The image contrast performance is often specified by MTF at a given spatial frequency. How to optimize it, this article will share a bit about it.
In the optical imaging system design, normally, the MTF tends to be badly behaved in the early stages, so that other optimization methods are needed until the system is very close to its final form. Optical Image Contrast Optimization largely solves these problems. Instead of calculating the full MTF, Contrast optimization measure the phase differences in the exit pupil across a distance corresponding to the desired spatial frequency in the MTF. This new Contrast Optimization technique allows for robust and efficient optimization on the system MTF at a given spatial frequency. The method minimizes the wavefront differences between pairs of rays separated by a pupil shift corresponding to the targeted spatial frequency, which maximizes the MTF.
In this example, we will show how to design imaging system by optimizing its contrast performance, which requires MTF> 40%@20lp/mm @full field. The rest of constraints are as below:
The starting point was a system of plane parallel plates, with a radius of curvature solve on the last surface to force an F/3 system. The design was done in visible wavelengths from 440 -770nm. The entrance pupil diameter is 44 mm.
We could select the Image Quality criterion “Contrast” in the Merit Function Wizard, specifies the spatial frequency of interest, and sets the weight of the sagittal MTF value versus the tangential. The wizard creates a Merit Function using the MECS and MECT operands which are designed to be used in pairs. MECS traces three rays: an unshifted ray, a ray shifted by d in the S direction in the pupil, and a ray shifted by d in the T direction in the pupil. MECS then reports the optical path difference (OPD) in waves for the sagittal pair of rays. MECT reports the OPD for the tangential pair of rays.
The remaining radii of curvature and spacings were set to variables for a Damped Least Squares optimization.
After 20s optimization, the layout and image contrast by MTF is as below:
We continue to run the Optical image contrast optimization by hammer optimization 20 hours.
The method is much faster and better-behaved than a direct optimization on the MTF value. And the optimization attempts to drive all the wavefront differences in the MECS and MECT operands to zero. This maximizes the MTF at the spatial frequency corresponding to the pupil shift, d.
The Optical Image Contrast Optimization technique allows for fast and robust optimization at specific spatial frequencies in the MTF which helps an optimization move quickly to the best solution.
The design file used in this particle is attached, please download it here.
Reference Source:
- https://www.zemax.com/
- Zemax Optical Design Program User’s Guide, Zemax Development Corporation
- https://en.wikipedia.org/wiki/Main_Page
