Angular resolution

Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object. It is thereby a major determinant of image resolution.

Contents

Definition of terms

Resolving power is the ability of an imaging device to separate (i.e., to see as distinct) points of an object that are located at a small angular distance. The term resolution or minimum resolvable distance is the minimum distance between distinguishable objects in an image, although the term is loosely used by many users of microscopes and telescopes to describe resolving power. In scientific analysis, in general, the term "resolution" is used to describe the precision with which any instrument measures and records (in an image or spectrum) any variable in the specimen or sample under study.

Explanation

The imaging system's resolution can be limited either by aberration or by diffraction causing blurring of the image. These two phenomena have different origins and are unrelated. Aberrations can be explained by geometrical optics and can in principle be solved by increasing the optical quality — and cost — of the system. On the other hand, diffraction comes from the wave nature of light and is determined by the finite aperture of the optical elements. The lens' circular aperture is analogous to a two-dimensional version of the single-slit experiment. Light passing through the lens interferes with itself creating a ring-shape diffraction pattern, known as the Airy pattern, if the wavefront of the transmitted light is taken to be spherical or plane over the exit aperture.

The interplay between diffraction and aberration can be characterized by the point spread function (PSF). The narrower the aperture of a lens the more likely the PSF is dominated by diffraction. In that case, the angular resolution of an optical system can be estimated (from the diameter of the aperture and the wavelength of the light) by the Rayleigh criterion invented by Lord Rayleigh:

Two point sources are regarded as just resolved when the principal diffraction maximum of one image coincides with the first minimum of the other.[1] If the distance is greater, the two points are well resolved and if it is smaller, they are regarded as not resolved. If one considers diffraction through a circular aperture, this translates into:

\sin \theta = 1.220 \frac{\lambda}{D}     

where

θ is the angular resolution in radians,
λ is the wavelength of light,
and D is the diameter of the lens' aperture.

The factor 1.220 is derived from a calculation of the position of the first dark ring surrounding the central Airy disc of the diffraction pattern. The calculation involves a Bessel function—1.220 is approximately the first zero of the Bessel function of the first kind, of order one (i.e., J_{1}), divided by π.

The formal Rayleigh criterion is close to the empirical resolution limit found earlier by the English astronomer W. R. Dawes who tested human observers on close binary stars of equal brightness. The result, θ = 4.56/D, with D in inches and θ in arcseconds is slightly narrower than calculated with the Rayleigh criterion: A calculation using Airy discs as point spread function shows that at Dawes' limit there is a 5% dip between the two maxima, whereas at Rayleigh's limit there is a 20% dip.[2] Modern image processing techniques including deconvolution of the point spread function allow resolution of even narrower binaries.

The angular resolution may be converted into a spatial resolution, Δl, by multiplication of the angle (in radians) with the distance to the object. For a microscope, that distance is close to the focal length f of the objective. For this case, the Rayleigh criterion reads:

\Delta l = 1.220 \frac{ f \lambda}{D}.

This is the size, in the imaging plane, of smallest object that the lens can resolve, and also the radius of the smallest spot to which a collimated beam of light can be focused.[3] The size is proportional to wavelength, λ, and thus, for example, blue light can be focused to a smaller spot than red light. If the lens is focusing a beam of light with a finite extent (e.g., a laser beam), the value of D corresponds to the diameter of the light beam, not the lens.Note Since the spatial resolution is inversely proportional to D, this leads to the slightly surprising result that a wide beam of light may be focused to a smaller spot than a narrow one. This result is related to the Fourier properties of a lens.

A similar result holds for a small sensor imaging a subject at infinity: The angular resolution can be converted to a spatial resolution on the sensor by using f as the distance to the image sensor; this relates the spatial resolution of the image to the f-number, f/#:

\Delta l \approx 1.220 \frac{f \lambda}{D} = 1.22 \lambda \cdot (f/\#).

Specific cases

Single telescope

Point-like sources separated by an angle smaller than the angular resolution cannot be resolved. A single optical telescope may have an angular resolution less than one arcsecond, but astronomical seeing and other atmospheric effects make attaining this very hard.

The angular resolution R of a telescope can usually be approximated by

R = \frac {\lambda}{D}

where

λ is the wavelength of the observed radiation
and D is the diameter of the telescope's objective.

Resulting R is in radians. Sources larger than the angular resolution are called extended sources or diffuse sources, and smaller sources are called point sources.

For example, in the case of yellow light with a wavelength of 580 nm, for a resolution of 0.1 arc second, we need D = 1.2 m.

This formula, for light with a wavelength of ca 562 nm, is also called the Dawes' limit.

Telescope array

The highest angular resolutions can be achieved by arrays of telescopes called astronomical interferometers: These instruments can achieve angular resolutions of 0.001 arcsecond at optical wavelengths, and much higher resolutions at radio wavelengths. In order to perform aperture synthesis imaging, a large number of telescopes are required laid out in a 2-dimensional arrangement.

The angular resolution R of an interferometer array can usually be approximated by

R = \frac {\lambda}{B}

where

λ is the wavelength of the observed radiation
and B is the length of the maximum physical separation of the telescopes in the array, called the baseline.

The resulting R is in radians. Sources larger than the angular resolution are called extended sources or diffuse sources, and smaller sources are called point sources.

For example, in order to form an image in yellow light with a wavelength of 580 nm, for a resolution of 1 milli-arcsecond, we need telescopes laid out in an array that is 120 m × 120 m.

Microscope

The resolution R (here measured as a distance, not to be confused with the angular resolution of a previous subsection) depends on the angular aperture \alpha:

R=\frac{1.22\lambda}{\mathrm{NA}_\text{condenser} %2B \mathrm{NA}_\text{objective}} where \mathrm{NA}=\eta\sin\theta.[4]

Here NA is the numerical aperture, \theta is half the included angle \alpha of the lens, which depends on the diameter of the lens and its focal length, \eta is the refractive index of the medium between the lens and the specimen, and \lambda is the wavelength of light illuminating or emanating from (in the case of fluorescence microscopy) the sample.

It follows that the NAs of both the objective and the condenser should be as high as possible for maximum resolution. In the case that both NAs are the same, the equation may be reduced to:

R=\frac{0.61\lambda}{\mathrm{NA}}\approx\frac{\lambda}{2\mathrm{NA}}

The practical limit for \theta is about 70°. In an air objective or condenser, this gives a maximum NA of 0.95. In a high-resolution oil immersion lens, the maximum NA is typically 1.45, when using immersion oil with a refractive index of 1.52. Due to these limitations, the resolution limit of a light microscope using visible light is about 200 nm. Given that the shortest wavelength of visible light is violet (\lambda ≈ 400 nm),

R=\frac{1.22 \times 400\,\mbox{nm}}{1.45\ %2B\ 0.95} = 203\,\mbox{nm}

which is near 200 nm.

Oil immersion objectives can have practical difficulties due to their shallow depth of field and extremely short working distance, which calls for the use of very thin (0.17mm) cover slips, or, in an inverted microscope, thin glass-bottomed Petri dishes.

However, resolution below this theoretical limit can be achieved using a diffraction technique called 4Pi STED microscopy. By destructively focusing two beams on a point source, objects as small as 30 nm have been resolved.[5]

Common photography

Image at left has a higher pixel count than the one to the right, but is still of worse angular resolution.
Further information: Image resolution

The angular resolution is a major determinant of the quality of common images such as raster digital images, film images, and other types of images. As with telescopy and microscopy, higher angular resolution in general means more image detail. For digital images, the pixel count is another major measure of image quality, and is often given instead of the angular resolution, mainly because of easier measuring.

Notes

1.^ In the case of laser beams, a Gaussian Optics analysis is more appropriate than the Rayleigh criterion, and may reveal a smaller diffraction-limited spot size than that indicated by the formula above.

See also

References

  1. ^ Born, Max; Wolf, Emil (October 1999). Principles of Optics. Cambridge: Cambridge University Press. p. 461. ISBN 0521642221. 
  2. ^ Lenny Abbey. "The Limits of Telescopic Performance". http://labbey.com/Articles/Limits/Limits.html. 
  3. ^ "Diffraction: Fraunhofer Diffraction at a Circular Aperture". Melles Griot Optics Guide. Melles Griot. 2002. https://www.cvimellesgriot.com/products/Documents/TechnicalGuide/fundamental-Optics.pdf. Retrieved 2011-07-04. 
  4. ^ Nikon MicroscopyU: Concepts and Formulas in Microscopy: Resolution
  5. ^ http://www.mpibpc.mpg.de/groups/hell/4Pi-STED.htm

External links