Airy disk
Note that for a given aperture the size of the Airy disk grows linearly with the wavelength formula_9, and consequently the peak intensity falls according to formula_10 so that the reference point for unity Strehl ratio is changed. Typically, as wavelength is increased, an imperfect optical system will have a broader PSF with a decreased peak intensity. However the peak intensity of the reference Airy disk would have decreased even more at that longer wavelength, resulting in a "better" Strehl ratio at longer wavelengths (typically) even though the actual image resolution is poorer.