1.  Field of the Invention
Embodiments of the present invention relate to an optical transmission element, an image guide or a light guide which includes a plurality of the optical transmission elements, and an endoscope including at least one of the image guide and the light guide. 
2.  Description of the Related Art
In a conventional endoscope, a glass optical fiber containing lead is passed through an insertion portion of the endoscope in order to secure brightness during observation.  Such a lead-containing optical fiber is used to transmit illumination light toward a tip part from a light source. 
The lead-containing optical fiber has excellent transmissivity and light distribution properties.  However, since detrimental substances such as lead are strictly regulated, the development of a lead-free optical fiber has been advanced.  However, the lead-free optical fiber has higher hardness in physical properties and lower flexibility than those of the lead-containing optical fiber.  For this reason, when the lead-free optical fiber is repeatedly severely bent in the tip part of the endoscope, the lead-free optical fiber is disadvantageously apt to be broken.  When the optical fiber is broken, the observation performance of the endoscope is deteriorated. 
An optical transmission element is disclosed as an optical fiber for solving such a problem in Japanese Patent No.  4229890.  In the optical transmission element, a covering layer including a fluorine-substituted alkyl group-containing organic silicon compound (hereinafter, a fluorinated alkylsilane layer) is formed on the outer periphery of a fiber.