A ratiometric sensor is formed of a first photodiode connected in series with an electronic component, such as a second photodiode or a resistor, between a source of first voltage and a source of second voltage. The ratiometric sensor supplies a "ratiometric voltage" on a ratiometric sensor output line...http://www.google.co.uk/patents/US5973313?utm_source=gb-gplus-sharePatent US5973313 - Method and apparatus for generating ratiometric control signals
