Aperture and field stops

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Aperture stop and image brightness

  • The aperture stop of an imaging system is the optical element that limits angle of rays passing through the system from a source on the optical axis.
  • For an off-axis object, the chief ray is the ray that passes through the center of the aperture stop, and the marginal rays are those that pass through the edge of the aperture stop.
  • The size of the aperture stop determines the light-gathering capability of an instrument, and thus the brightness (or irradiance in W/m2) of its images.
  • Typically in a microscope, as opposed to a photographic camera, the aperture stop will be the objective lens rather than an aperture before the lens as shown here.
The aperture stop in system can be (top) the left diaphragm, (middle) the first, left lens, or (bottom) the second, right lens.
Light-gathering power of oil-immersion and air-immersion lens, showing that $ \alpha_{oil} > \alpha_{air} $.

Numerical aperture

  • The numerical aperture (NA) is another important measure that characterizes the light-gathering power of a lens.
$ NA = n\ \sin \alpha $
where $ n $ is the index of refraction of the medium between the object (sample) and the lens and $ \alpha $ is the half-angle defined by the limiting ray. The limiting ray is is the last ray not blocked by any aperture.
  • An oil-immersion objective lens will thus have a greater NA than an air-immersion objective lens ($ n_{oil} > n_{air} $ and $ \alpha_{oil} > \alpha_{air} $; indeed, little refraction occurs at the glass-oil interface where $ n_{oil} \approx n_{glass} $). Notice the much smaller angle of the limiting ray (drawn, but not labeled, near the point of origin in the diagram) in the case of air-immersion lens.

Field stop and field of view

  • The field stop is the optical element that blocks off-axis rays passing through the center of the aperture stop (i.e. the chief rays).
  • The field stop determines how much of the object can be viewed, in other words: the field of view of the imaging system.
  • In your 20.309 microscope, the field stop will be the CCD camera.
  • A finite-size lens can also act as a field stop. Below is an illustration of this vignetting effect: a cone of light rays from an off-axis object is not transmitted in its entirety, but rather partially cut off by the field stop of the system, L2's rim.
20.309 130822 FieldStop.png