Image Quality –Which Metric Should I Choose? Jed Hancock Optical Sciences 521
Image Quality Criteria Image quality criteria defines the imaging performance of an optical system –This is how you know you are done Most often a complex process –The criteria is usually described by multiple image quality metrics (WFE, MTF, etc…)
Image Quality Metrics Evaluation of metrics –Typically modeled (ray trace codes) then tested (calibration) Image quality is affected by the mechanics Optical engineers tool bag –Opto-mechanics: Tolerancing, statics, deflections, thermal effects, vibration isolation, material properties, precision adjustments, mounting of lenses, windows, mirrors…..
Spot Diagrams Effective for applications where (≥ ¼ λ) Specific types of aberrations can be determined Geometric or RMS radius Geometric metric
Ensquared Energy Detector outline Geometric estimate of energy on a pixel Imaging and non-imaging applications where (≥¼ λ) Geometric metric
Wavefront Error Optical path difference between the actual wave and reference wave Measured in the exit pupil plane Aberrations can be determined by wave fan plots Geometric metric
Modulation Transfer Function The optical transfer function (OTF) is the autocorrelation of the complex pupil function and it is also the Fourier transform of the point spread function The MTF is the modulus of the OTF Extended scenes, small aberrations, complete system analysis Geometric and diffraction metric
Point Spread Function The point spread function captures diffraction effects or the wave nature of light Small aberrations (≤ ¼ λ) Young’s double pin hole experiment Diffraction metric
Strehl Ratio Best for small aberrations (≤ ¼ λ)
Geometric vs. Diffraction GeometricDiffraction MTF % Ensquared energy RMS WFE--- Spot DiagramPSF
Choose Appropriate Metrics Often times more than one image quality metric should make up the criteria, this depends on the application If WFE were the only image quality metric chosen, the effect of an obscured aperture on image quality could be overlooked