Apparatus for measuring the optical transmissive power of the atmosphere

• US 4,225,245 A
• Filed: 05/26/1978
• Issued: 09/30/1980
• Est. Priority Date: 05/31/1977
• Status: Expired due to Term

• Current Assignee: Societe Nationale D'Etude Et De Construction De Moteurs D'Aviation
• Original Assignee: Societe Nationale D'Etude Et De Construction De Moteurs D'Aviation
• Inventors: Salles, Aime, Roiret, Michel, Dupitier, Michel
• Primary Examiner(s): Corbin, John K.
• Assistant Examiner(s): Arnold, Bruce Y.
• Application Number:
  US05/909,953
• Time in Patent Office: 858 Days
• Field of Search: 356/432-435, 356/437-442, 356/414, 250/205
• US Class Current: 356/437
• CPC Class Codes: G01N 21/538 for determining atmospheric attenuation and visibility

Abstract

An apparatus for measuring the optical transmissive power of the atmosphere, having particular application to measuring visibility at airports, in which luminous flux from a source passes to a remote first detector through the atmosphere, and flux from the source also passes to a second detector through a fibre optic arrangement. The output of the second detector is compared with a reference signal and the difference signal from the comparator is fed to a variable gain amplifier which controls the light source to remove errors in the output of the first detector which would otherwise occur by virtue of aging of the detectors and/or by virtue of changes in the luminosity of the source. Also described is an arrangement for compensating for accumulation of dirt on the detectors and/or the luminous flux source without cleaning off the dirt.

Claims

1. An apparatus for measuring optical transmissive power of the atmosphere comprising:

(a) an electrically driven source for producing luminous flux;

(b) a current supply device arranged to supply an electric current to drive said source,

(c) a receiver station remote from said source and including first and second detectors for producing electrical signals indicative of luminous flux incident thereon from said source, said detectors having substantially the same operating characteristics, said first detector being arranged to receive luminous flux from said source through the atmosphere;

(d) a fibre optic light guide arranged to supply light directly from said source to said second detector;

(e) means for establishing a substantially constant reference signal;

(f) a comparator for comparing the signal produced by said second detector with the said reference signal, the comparator being arranged to produce an output signal in accordance with said comparison; and

(g) means for varying the electric current supplied by said current supply device in accordance with said output signal and in such a manner as to cancel the difference between electrical signals produced by said second detector and said reference signal.

2. Apparatus as claimed in claim 1 including a filter having a spectral response proportional to the spectral response of the light guide and situated in the path of luminous flux from said source which impinges on said first detector.

3. Apparatus as claimed in claim 1 including a filter arranged in the path of light travelling in said fiber optic guide from said source to said second detector, said filter having a spectral response so arranged to render the spectral bandwidth of the flux transmitted to the second detector independent of the spectral bandwidth of the luminous flux produced by said source.

4. Apparatus as claimed in claim 1 including variable gain amplifier means arranged to amplify the signal produced by said first detector, and means for selectively varying the gain of the amplifier means in dependence upon operating conditions of the apparatus.

5. Apparatus as claimed in claim 4 including a store adapted to store predetermined gains for said amplifier, and operator controlled means for selecting a particular one of the stored gains from the store.

6. Apparatus as claimed in claim 5 including means for automatically selecting a predetermined one of said gains in response to a cessation of operation of the apparatus.

7. Apparatus as claimed in claim 5 including a control loop for controlling the gain of the amplifier in accordance with the selected stored value.