Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1992-07-31
1993-09-07
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
340583, G02F 101
Patent
active
052431851
ABSTRACT:
An apparatus and method that accurately and reliably detects ice on a target surface (20). Intrinsic birefringent properties of hexagonal ice crystals are used to detect ice by analyzing elliptically-polarized reflection of linearly-polarized radiation from ice. A linearly-polarized radiation beam (15) is focused on the target surface (20), and returned radiation (25) is filtered (30) as a function of its elliptical polarization. Filtered radiation (35) is directed to a sensor (40) and quantified according to the intensity of radiation received at various points (41.sub.1 -41.sub.n) on the filter (30), and an output signal for each point is produced (45.sub.1 -45.sub.n). The presence or formation of ice is determined by a signal processor (50) that detects the variance among the output signals (45.sub.1 -45.sub.n)--a variance (55) reaching a threshold value being indicative of ice. Filtering unit (30) may be composed of two polarizing filters (31 and 33), rotatable with respect to each other on a common axis and disposed in a parallel plane therewith. A reading is taken from all the output signals (45.sub.1 -45.sub.n), and an output signal variance (55) is calculated. The filters (31 and 33) are then rotated with respect to each other (e.g., several degrees), another reading is taken, and another variance (55) is calculated. This read-calculate-rotate cycle is repeated continuously. Thus, this embodiment is more accurate since it relies on a greater number of samples, and such samples are taken over numerous time periods. Also, because the output from the filter (33) is dynamically changing as the two filters (31 and 33) are rotated, the presence of ice is easier to detect visually.
REFERENCES:
patent: 4327286 (1982-04-01), Thoma
patent: 4668860 (1987-05-01), Anthon
patent: 4701052 (1987-10-01), Schoen
patent: 4725145 (1988-02-01), Azzam
patent: 5170049 (1992-12-01), De Jonge et al.
Le Que T.
Loral Aerospace Corp.
Nelms David C.
Radlo Edward J.
Sueoka Greg T.
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