Paraxial approximation

In geometric optics, the paraxial approximation is an approximation used in ray tracing of light through an optical system (such as a lens).

A paraxial ray is one which makes a small angle (θ) to the optical axis of the system, and lies close to the axis throughout the system. Generally, this allows the approximation (for θ in radians) sin(θ)≈tan(θ)≈θ and cos(θ)≈1 for calculation of the ray's path.

The approximation is known as first-order raytracing or Gaussian optics. Ray transfer matrix analysis is one method that uses the approximation.