System for detecting ice or snow on surface which specularly reflects light

Apparatus for detecting the presence of a polarization-altering substance on a specularly reflecting surface comprising:

• means for transmitting light over a first path to said surface,
• means for receiving transmitted light returning on a second path from said surface, and
• optical means in at least one of said first path and said second path;
• said optical means including means for alternating between an optical isolator state and an optical non-isolator state for producing a difference in an intensity of said returning transmitted light between said returning transmitted light from said polarization-altering substance and said returning light from said specularly reflecting surface such that, when said substance is present on said surface, a difference in an intensity of light received at said means for receiving exists between said states, said difference resulting from more light being received from one of said surface and said substance than from the other thereof in one of said states than in the other of said states;
• and
• means for detecting said difference, whereby the presence of said polarization-altering substance is detectable.

Apparatus for detecting the presence of a dielectric substance on a specularly reflecting surface comprising:

• means for transmitting light over a transmitting path to said surface;
• means for receiving returning light on a receiving path from said surface;
• optical means in at least one of said transmitting path and said receiving path;
• and
• means for alternating said optical means between an optical isolator state and an optical non-isolator state, whereby a brightness of said returning light from said specularly reflecting surface changes in a manner different from a change in brightness of returning light from said dielectric substance.

Apparatus for detecting the presence of a dielectric substance on a specularly reflecting surface comprising:

• means for transmitting light over a transmitting path to said surface;
• first means for receiving returning light on a first receiving path from said surface;
• second means for receiving returning light on a second receiving path from said surface;
• optical means in at least one of said transmitting path, said first receiving path and said second receiving path;
• said optical means being effective for creating an optical isolator state in one of said first and second receiving paths, and an optical non-isolator state in the other of said first and second receiving paths, said states differing in an intensity of light received at said first and second means for receiving said difference resulting from more light being received from one of said surface and said substance than from the other thereof in one of said states than in the other of said states;
• and
• means for detecting said difference.