# 11. Glossary¶

A glossary of the key concepts used in Raysect.

- Affine Matrix
A 4x4 matrix which defines a transformation between two coordinate systems. See

`AffineMatrix3D`

.- Flux
See radiant flux.

- Intensity
The radiant flux per solid angle. Note that this is not the same as radiance, and in practice not very useful except for comparing point sources.

- Intersection
A ray-primitive intersection that describes the point of intersection between the given ray object and a piece of geometry (a primitive) in the scene

`Intersection`

.- Irradiance
The area density of radiant flux passing through a surface, symbol \(E\), with units W/m^2.

- Material
A material object provides an associated primitive with physical surface and volume properties. e.g.

`core.material.Material`

and`optical.material.Material`

- Node
A fundamental element of the scene-graph with its own local coordinate system

`Node`

.- Observer
An observing node of the the scene-graph that generates rays to sample the scene. Observers can be 0D (Pixel, FibreOptic) or 2D (PinholeCamera). Observers are responsible for all the geometric effects of the observing process, such as the pixel area and solid angle sensitivity that define an effective etendue for the observer. After ray-tracing the results of all the samples are passed to a list of connected pipeline objects for further data procressing. See Observers for more examples.

- Pipeline
Pipelines are responsible for the data analysis of observers after ray-tracing. They process the mean spectral radiance at each sample location (could be 0D or 2D) into the more specific data type the user is interested it. For example, users may be interested in converting mean spectral radiance values to the more human friendly RGB colour or raw power values. See Observers for more examples.

- Primitive
A geometric structure that interacts with rays and is also a scene-graph node. May define a closed or open surface, and a volume. Examples of primitives are geometric shapes, such as the Box and Cylinder, as well as structured meshes. All primitives have a material applied to their surface and volume. See the Primitives Module.

- Radiance
Radiance is the flux density per unit area per steradian, W/m^2/str, given the symbol \(L\). Radiance is the most fundamental radiometric quantity since all other quantities can be calculated as integrals of radiance over areas and solid angles. Radiance is the natural quantity for ray-tracing because it remains constant along rays in empty space.

- Radiant Flux
Also known as power, the total amount of energy passing through a surface in one second, i.e. units of watts (W) with the symbol (\(\Phi\)).

- Ray
Describes a line in space with an origin and direction. Ray’s are used to trace() the world (e.g.

`core.ray.Ray`

and`optical.ray.Ray`

).- Scene
A particular scene-graph instance consisting of geometry and cameras that correspond to a physical scene.

- Scenegraph
A tree structure that represents a nested set of coordinate systems. Changes to the coordinate system at any node level are cascaded to all children. For example, suppose you have a car node that contains separate child nodes for each of the wheels and car body. Any transform applied to the car node, would also be applied to each of the cars component parts. The scenegraph makes it easy to manage complex scenes by allowing complex nodes to have their own local coordinate system. Any instanced node objects (e.g. boxes, lenses, etc) that are not connected to the scene-graph will not be included in the ray-tracing. It is possible to have multiple worlds (scene-graphs) in the same script for cases where you need to trace two different scenes and compare the results.

- Solid Angle
The two dimensional angle in three dimensional space that an object subtends at a point. It represents how large an object appears to an observer at that point. Measured in units of steradian (str). There are \(4\pi\) steradians in a sphere.

- Transform
A 4x4 affine matrix which defines a transformation between two coordinate systems. See

`AffineMatrix3D`

.- World
The root node of the scene-graph on which ray-tracing is performed, it also holds the acceleration structures used by the ray-tracer. See

`core.scenegraph.world.World`

and`optical.scenegraph.world.World`

.