Method for measuring optical attenuation in a liquid medium
Inventors
Assignees
US Department of Navy • Government of the United States of America
Publication Number
US-9983055-B1
Publication Date
2018-05-29
Expiration Date
2036-05-13
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Abstract
The present invention provides a meter and method of use for measuring an optical attenuation coefficient in a liquid medium. In operation, a collimated beam, produced by a laser of the attenuation meter apparatus, propagates thru the liquid medium with filtered back-scattered light arriving at a camera of the meter. A light image is formed at a focal plane of the camera. The light image is recorded and analyzed by a microcomputer to provide optical beam attenuations coefficients.
Core Innovation
The invention provides a meter and method for measuring an optical attenuation coefficient in a liquid medium. In operation, a collimated laser beam propagates through the liquid medium, generating filtered back-scattered light that arrives at a camera within the attenuation meter. A light image is formed at the focal plane of the camera, recorded, and analyzed by a microcomputer to determine the optical beam attenuation coefficient.
The problem addressed by the invention arises due to limitations in prior art meters, which use optical paths less than one meter, insufficient for accurate measurements in clear water where attenuation lengths can exceed eight meters. Short path meters struggle with maintaining optical surface cleanliness, measurement electronics accuracy, and calibration precision, leading to non-repeatable and unusable measurements in clear water environments.
To solve these issues, the invention utilizes a measurement approach with long propagation paths (approximately one half of the expected maximum attenuation length) and controls the laser beam diameter to maintain collimation while minimizing forward scattering contributions. This enables accurate measurement of optical beam attenuation coefficients even in clear water or other liquid mediums with low attenuation rates.
Claims Coverage
The patent contains one independent claim focusing on a method for measuring optical attenuation in a liquid medium with multiple inventive features.
Method for measuring optical attenuation using a specialized attenuation meter
The method employs an attenuation meter comprising an optical window separating the interior from the liquid, a continuous wave laser inside the meter, an optical filter that passes laser wavelength light after passing through the window, a camera with a focal plane, sensors connected to the focal plane, and a microcomputer connected to the sensors.
Choosing a maximum water range as half of a maximum attenuation length
The method specifies selecting a maximum water range equal to half of the anticipated maximum attenuation length to optimize measurement accuracy.
Projecting a collimated laser beam and using back-scattered light
The method includes projecting a collimated laser beam into the liquid medium, generating back-scattered light within the medium, passing this back-scattered light through the optical filter, and forming a light image of the filtered back-scattered light.
Sensing and analyzing the light image to determine the beam attenuation coefficient
Sensors detect the light image, transmit corresponding data to the microcomputer, which analyzes the data and determines the beam attenuation coefficient in the liquid medium based on the transmitted data.
Maintaining beam collimation and managing forward scattering
Further steps include producing the laser beam diameter to maintain collimation over one half the inverse of the beam attenuation coefficient and managing to achieve a maximum amount of forward scattering by the laser beam.
Graphing electron counts per pixel and fitting to attenuation coefficients
Graphing the number of electrons within the light image per pixel as a function of water range and fitting data from these graphs to predicted results is used to refine measurement of attenuation coefficients.
These inventive features collectively define a method and apparatus that measure optical attenuation coefficients in a liquid medium with enhanced accuracy by managing beam propagation distance, laser beam properties, and analyzing back-scattered light images.
Stated Advantages
Ability to obtain measurements over long propagation paths, thus providing accurate measurements of optical beam attenuation coefficients.
Use of relatively low-cost continuous wave lasers instead of short duration pulsed lasers.
Capability of deployment as a self-contained module powered by batteries and operable on various platforms such as unmanned underwater vehicles or remote platforms via umbilical cables.
Ability to contain multiple color light sources to provide measurements at more than one wavelength.
Documented Applications
Measurement of optical beam attenuation coefficients in clear water environments.
Measurement of optical attenuation in other liquid mediums that have backscattering based on density fluctuations producing an index of refraction.
Deployment on vehicles such as unmanned underwater vehicles or from separate platforms with power and data access through umbilical cables.
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