Prism Block for Suppressing Decrease of Amount of Light by Medium, Optical Unit, and Scanner Optical System

An optical unit comprising an optical element and a prism block,

• the prism block being configured to reflect a light flux, entering the prism block via a first portion of the prism block, at least once in the prism block to exit from the prism block via a second portion of the prism block,
• the prism block being provided with at least one notch to propagate the light flux in air on an optical path from the entrance of the light flux via the first portion to the exit of the light flux via the second portion,
• the notch being configured such that the light flux exiting from inside of the prism block via a first interface of the notch between the prism block and the air re-enters the prism block via a second interface between the air and the prism block, the second interface facing the first interface,
• the optical unit having at least one of a first optical element and a second optical element, the first optical element being disposed at a stage before the first portion, the second optical element being disposed at a stage after the second portion.

A scanner optical system comprising:

• a light source configured to emit a light flux to a document to be scanned;
• a prism block configured to permit the light flux reflected by the document to enter the prism block via a first portion of the prism block and to reflect the light flux at least once in the prism block to exit from the prism block via a second portion of the prism block, the first portion having a long side direction in a first direction, the second portion having a long side direction in the first direction;
• and
• a light receiving portion configured to detect the light flux exiting from the prism block,
• the prism block being provided with at least one notch to propagate the light flux in air on an optical path from the entrance of the light flux via the first portion to the exit of the light flux via the second portion, the notch having a long side direction in the first direction,
• the notch being configured such that the light flux exiting from inside of the prism block via a first interface of the notch between the prism block and the air re-enters the prism block via a second interface between the air and the prism block, the second interface facing the first interface.