Expendable beam transmissometer

• US 20030174317A1
• Filed: 01/22/2003
• Published: 09/18/2003
• Est. Priority Date: 01/25/2002
• Status: Active Grant

• Current Assignee: James T. Velky, Karen D. Rennich, Kirk S. Decker, Thomas M. Murdock
• Original Assignee: James T. Velky, Karen D. Rennich, Kirk S. Decker, Thomas M. Murdock
• Inventors: Decker, Kirk S., Murdock, Thomas M., Rennich, Karen D., Velky, James T.
• Granted Patent: US 7,068,362 B2
• Time in Patent Office: Days
• US Class Current: 356/218
• CPC Class Codes: G01J 1/42 using electric radiation detectors optical or mechanical part G01J1/04; by comparison with a reference light or electric value G01J1/10
• CPC Class Codes: G01N 21/538 for determining atmospheric attenuation and visibility

Abstract

The present invention provides apparatus for a beam transmissometer. In an embodiment, the beam transmissometer includes a LED signal source, collimating apparatus, a retroreflector that directs the projected beam, imaging apparatus that directs the beam onto a detector that converts the projected beam into an electrical signal, and signal processing circuitry that enhances the reception of the optical beam. The LED signal source is modulated in a pulsed fashion so that the effects of beam reflection are ameliorated. Signal processing circuitry amplifies, filters, and synchronizes to the electrical signal. A calculating unit uses the processed signal to determine a beam attenuation coefficient, which is indicative of the visibility of a medium in which the transmissometer is immersed. A method is also provided for aligning optics of a beam transmissometer. The method determines an offset and a rotation of a retroreflector housing that causes an optical beam to be centered.

Claims

1. An apparatus for determining a beam attenuation coefficient of a medium, comprising:

a signal source that generates a generated optical beam;

a first arrangement that collimates the generated optical beam to form a projected optical beam;

a retroreflector that directs the projected optical beam;

a second arrangement that receives the projected optical beam from the retroreflector and that focuses the projected optical beam to form a focused optical beam;

a detector that detects a portion of the focused optical beam in order to produce an electrical signal so that the beam attenuation coefficient can be determined.

2. The apparatus of claim 1, further comprising:

a synchronizer that causes the signal source to generate the generated optical beam in a pulsed pattern and that causes the detector to detect the portion of the focused optical beam in concert with the pulsed pattern.

3. The apparatus of claim 2, wherein the signal source comprises a light emitting diode (LED) and wherein the synchronizer comprises:

a timer that causes the LED to generate the generated optical beam in concert with a pulse rate;

a lock-in amplifier that processes the electrical signal in concert with the pulse rate, whereby an effect of distortion that is incurred by the generated optical beam traversing between the LED and the first arrangement, by the projected optical beam traversing between the first arrangement and the second arrangement, and by the focused optical beam traversing between the second arrangement and the detector is ameliorated.

4. The apparatus of claim 3, wherein the lock-in amplifier comprises:

a bandpass filter that amplifies a pulsed component of the electrical signal and attenuates a noise component of the electrical signal;

a mixer that samples the pulsed component of the electrical signal in order obtained a locked signal; and

a low pass filter that converts the locked signal into a baseband signal.

5. The apparatus of claim 4, further comprising:

a transmitter that receives the baseband, determines information about the beam attenuation coefficient, and sends the information to a calculating unit.

6. The apparatus of claim 5, wherein the beam transmissometer further comprises the calculating unit.

7. The apparatus of claim 6, wherein the calculating unit comprises a processor and wherein the processor is configured to perform:

(a) determining the beam attenuation coefficient from the information, wherein the beam attenuation coefficient is determined from a pulse period measurement; and

(b) averaging the beam attenuation coefficient for a plurality of pulse period measurements.

8. The apparatus of claim 7, wherein the processor is configured to perform:

(c) displaying the beam coefficient on a display that is associated with the calculating unit.

9. The apparatus of claim 1, wherein the signal source comprises a light emitting diode (LED).

10. The apparatus of claim 9, wherein the generated optical beam has a wavelength between approximately 455 to 575 nanometers.

11. The apparatus of claim 1, further comprising:

a processor that receives the electrical signal from the detector, the processor configured to perform;

(a) pulsing the signal source when generating the generated optical beam; and

(b) synchronizing the detector with the generated optical beam.

12. The apparatus of claim 11, wherein (b) comprises:

(i) amplifying a pulsed component of the electrical signal in order to reduce a noise component of the electrical signal;

(ii) sampling the pulsed component in order to obtain a locked signal in concert with the generated optical beam; and

(iii) converting the locked signal to a baseband signal.

13. The apparatus of claim 12, wherein the processor configured to perform:

(c) calculating the beam attenuation coefficient from the baseband signal.

14. The apparatus of claim 1, wherein the detector comprises:

a photodiode that converts the portion of the focused optical beam.

15. The apparatus of claim 14, further comprising:

a amplifier that compensates for a capacitance associated with the photodiode.

16. The apparatus of claim 1, wherein the medium is selected from the group consisting of a sea water medium, a fresh water medium, and an air medium.

17. The apparatus of claim 1, wherein the apparatus utilizes a diverging collimated beam (DCB) approach.

18. The apparatus of claim 1, wherein the apparatus utilizes a cylindrically limited beam (CLB) approach.

19. The apparatus of claim 1, wherein the retroreflector is situated in a noise/retroreflector housing, and wherein the signal source, the first arrangement, the second arrangement, and the detector are situated in an electro-optics housing, the beam transmissometer further comprising:

a housing support frame.

20. The apparatus of claim 1, further comprising:

a window that seals the first arrangement and the second arrangement from the medium.

21. A beam transmissometer for determining a beam attenuation coefficient of a medium, comprising:

a signal source that generates an optical beam;

a detector that detects a portion of the optical beam to obtain an electrical signal; and

a synchronizer that causes the signal source to generate the optical beam in concert with a pulsed pattern and that causes the detector to detect a portion of the optical beam in concert with the pulsed pattern.

22. The beam transmissometer of claim 21, wherein the synchronizer comprises:

a timer that causes the signal source to generate the optical beam in concert with a pulse rate;

a lock-in amplifier that processes the electrical signal in concert with the pulse rate.

23. The transmissometer of claim 22, wherein the lock-in amplifier comprises:

a bandpass filter that amplifies a pulsed component of the electrical signal and attenuates a noise component of the electrical signal;

a mixer that samples the pulsed component of the electrical signal in order obtained a locked signal; and

a low pass filter that converts the locked signal into a baseband signal.

24. The beam transmissometer of claim 21, wherein the signal source is selected from the group consisting of a light emitting diode (LED) and an optical laser.

25. The beam transmissometer of claim 21, wherein the pulsed pattern is periodic.

26. The beam transmissometer of claim 25, wherein the pulsed pattern has a corresponding frequency between approximately 100 Hz and 500 Hz.

27. The beam transmissometer of claim 21, wherein the pulsed pattern is aperiodic.

28. A beam transmissometer for determining a beam attenuation coefficient in a water medium, comprising:

a light emitting diode (LED) that generates a generated optical beam;

a first arrangement that collimates the generated optical beam to form a projected optical beam;

a retroreflector that directs the projected optical beam;

a second arrangement that receives the projected optical beam from the retroreflector and that focuses the projected optical beam to form a focused optical beam;

a photodiode that detects a portion of the focused optical beam in order to produce an electrical signal;

a synchronizer that causes the LED to generate the generated optical beam in a pulsed pattern and that causes the photodiode to detect the portion of the focused optical beam in concert with the pulsed pattern, wherein the synchronizer comprises;

a timer that causes the LED to generate the generated optical beam in concert with a pulse rate;

a bandpass filter that amplifies a pulsed component of the electrical signal and attenuates a noise component of the electrical signal;

a mixer that samples the pulsed component of the electrical signal in order obtained a locked signal; and

a low pass filter that converts the locked signal into a baseband signal; and

a transmitter that receives the baseband signal, determines information about the beam attenuation coefficient, and sends the information to a calculating unit.

29. A method for aligning optical components of a beam transmissometer, the method comprising the steps of:

(a) constraining a housing support frame of the beam transmissometer to minimize movement of the housing support frame;

(b) activating a signal source in order to produce an optical beam;

(c) placing a retroreflector mounting into a far side of the housing support frame, wherein a corner cube has a first offset with respect to a centerline of the housing support frame;

(d) positioning an electro-optics housing on a near side of the housing support frame, wherein the electro-optics housing comprises projecting and receiving apparatus for the optical beam;

(e) verifying that the optical beam is centered in a receiving bore of the electro-optics housing;

(f) in response to step (e), if the optical beam is not centered, rotating the retroreflector mounting;

(g) in response to step (f), if the optical beam cannot be centered by rotating the retroreflector mounting, reconfiguring the retroreflector mounting, wherein the corner cube has a second offset; and

(h) in response to step (g), repeating steps (e)-(g).

30. The method of claim 29, further comprising the step of:

(i) in response to step (h), anchoring the corner cube in the retroreflector housing.

31. The method of claim 30, further comprising the step of:

(j) in response to step (i), mounting a receiving diode in the receiving bore of the electro-optics housing.

32. The method of claim 29, wherein the first offset and the second offset are selected from the group consisting of approximately zero inches, 0.005 inches, 0.010 inches, and 0.015 inches.