27.6 Limits of Resolution: The Rayleigh Criterion

Light diffracts as it moves through space, bending around obstacles, interfering constructively and destructively. While this can be used as a spectroscopic tool—a diffraction grating disperses light according to wavelength, for example, and is used to produce spectra—diffraction also limits the detail we can obtain in images. Figure 27.25(a) shows the effect of passing light through a small circular aperture. Instead of a bright spot with sharp edges, a spot with a fuzzy edge surrounded by circles of light is obtained. This pattern is caused by diffraction similar to that produced by a single slit. Light from different parts of the circular aperture interferes constructively and destructively. The effect is most noticeable when the aperture is small, but the effect is there for large apertures, too.

How does diffraction affect the detail that can be observed when light passes through an aperture? Figure 27.25(b) shows the diffraction pattern produced by two point light sources that are close to one another. The pattern is similar to that for a single point source, and it is just barely possible to tell that there are two light sources rather than one. If they were closer together, as in Figure 27.25(c), we could not distinguish them, thus limiting the detail or resolution we can obtain. This limit is an inescapable consequence of the wave nature of light.

There are many situations in which diffraction limits the resolution. The acuity of our vision is limited because light passes through the pupil, the circular aperture of our eye. Be aware that the diffraction-like spreading of light is due to the limited diameter of a light beam, not the interaction with an aperture. Thus light passing through a lens with a diameter $D$ shows this effect and spreads, blurring the image, just as light passing through an aperture of diameter $D$ does. So diffraction limits the resolution of any system having a lens or mirror. Telescopes are also limited by diffraction, because of the finite diameter $D$ of their primary mirror.

Just what is the limit? To answer that question, consider the diffraction pattern for a circular aperture, which has a central maximum that is wider and brighter than the maxima surrounding it (similar to a slit) [see Figure 27.26(a)]. It can be shown that, for a circular aperture of diameter $D$, the first minimum in the diffraction pattern occurs at $\theta =1\text{.}\text{22}\lambda /D$ (providing the aperture is large compared with the wavelength of light, which is the case for most optical instruments). The accepted criterion for determining the diffraction limit to resolution based on this angle was developed by Lord Rayleigh in the 19th century. The Rayleigh criterion for the diffraction limit to resolution states that two images are just resolvable when the center of the diffraction pattern of one is directly over the first minimum of the diffraction pattern of the other. See Figure 27.26(b). The first minimum is at an angle of $\theta =1\text{.}\text{22}\lambda /D$, so that two point objects are just resolvable if they are separated by the angle

where $\lambda$ is the wavelength of light (or other electromagnetic radiation) and $D$ is the diameter of the aperture, lens, mirror, etc., with which the two objects are observed. In this expression, $\theta$ has units of radians.

Diffraction is not only a problem for optical instruments but also for the electromagnetic radiation itself. Any beam of light having a finite diameter $D$ and a wavelength $\lambda$ exhibits diffraction spreading. The beam spreads out with an angle $\theta$ given by the equation $\theta =1\text{.}\text{22}\frac{\lambda }{D}$. Take, for example, a laser beam made of rays as parallel as possible (angles between rays as close to $\theta =\mathrm{0º}$ as possible) instead spreads out at an angle $\theta =1\text{.}\text{22}\lambda /D$, where $D$ is the diameter of the beam and $\lambda$ is its wavelength. This spreading is impossible to observe for a flashlight, because its beam is not very parallel to start with. However, for long-distance transmission of laser beams or microwave signals, diffraction spreading can be significant (see Figure 27.29). To avoid this, we can increase $D$. This is done for laser light sent to the Moon to measure its distance from the Earth. The laser beam is expanded through a telescope to make $D$ much larger and $\theta$ smaller.

In most biology laboratories, resolution is presented when the use of the microscope is introduced. The ability of a lens to produce sharp images of two closely spaced point objects is called resolution. The smaller the distance $x$ by which two objects can be separated and still be seen as distinct, the greater the resolution. The resolving power of a lens is defined as that distance $x$. An expression for resolving power is obtained from the Rayleigh criterion. In Figure 27.30(a) we have two point objects separated by a distance $x$. According to the Rayleigh criterion, resolution is possible when the minimum angular separation is

where $d$ is the distance between the specimen and the objective lens, and we have used the small angle approximation (i.e., we have assumed that $x$ is much smaller than $d$), so that $\text{tan}\theta \approx \text{sin}\theta \approx \theta$.

Therefore, the resolving power is

Another way to look at this is by re-examining the concept of Numerical Aperture ($\text{NA}$) discussed in Microscopes. There, $\text{NA}$ is a measure of the maximum acceptance angle at which the fiber will take light and still contain it within the fiber. Figure 27.30(b) shows a lens and an object at point P. The $\text{NA}$ here is a measure of the ability of the lens to gather light and resolve fine detail. The angle subtended by the lens at its focus is defined to be $\theta =\mathrm{2\alpha }$. From the figure and again using the small angle approximation, we can write

The $\text{NA}$ for a lens is $\text{NA}=n\text{sin}\alpha$, where $n$ is the index of refraction of the medium between the objective lens and the object at point P.

From this definition for $\text{NA}$, we can see that

In a microscope, $\text{NA}$ is important because it relates to the resolving power of a lens. A lens with a large $\text{NA}$ will be able to resolve finer details. Lenses with larger $\text{NA}$ will also be able to collect more light and so give a brighter image. Another way to describe this situation is that the larger the $\text{NA}$, the larger the cone of light that can be brought into the lens, and so more of the diffraction modes will be collected. Thus the microscope has more information to form a clear image, and so its resolving power will be higher.

One of the consequences of diffraction is that the focal point of a beam has a finite width and intensity distribution. Consider focusing when only considering geometric optics, shown in Figure 27.31(a). The focal point is infinitely small with a huge intensity and the capacity to incinerate most samples irrespective of the $\text{NA}$ of the objective lens. For wave optics, due to diffraction, the focal point spreads to become a focal spot (see Figure 27.31(b)) with the size of the spot decreasing with increasing $\text{NA}$. Consequently, the intensity in the focal spot increases with increasing $\text{NA}$. The higher the $\text{NA}$, the greater the chances of photodegrading the specimen. However, the spot never becomes a true point.