optical square [′äp·tə·kəl ′skwer]
Optical Square
(engineering)
A surveyor's hand instrument used for laying of right angles; employs two mirrors at a 45° angle.
Optical Square
a very simple geodetic instrument that is used to lay off angles that are multiples of 90° or of 45°.
In a prism square (see Figure 1), the direction of a ray originating from the flag on the right is changed by 90° after refraction and internal reflection. To lay off the angle by which the direction of the ray is changed, a second flag is set up in such a way that, when it is seen over the instrument, it is exactly above the image of the first flag in the instrument.
Figure 1. A prism square
Optical squares are used to solve geometric problems on a terrain and to carry out plane surveys.
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