Introduction:
OpticStudio Zemax is a geometrical ray-tracing software. When we look at light as a wave we can make propagating light through different optical elements easier by using rays. A ray travels from left to right, which is the Z-axis. Rays represent the direction the wavefront is propagating and are defined from the normal at a particular location on the wave. When the ray encounters a surface of an optical element we can use the angle of incidence and the indices of the mediums to determine the angle of exitance via Snell's Law.
To get started in Zemax you do not need to understand all 50+ of the surface types to start designing lens systems. This document will summarize the basic surface types to help you get started.
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We will look at:
Zemax Surface Types (Sequential Mode):
Each row in the Lens Data Editor is a surface where light will either reflect or refract (or diffract, scatter, split, but that is for other surface types that are more advanced optical systems).
The purpose of an Optical System is to record the scene exactly as it really is. However, there are limiting factors that create blur. The image of a perfect point source is not possible. There are two sources of blur that will spread the light out (point spread function, PSF): diffraction and aberrations. Diffraction is a physical blur that the laws of physics dictate and can be summarized as light bends around corners and causes blur. Aberrations are introduced from imperfections in the optical elements that cause deviations from a perfect spherical wavefront (converging to a focus) and result in blur. Diffraction-limited means you are a great lens designer. Aberration-limited means the optics could be better.
Rays leave the source and are bent when incident on an optical surface. The amount of bending is a function of the angle of incidence and the index of refraction of the optical glass. A spherical lens will have a radius of curvature, and that radius of curvature creates a focus. More curvature means more optical power and a faster focus (faster focus means the focal length is short - small F/#=effective focal length/entrance pupil diameter, slower focus means the focal length is long - large F/#).
In Zemax, the rays representing the wavefront converging to a focus is compared to a perfect reference sphere in the exit pupil (the image of the stop in Image Space). This is a great place to compare your optical system because it is the last defined location before the focus. The Optical Path Difference of each ray is compared to its corresponding ray of a perfect reference sphere (a perfect wavefront converging to a focus is a sphere).
1. Standard Surface
Using the Standard surface we created a singlet lens and a scanning Mirror.
2. Even Asphere
Here we are using a Standard surface to create a Plano-Convex Lens. You can see the Spherical Aberration is strong. That is when rays near the edge of the pupil focus faster than rays near the center. You can read the full blog on aberrations here.
One way of fixing this is to change surface type to Even Asphere. We want to release the 2nd order term and 4th order term to correct for spherical aberration using a custom Merit function for EFFL at the same focal length and SPHA at 0. The result is a much tighter focus where all rays in the pupil focus at the same point. See below.
3. Toroidal
This allows for you to create a focus in the X-axis that is different than the Y-axis.
In the Y-Z Plane (Left) we have a Standard Spherical surface and in the X-Z Plane (Right) we have a Planar surface with no curvature and therefore no power to bend the light. The result is the strip or line of light on the target.
4. Conic Surface
A great example of this is when we use the Standard Surface with a Conic = 0, provides a focus that is much larger than the airy disc (it is not diffraction limited).
When we apply a parabolic Conic = -1, we can significantly improve the Blur Spot and achieve a diffraction limited design, see below.
5. Coordinate Break
Below is an example of using coordinate breaks in their most simple function. The Layout shows the decentered and tilted Lens 2 and overlayed it with the unperturbed system.
Other Surface Types
Start with the 5 basic surface types first and Master them. If these don't meet your needs use the help in Zemax to understand the different options. There are many advanced surface types (Freeform, Binary, Diffractive, User DLLs, etc.)
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