Lateral surface sensor and imaging optical system therefor

An imaging optical system for directing a light beam along a path emanating from peripherally extending lateral surfaces of an object to an imaging plane comprising:

• a reflecting element;
• a first angle-reducing element arranged downstream along the path in the direction of the light beam after said reflecting element, said first angle-reducing element being capable of reducing the angle between the light beam and an optical axis;
• and
• a second angle-reducing element arranged upstream from said reflecting element along the path against the direction of the light beam, said second angle-reducing element being capable of reducing the angle between the light beam and the optical axis;
• wherein said first angle-reducing element has a surface being one of a reflecting surface and a refracting surface, said surface being a peripheral surface of a cone and is so oriented that angle reduction is effected by one of reflection and refraction of the light beam, and wherein said reflecting element has a reflection surface formed by a boundary surface of a transparent cylinder.