Night glass

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Fig. 1: Night glass 9x63 (9x magnification, 63 mm aperture)

A night binoculars is a binoculars with larger exit pupil , which is optimized for twilight and night vision. In contrast to the night vision device , the optics are passive, i.e. without light amplification.

function

The exit pupil AP of a telescope is as large as the entry pupil EP , reduced by the magnification factor V :

AP = EP / V

The eye only sees the part of the beam that passes through the eye pupil . During the day the pupil of the eye measures approx. 2 mm. At night it is widened to 5 to 8 mm, correspondingly the exit pupil of a night lens is adapted to 5–7 mm. It is more than twice the size of normal binoculars, and the objective lenses are enlarged accordingly.

Typical night glasses have an exit pupil of 7 mm and the dimensions 7x50, 8x56 and 9x63.

Despite the great surface brightness that you achieve in the dark, night glasses are usually not the first choice for astronomical purposes, as the residual light, which can come from light pollution , the moon or the sun, is also amplified by the night glass, so no more contrast is achieved .

Observing the nocturnal environment with good night glasses allows a wealth of details to be recognized even where observation with the naked eye does not suggest any details at all. Due to this property, night glasses are of particular importance in marine and hunting.

Examples

Fig. 2: Exit pupils of a telescope (left) and a night glass.

The 25x30 telescope on the left in Figure 2 is optimized for daylight because the exit pupil is smaller than 2 mm. It has a large twilight factor of 27, but because of the small exit pupil it depicts the surroundings darker than without aids. The 9x63 binoculars to the right of the telescope are night glasses. The exit aperture of approx. 7 mm can be clearly seen. The twilight factor is also approx. 25, but is adapted to the dilated pupil of a night-adapted eye.

Individual evidence

  1. ^ Paul Dobrinski , Gunter Krakau, Anselm Vogel: Physics for engineers . Vieweg + Teubner, Wiesbaden 2006, ISBN 3-8351-0020-3 , page 372.