  All next generation ground-based and space-based solar telescopes require a
good quality assessment metric in order to evaluate their imaging performance.
In this paper, a new image quality metric, the median filter gradient
similarity (MFGS) is proposed for photospheric images. MFGS is a
no-reference/blind objective image quality metric (IQM) by a measurement result
between 0 and 1 and has been performed on short-exposure photospheric images
captured by the New Vacuum Solar Telescope (NVST) of the Fuxian Solar
Observatory and by the Solar Optical Telescope (SOT) onboard the Hinode
satellite, respectively. The results show that: (1)the measured value of MFGS
changes monotonically from 1 to 0 with degradation of image quality; (2)there
exists a linear correlation between the measured values of MFGS and
root-mean-square-contrast (RMS-contrast) of granulation; (3)MFGS is less
affected by the image contents than the granular RMS-contrast. Overall, MFGS is
a good alternative for the quality assessment of photospheric images.
