9. Surface Overview

Surfaces are core elements of Optiland’s optical system. Each surface represents an optical interface, defined by its geometry, material properties, coatings, and optional apertures. Surfaces are organized into a Surface Group to manage operations on multiple surfaces.

9.1. Surface Components

A surface consists of several components that define its optical properties:

  • Geometry: The shape of the surface (e.g., planar, spherical, aspheric, freeform). This includes the surface’s coordinate system.

  • Materials: The material type before and after the surface, which determines the refractive index and extinction coefficient.

  • Coatings: Coatings (e.g., thin films) applied to the surface for modifying reflection, transmission, and/or polarization properties.

  • Stop Surface Flag: Indicates if the surface is the aperture stop of the system.

  • Interaction Model: Defines how a ray interacts with the surface (e.g., refraction, reflection). For more details, see the Interaction Models section.

  • Physical Aperture (optional): A physical or virtual aperture defining the area where rays can interact with the surface.

  • BSDF (optional): Bidirectional Scattering Distribution Function for modeling scattering behavior.

9.2. Paraxial Surfaces

Optiland handles paraxial surfaces not as a distinct class, but through a ThinLensInteractionModel that can be applied to a standard Surface. This model simplifies the surface to an ideal thin lens with a given focal length. This approach allows for first-order layouts and analysis. A Surface with a ThinLensInteractionModel can be converted into a thick lens equivalent using the convert_to_thick_lens function in optiland.surfaces.converters.

9.3. Ray Interaction with Surfaces

When a ray interacts with a surface, the following steps are typically performed:

  1. Intersection: The ray’s path is intersected with the surface’s geometry.

  2. Aperture Check: The ray’s intersection point is checked against the surface’s aperture to determine if the ray is blocked.

  3. Refraction/Reflection: The ray’s direction is updated based on Snell’s law or the law of reflection, and the ray properties may be affected by the surface’s material/coating properties.

4. Scattering: If the surface has a BSDF, the ray may be scattered based on the scattering distribution function. - Recording: During a trace, each Surface temporarily stores the ray data at the intersection point.

9.4. Surface Group

Surfaces are combined into a Surface Group, which manages a collection of surfaces and facilitates operations like ray tracing. The Surface Group:

  • Tracks the ordered list of surfaces in the optical system.

  • Propagates rays through the system, invoking surface-specific logic at each step.

  • Aggregates ray trace history from all surfaces, providing a complete picture of the ray paths.

  • Exposes methods for adding, removing, and modifying surfaces in the system.

Tip

The Surface Group allows efficient iteration over multiple surfaces, simplifying complex ray tracing operations.

9.5. Surface Factories

To streamline surface creation, Optiland uses a multi-factory pattern, with distinct factories for different components:

  • SurfaceFactory: The main factory that orchestrates the creation of Surface objects.

  • GeometryFactory: Creates different geometry types (e.g., StandardGeometry, ToroidalGeometry).

  • MaterialFactory: Handles the creation of materials.

  • CoatingFactory: Manages the creation of coatings.

This granular approach makes the system more modular and extensible.

9.6. Extensibility

The surface framework is designed for extensibility:

  • Custom geometries, coatings, or aperture definitions can be added by subclassing existing components. These may be added to any surface instance.

  • The factories can be extended to handle new component types.

For more detailed information on surface geometry and coatings, see their dedicated sections in this guide.