A measurement of optical precision that expresses the fraction of light that the mirror concentrates into the airy disk. A perfect mirror, therefore, has a Strehl ratio of 1--it reflects all of the light into the airy disk. Back to top of glossary

A figure of merit for beam propagation. For tilt corrected images it is the ratio of uncorrected on-axis intensity to corrected on-axis intensity. For non-tilt corrected images it is the same ratio in the axis normal to the plane of the tilt, rather than parallel to the z-axis.

The ratio of the peak intensity in the diffraction pattern of a point image with aberration to the peak intensity in the diffraction pattern of the same point image, but without aberration.

The ratio of the peak brightness of a stellar image to that produced by an ideal optical system.

The ratio of the illuminance at the peak of an aberrated diffraction pattern to that at the peak of an aberration-free system.

The modern definition of the Strehl ratio is the ratio of the observed peak intensity at the detection plane of a telescope or other imaging system from a point source compared to the theoretical maximum peak intensity of a perfect imaging system working at the diffraction limit. This is closely related to the sharpness criteria for optics defined by Karl StrehlSTREHL, K. 1895, Aplanatische und fehlerhafte Abbildung im Fernrohr, Zeitschrift für Instrumentenkunde 15(Oct.), 362-370.