Multi-wavelength absorption water sensor with high ultraviolet wavelength resolution and extended visible and near infrared measurement range
An apparatus and method for measuring absorption of a liquid at several highly resolved wavelengths in the ultraviolet light spectrum with additional wavelengths or wavelength groups in the visible and/or near infrared light spectrum is provided. The apparatus includes a submersible housing that incorporates a broad band light source that emits ultraviolet, visible, and near infrared light, optical windows defining a measurement region of the liquid, an optical chopper unit to alternate between reference and measured light beams, an optical sensor to detect a specific wavelength or group of wavelengths in the visible or near infrared light spectrum from the broadband light source, a slit to define the broadband light geometry onto a diffraction grating, the diffraction grating to modify the broad band light beam into highly wavelength resolved ultraviolet wavelengths, and a line sensor to detect the highly wavelength resolved ultraviolet light.
1 . A submersible apparatus for measuring light absorption of a liquid sample surrounding the submersible apparatus, the submersible apparatus comprising:
a broadband light source configured to output broadband light in an ultraviolet light spectrum and light in one or more of a visible light spectrum and a near infrared light spectrum;
a measurement region configured to include the liquid sample, which propagates the broadband light emitted from the broadband light source;
a light beam splitter configured to split the broadband light, after passing through the measurement region, into a first light beam and a second light beam;
a light sensor configured to detect an intensity of visible or near infrared light from the first light beam;
an optical slit configured to produce an approximate point source of light from the second light beam;
a diffraction grating configured to resolve the second light beam propagating from the optical slit into a plurality of resolved and substantially adjacent narrow ultraviolet bandwidth regions; and
a line sensor array configured to detect a plurality of individual light intensities corresponding to the substantially adjacent narrow ultraviolet bandwidth regions produced by the diffraction grating.
2 . The submersible apparatus of claim 1 , further comprising an optical splitter configured to substantially split the broadband light emitted from the broadband light source, wherein:
a first part of the emitted light is directed to propagate through the measurement region containing the liquid sample and constitutes a measurement beam; and
a second part of the emitted light is directed around the measurement region and constitutes a reference beam.
3 . The submersible apparatus of claim 2 , wherein:
the measurement beam and the reference beam comprise substantially identical light bandwidths.
4 . The submersible apparatus of claim 2 , wherein:
the measurement beam and the reference beam comprise substantially identical amplitudes.
5 . The submersible apparatus of claim 2 , wherein:
the optical splitter comprises ultraviolet transmitting fused silica fiber optics.
6 . The submersible apparatus of claim 2 , further comprising an actuator and shutter assembly, wherein the actuator and shutter assembly is configured to alternately move between:
a first position whereby the shutter substantially blocks propagating light of the measurement beam; and
a second position whereby the shutter substantially blocks propagating light of the reference beam.
7 . The submersible apparatus of claim 6 , further comprising a controller connected to the broadband light source, the light sensor, and the line sensor array, wherein the controller is configured to:
record and output the intensity measurements of visible or near infrared light detected by the light sensor;
record and output the intensity of the plurality of individual light intensities corresponding to the ultraviolet bandwidth regions detected by the line sensor array; and
calculate an absorbance of light by the liquid sample using the recorded intensity measurements of ultraviolet and visible or near infrared light.
8 . The submersible apparatus according to claim 7 , wherein the controller is further configured to:
separately record intensity measurements during periods when the shutter is in the first position and during periods when the shutter is in the second position; and
calculate the absorption of the liquid sample with compensation for changes in the raw light output of the broadband light source.
9 . The submersible apparatus of claim 1 , further comprising a controller connected to the broadband light source, the light sensor, and the line sensor array, wherein the controller is configured to:
record and output the intensity measurements of visible or near infrared light detected by the light sensor, and the intensity of the plurality of individual light intensities corresponding to the ultraviolet bandwidth regions detected by the line sensor array; and
calculate an absorbance of light by the liquid sample using the recorded intensity measurements of ultraviolet and visible or near infrared light.
10 . The submersible apparatus of claim 1 , wherein:
the diffraction grating is further configured to resolve the second light beam propagating from the optical slit into at least ten resolved and substantially adjacent narrow ultraviolet bandwidth regions, wherein each bandwidth region is not more than two nanometers, and
the line sensor array is further configured to detect at least ten individual light intensities corresponding to the ultraviolet bandwidth regions produced by the diffraction grating.
11 . The submersible apparatus of claim 1 , wherein:
the broadband light source is further configured to output the broadband light only in the ultraviolet light spectrum, along with at least one narrow band light in the visible light spectrum or the near infrared light spectrum.
12 . The submersible apparatus of claim 11 , wherein:
the at least one narrow band light is in the visible light spectrum.
13 . The submersible apparatus of claim 1 , further comprising:
an optical filter configured to produce a narrow band of visible or infrared light from the first light beam,
wherein the light sensor is configured to detect an intensity of visible or near infrared light produced by the optical filter.
14 . A method operable by a controller of a submersible apparatus for measuring light absorption of a liquid sample surrounding the submersible apparatus, wherein:
the submersible apparatus comprises:
a broadband light source configured to output broadband light in an ultraviolet light spectrum and light in one or more of a visible light spectrum and a near infrared light spectrum;
a measurement region including the liquid sample, which propagates the broadband light emitted from the broadband light source;
a light beam splitter configured to split the broadband light, after passing through the measurement region, into a first light beam and a second light beam;
a light sensor configured to detect an intensity of visible or near infrared light from the first light beam;
an optical slit configured to produce an approximate point source of light from the second light beam;
a diffraction grating configured to resolve the second light beam propagating from the optical slit into a plurality of resolved and substantially adjacent narrow ultraviolet bandwidth regions; and
a line sensor array configured to detect a plurality of individual light intensities corresponding to the ultraviolet bandwidth regions produced by the diffraction grating, and
the method comprises:
recording and outputting the intensity measurements of visible or near infrared light detected by the light sensor;
recording and outputting the intensity of the plurality of individual light intensities corresponding to the ultraviolet bandwidth regions detected by the line sensor array; and
calculating an absorbance of light by the liquid sample using the recorded intensity measurements of ultraviolet and visible or near infrared light.
15 . The method of claim 14 , wherein the submersible apparatus further comprises an optical splitter configured to substantially split the broadband light emitted from the broadband light source, and wherein:
a first part of the emitted light is directed to propagate through the measurement region containing the liquid sample and constitutes a measurement beam; and
a second part of the emitted light is directed around the measurement region and constitutes a reference beam.
16 . The method of claim 15 , wherein:
the submersible apparatus further comprises an actuator and shutter assembly,
the actuator and shutter assembly are configured to alternately move, under direction of the controller, between:
a first position whereby the shutter substantially blocks propagating light of the measurement beam; and
a second position whereby the shutter substantially blocks propagating light of the reference beam, and
the method further comprises:
separately recording intensity measurements during periods when the shutter is in the first position and during periods when the shutter is in the second position; and
calculating the absorption of the liquid sample with compensation for changes in the raw light output of the broadband light source.
17 . The method of claim 15 , wherein:
the diffraction grating is further configured to resolve the second light beam propagating from the optical slit into at least ten resolved and substantially adjacent narrow ultraviolet bandwidth regions, wherein each bandwidth region is not more than two nanometers, and
the line sensor array is further configured to detect at least ten individual light intensities corresponding to the ultraviolet bandwidth regions produced by the diffraction grating.
18 . The method of claim 15 , wherein:
the broadband light source is further configured to output the broadband light only in the ultraviolet light spectrum, along with at least one narrow band light in the visible light spectrum or the near infrared light spectrum.
19 . The method of claim 15 , wherein:
the at least one narrow band light is in the visible light spectrum.
20 . The method of claim 15 , wherein:
the submersible apparatus further comprises:
an optical filter configured to produce a narrow band of visible or infrared light from the broadband light emitted by the broadband light source,
wherein the light sensor is configured to detect an intensity of visible or near infrared light produced by the optical filter.