are selected, the default coefficients for each of the built-in dispersion models give nrel = 1 (or, n = 1, if Pref = 0). See Optical Dispersion Models for further details.

If ray intensity or power is computed, specify the k (dimensionless). By default the k uses values . For enter a value or expression. The convention followed by the Geometrical Optics interface is that an the imaginary part of the refractive index is negative in absorbing media. A positive value indicates a gain medium in which the intensity increases as the ray propagates.

How to Automatically Detect Optical Dispersion Models

The option is unique because it allows a single node to detect different optical dispersion models from a large number of different materials having different domain selections. For example, the materials used in the Petzval Lens tutorial are shown in Figure 3-2 below. To automatically detect the optical dispersion models of the loaded glasses, take the following steps:

Add materials to the model using the Optical Material Library. Most of the glasses in this material library use an optical dispersion model, specified by the glass manufacturer, to define the refractive index. As the materials are added, the coefficients used by the optical dispersion model are automatically loaded.

Locate the default s node.

From the list, select . If the glasses loaded in step 1 also provide thermo-optic coefficients, it is important to specify an accurate value of the as well.

Figure 3-2: Workflow for automatically detecting optical dispersion models from the glasses in an optical prescription.

Thermo-Optic Dispersion Models

A temperature-dependent offset in the may be specified using a thermo-optic dispersion model. Two options are possible:

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(the default): no offset will be applied.

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: this is the only built-in thermo-optic dispersion model available. The coefficients used to compute a temperature dependent offset in the refractive index can either be taken (the default), or . A reference temperature is also required. This may also be taken (the default), or

For further details, see Thermo-Optic Dispersion Models.