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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
FIELD
POSITION
Ensquared Energy
•Geometric estimate of energy on a pixel
•Imaging and non-imaging applications where (≥¼ λ)
•Geometric metric
0.00, 0.00
0.000,0.000 DG
Detector outline
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
Geometric
Diffraction
MTF
MTF
% Ensquared energy
% Ensquared energy
RMS WFE
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Spot Diagram
PSF
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