Medium Properties
Use the Medium Properties node to specify the refractive index of the medium. An instance of this feature is created by default, including all of the selected domains for the physics interface. For the region outside the geometry and for any domains not included in the physics interface selection, the refractive index is instead controlled by the Optical dispersion model in the physics interface Material Properties of Exterior and Unmeshed Domains section.
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 Refractive index, real part, n (dimensionless) using one of several optical dispersion models. The options are:
Specify refractive index (the default): use this option to enter a value or expression for the real part of the refractive index n directly. By default the Refractive index, real part uses values From material. It is also possible to enter a User defined value or expression. Only ray propagation in isotropic media can be modeled with the Geometrical Optics interface.
Get dispersion model from material: the dispersion model in each domain is automatically deduced from the Material nodes and their selections. This allows different optical dispersion models to be used in different domains in the geometry.
For example, suppose that a model contains two lenses consisting of two different glasses, and that the first glass is defined using Sellmeier coefficients whereas the second glass is defined using Schott coefficients. Then selecting Get dispersion model from material will automatically express the refractive index using the Sellmeier equation in the first lens and the Schott (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 Medium Properties node and select the dispersion models manually.
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For more information on each optical dispersion model, see Table 3-3 in Theory for the Geometrical Optics Interface, Optical Dispersion Models section.
The coefficients for each of these dispersion models are taken From material by default. Alternatively, User defined 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 Specify refractive index and Temperature-dependent Sellmeier, 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 Reference temperature Tref (SI unit: K) and Reference pressure Pref (SI unit: Pa) can either be taken From material (the default) or User defined.
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 the Edlén model (Ref. 2) for the refractive index of air. The Temperature-dependent Sellmeier model always returns absolute indices. If User defined 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 Refractive index, imaginary part k (dimensionless). By default the Refractive index, imaginary part k uses values From material. For User defined enter a value or expression. The convention followed by the Geometrical Optics interface is that 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 Get dispersion model from material is unique because it allows a single Medium Properties 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:
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Locate the default Medium Properties node.
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From the Optical dispersion model list, select Get dispersion model from material. If the glasses loaded in step 1 also provide thermo-optic coefficients, it is important to specify an accurate value of the Temperature 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 Refractive index may be specified using a thermo-optic dispersion model. Two options are possible:
None (the default): no offset will be applied.
Schott: 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 From material (the default), or User defined. A reference temperature is also required. This may also be taken From material (the default), or User defined.
For further details, see Thermo-Optic Dispersion Models.