# How is the Rayleigh criterion connected to the Abbe limit

I am interrested whether one can derive a formula for the point resolution (like Abbe did) of an optical system from the Rayleigh criterion (without the use of small angle approximation i.e. $\rm{sin}(\alpha)=\rm{tan}(\alpha)$ which is not really suitable e.g. for microscopy).

And if so whether formula is directly comparable to the Abbe limit for point (or rather line) resolution.

The Rayleigh criterion is given as: $$\theta_{min}=1.22\frac{\lambda}{D}$$ where $\theta_{min}$ is the smallest resolvable angle, $\lambda$ is the wavelength of the used lightsource and $D$ is the diameter of the used aperture (or of the used lens).

And the Abbe limit is given as:

$$d=\frac{\lambda}{2\,n\,\rm{sin}(\alpha)}=\frac{\lambda}{2\,\rm{NA}}$$

where $d$ is the smallest resolvable distance, $n$ is the refractive index of the medium between the object and the optical system, $\alpha$ is the biggest scattering angle (inicient on the optical system) and $\rm{NA}$ is the numerical aperture.

• Obviously you are missing, at the very least, the index of refraction. Other than that, the two are virtually the same, one is basically predicting an angular resolution for an object that is infinitely far away, the other a spatial resolution for an object that is close. Since the distance to the lens is implicitly included in the numerical aperture, you could bring that over to the left hand side in the Abbe limit and get a similar formula for angular resolution (which is less useful). Today we have replaced both with point spread functions, anyway. – CuriousOne May 24 '16 at 20:21

Both equations are in fact structurally similar with Abbe limit given by $d= \dfrac{\lambda}{2\mathrm{NA}}$
And Rayleigh limit given by $d =1.22 \dfrac{\lambda} {2\mathrm{NA}}= 0.61\dfrac{\lambda} {\mathrm{NA}}$
where lambda is the wavelength and $\mathrm{NA}$ the numerical aperture of the light collecting lens.
Rayleigh criterion is thus a modification of the Abbe Resolution limit. The Rayleigh criterion states that in order for 2 closely placed PSF to be resolved, the central maxima of one should lie exactly at the first minima of the second one. Since the Airy pattern is defined by the Bessel function, the minimum separation between the 2 patterns should be $1.22 \lambda/ 2\mathrm{NA}$ instead of just $\lambda/2\mathrm{NA}$ considering that the first minima will be at 1.22 times the unit from the central maxima.