A lens having at least one surface that is formed like a portion of a cylinder.Cylindrical lenses are used to correct astigmatism in the eye,and,in rangefinders,to produce astigmatism,stretching a point of light into a line.
An optical device for dividing a beam into two or more separate beams.A simple beamsplitter may be a very thin sheet of glass inserted in the beam at an angle to divert a portion of the beam in a different direction.A more sophisticated type consists of two right-angle prisms cemented together at their hypotenuse faces.The cemented face of one prism is coated,before cementing,with a metallic or dielectric layer having the desired reflecting properties,both in percentage of reflection and the desired color.
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