Meaning
An optical performance metric defined as the ratio of the peak aberrated intensity of a point source image to the peak intensity of a perfect diffraction-limited system. In evaluation of optical systems, the strehl ratio quantifies how closely a real system approaches its theoretical performance limit. This value is expressed as a decimal between zero and one, with higher values representing better imaging performance.
The metric is a key requirement in adaptive optics and high-resolution imaging configurations, guiding the acceptance tests of astronomical instruments. Development teams utilize this metric to verify alignment precision.
Performance Metric
A value above eighty percent is typically considered diffraction-limited, meaning that the wavefront quality is sufficient for high-precision scientific applications. When the strehl ratio drops below this threshold, the image loses contrast and energy spreads into the surrounding diffraction rings. This reduction in peak intensity makes it difficult to resolve closely spaced features or detect weak signals.
Wavefront Error
The relationship between phase variations and the resulting image quality is mathematically approximated using the Marechal approximation for small errors. If the strehl ratio is calculated from the root-mean-square wavefront error, the estimation remains highly accurate for values above sixty percent. For larger aberrations, the approximation breaks down and full wave-optic simulations are required.
Calibration Baseline
To establish an accurate baseline, the measurement must account for the excitation of the testing source and the internal aberrations of the sensor. When the strehl ratio is measured in the field, a reference star or a calibrated fiber source is used to isolate the system response. This calibration ensures the validity of the reported data.
