Exit pupil

The Exit Pupil is the image-side image of the aperture stop of an optical system. Is conjugated to the entrance pupil. For optical devices for direct visual observation - eg telescopes and binoculars - is called the exit pupil, or exit pupil diameter of the beam leaving the eyepiece.

Lenses

The distance of the exit pupil to the image plane determines the angle θ in which the chief rays are incident on the image plane:

In the image side and both sides telecentric lenses, the exit pupil is at infinity, so is the entire image plane that is the main beams hit everywhere perpendicular to the image plane.

Conversely, at the object side, telecentric lenses, the exit pupil in the focal plane and thus very close to the image plane. Therefore, the main jets collide with increasing distance from the image center quickly very flat on the picture plane. Would the image plane are also located in the focal plane, that is, would the main beam would be parallel to the image plane and therefore never encounter this. Therefore, can be achieved only in close- sharp images with object- side telecentric systems.

Devices for visual observations

As mentioned, in telescopes and binoculars, is the diameter of the beam which emerges from the eyepiece, as the exit pupil ( exit pupil ) respectively. If the magnification of the instrument is increased, then the exit pupil of the eyepiece reduced.

Determining the size of the exit pupil

You point the instrument on a light surface, preferably in the daytime sky, but not in the immediate vicinity of the Sun. It is focused on a distant object as possible. In telescopes of the type Maksutov or Schmidt -Cassegrain is important to note that changes the focal length depending on the focus position. Therefore telescopes of this type should be focused on "infinity". The size of the exit pupil can be easily determined coarse when viewed at a distance of about 30 cm on the eyepiece of the instrument. The exit pupil appears as a bright light in the eyepiece slices.

More accurate determination of the size of the exit pupil

If the exit pupil can be measured more accurately, a screen is held in front of the eyepiece. With enough light, a thin sheet of graph paper suffice. It changes the distance of the focusing screen or the paper to the eyepiece until a bright slices with sharp- lined edge maps. The use of graph paper is obtained by counting the millimeter lines the value for the size of the exit pupil ( in millimeters). This value should be as large as the pupil of the eye - 1 to 2 mm in daylight seeing, between 6 and 8 mm in night vision.

Is the exit pupil is not adapted to the entrance pupil of the eye, an object with the optical device appears dark as seen with the naked eye.

The pupil of the eye forms the aperture of the eye. A small exit pupil of an optical device it does not fully light. It limits according to the aperture and reduces the diffraction-limited resolution.

Meaningful exit pupil

At the exit pupil of an instrument is to be noted that a maximum of the pupil dilates of a young, healthy eye to about 7 mm, and on the other hand barely bring on the retina exit pupil less than 1 mm a perceptual gain. 0.5 mm present here represents the absolute limit, the only good eye and a very good instrument, however, are still usable.

, The exit pupil is limited to the eye, both the minimum as well as the maximum usable magnification of an optical instrument.

The exit pupil of a given eyepiece is in telescopes and binoculars from the aperture ratio determined (focal length / lens opening) of the instrument.

The diameter of the exit pupil AP (in mm ) was calculated as the ratio of the lens aperture D ( in mm ) and the magnification M of an instrument:

The geometric luminosity LG of an optical instrument is defined as the square of the diameter of the exit pupil (AP ²). Example: A 8x40 binoculars (8x magnification and 40 mm entrance pupil ) has a brightness index of 25, according to:

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Related Topics

• Entrance pupil
• Aperture (optics)
• Telescope
• Telecentric lens