Ruggedized homogeneous thin dielectric film focus sensor

Focus detecting apparatus comprising:

• a source of light for producing a polarized beam;
• beam splitter means receiving the polarized beam and transmitting a part thereof to a first reflecting surface for reflection back to the beam splitter means to produce first and second beam portions, the first beam portion being reflected for use by a power monitor and the second portion being transmitted through lens means to a recording medium, the recording medium reflecting light back through the lens means, the lens means operable to collimate the reflected light from the recording medium when the recording medium is located a first distance from the lens means, the lens means transmitting the reflected light to the beam splitter means to produce third and fourth beam portions, the third beam portion being reflected for use by a data detector and the fourth portion being reflected in a first path;
• first means having a first index of refraction positioned in the first path to receive light in the fourth portion from the beam splitter means, said first means having a first surface positioned at a first angle with respect to light in the fourth portion received from the lens means;
• second means having the first index of refraction to receive the light passing through the first surface and having a second surface parallel to but displaced from the first surface by a medium having a second index of refraction, the first and second means being positioned so that the incidence angle at the first surface provides substantially total internal reflection;
• first and second detector means for receiving the light reflected from the first surface and producing first and second outputs in accordance therewith, said first and second detector means for receiving substantially the same amount of light when light from the lens means is collimated but different amounts of light when not collimated;
• third and fourth detector means for receiving the light transmitted through the first and second surfaces and producing third and fourth outputs in accordance therewith, the third and fourth detector means receiving substantially the same amount of light when light from the lens means is collimated but different amounts of light when not collimated;
• and
• signal receiving means connected to the first, second, third and fourth detectors to receive the first, second, third and fourth outputs to provide a resultant output indicative of the receipt of light in different amounts by the first and second detectors and by the third and fourth detectors as an indication that the object is at other than the predetermined distance from the lens means.