CA2054887A1 - Critical angle refractometer for measuring refractive index of seawater - Google Patents
Critical angle refractometer for measuring refractive index of seawaterInfo
- Publication number
- CA2054887A1 CA2054887A1 CA 2054887 CA2054887A CA2054887A1 CA 2054887 A1 CA2054887 A1 CA 2054887A1 CA 2054887 CA2054887 CA 2054887 CA 2054887 A CA2054887 A CA 2054887A CA 2054887 A1 CA2054887 A1 CA 2054887A1
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- Canada
- Prior art keywords
- light
- light emitting
- emitting means
- phototransistor
- measuring surface
- Prior art date
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Abstract
ABSTRACT
A high precision linear solid state opto-electronic critical angle refractometer which is adapted to continuously measure the refractive index of a liquid over a narrow range of refractive indices when immersed into liquid being measured. The device is adapted for use in both industrial and oceanic remote sensing applications. The device is compact and is comprised of a broad beam monochromatic infra-red semiconductor light source, a planar prism with one measuring plane and a reflective plane per-pendicular to the measuring surface, and a small aperture semi-conductor light detector shielded by an infra-red band-pass optical filter. The device is capable of measuring small changes in refractive index linearly with resolutions of 1 ppm or better and can be manually adjusted to measure bands of refractive index contained within a wide range of indices.
A high precision linear solid state opto-electronic critical angle refractometer which is adapted to continuously measure the refractive index of a liquid over a narrow range of refractive indices when immersed into liquid being measured. The device is adapted for use in both industrial and oceanic remote sensing applications. The device is compact and is comprised of a broad beam monochromatic infra-red semiconductor light source, a planar prism with one measuring plane and a reflective plane per-pendicular to the measuring surface, and a small aperture semi-conductor light detector shielded by an infra-red band-pass optical filter. The device is capable of measuring small changes in refractive index linearly with resolutions of 1 ppm or better and can be manually adjusted to measure bands of refractive index contained within a wide range of indices.
Description
2~5~7 FIELD OF INVENTION
This relates to critical angle refractometers for measuring the concentration of solutions.
OBJECT OF INVENTION
It is an object of the invention to provide a compact high resolution opto-electronic refractometer designed specifical-ly for the linear, continuous measurement of the refractive index of liquids within narrow limits or bands. More particularly it is an object of the invention to provide a device which operates on similar principles as an Abbe type refractometer but which uses a specific balance of critical dimensions for measuring small changes in refractive index, linearly. Although the device measures narrow bands of refractive index continuously the positioning of the bands can be manually adjusted. This allows the device to be used to measure small changes in the refractive index of a large number of different liquids.
It is a further object of the invention to provide a device the applications for which range from the on-line monitor-ing of beer and wine sugar concentrations to the remote sensing of oceanic salinity and density. In many of said industrial applica-tions, narrow limits in the concentrations of dissolved sugars or salts must be maintained and accurate monitoring of the solutions is required. The invention is adapted to provide for such monitoring and for easy use through its simple linear response, to control mixing processes for a wide range of applications.
SUMMARY OF INVENTION
In accordance with the present invention there is ~5~87 provided a refractometer comprising (a) a light conducting element comprising a light trans-parent material (i) said light conducting element including a measuring surface adapted to provide an interface between liquid, the re-fractive index of which is to be measured, and said light trans-parent material, (b) light emitting means for emitting a diverging beam of substantially monocromatic light, (c) a photo sensitive means having a small light admitting aperture for measuring light admitted by said aperture, (i) means for providing a signal representative of the level of light received by said photosensitive means, (d) mèans for fixing the position of said light emitting means so that said beam of light emitted from it is transmitted through said light conducting element and impinges on said measur-ing surface at a range of angles over the face of the beaml (i) said range of angles encompassing the critical angle of incline which lies between angles of incidence at which said light is reflected by said interface and angles of incidence at which said light is transmitted through said interface into the liquid, (e) means for fixing the position of said photosensitive means so that a first portion of said aperture receives light reflected from said measuring surface and a second portion of said aperture does not receive reflected light due to the fact that the corresponding portion of the beam has been transmitted through the interface into the liquid.
2~ ~887 BRIEF DF.SCRIPTION OF DRAWI~GS
Figure lA of the drawings i5 a cross sectional view of the assembly containing the optical elements of one embodiment of the invention.
Figure lB of the drawings is an end view of the assembly of Figure lA.
Figure 2 of the drawings is a top, a side and an end view of a prism in accordance with this invention showing suitable dimensions.
Figure 3 is a curve showing level of light emitted by the light emitting diode (LED) over the range of the angle of divergence of the light beam emitted from the LED.
Figure 4 is a curve showing the electrical current conducted by the photodetector verses the range of angles at which the device provides linear response of light beam emitted by the LED.
Figure 5 illustrates circuitry for use in measuring current conducted by the photodetector.
DESCRIPTION OF INVENTION
With reference to Figure lA of the drawings, the assem-bly containing the optical components of the device shown general-ly by drawing reference 32 which includes a prism 1 and a cap unit 3. Since the prism is used as a fixed reference in the measuring process the prism is composed of a material which displays a stable refractive index as a function of ambient temperature and pressure. It has been found that borosilicate glass displays a suitable index of refraction and has a suitably stable refractive index with changes of temperature and pressure. The prism 1 includes an entry/exit window face 22 at one end thereof, which is substantially perpendicular to the longitudinal axis of the prism.
At the end, the prism remote from the window 22, there is a polished planar surface 11 which serves as the measuring surface and is adapted to from an interface with the liquid being measured. The measuring surface 11 is situated at an angle ~ to the major axis of the prism. The angle ~ = 90 ~CRIT (where 0CRIT is the critical angle limit of Snell's law when the prism is immersed in fresh water at room temperature) and is fixed close to 27 degrees.
With reference to window face 22, as illustrated in Figures lA and 2 the vertical dimension A must be greater than that of B, to allow all the light rays reflected by the reflecting surface 2 to be incident upon a region through which the detector 7 can be moved. Such proportionings provides the refractometer with the optimum range for the measurement of refractive index when the angle ~ of the LED 6 is altered. In the embodiment shown in Figures lA and 2, the dimension A is approximately twice tnat of B.
Situated perpendicular to the measuring surface 22 is planar reflecting surface 2 which is adapted to reflect the light beam emitted by the LED 6 onto the measuring surface 11. In order to provide the necessary reflectivity the surface 2 is plated with a reflective material. Gold has been found to be a suitable re-flective material for this purpose since it is least affected by corrosion by seawater and is easily evaporated onto the planar surface 2.
2 ~
At the end of the prism 1 remote from the measuring and reflecting surfaces 11 and 2 a cap unit 3 is mounted over the end of the prism, as shown in Figure lA. Mounted within the cap unit 3 is light source 6 which consists of a gallium arsenide light emitting diode (LED) having an isotropic beam dispersion within the range of angles used for measurements, i.e. plus and minus 10 degrees from the axis of the device. The preferred type of infra-red emitting diode has a narrow spectral emission peak at 930 nm.
and can be considered monochromatic. The use of monochromatic light avoids the non-linear effect resulting in the degradation of the precision of the instrument which would result from use of non-monochromatic light.
In applications which involve immersion of the instru-ment in seawater, solar infra-red light is not present at depths of more than 1 meter and the refractometer is able to function without interference from ambient light through the use of a band-pass or long pass optical filter positioned in front of the light sensing devices.
The diode used as a light source in the present inven-tion provides an semi-isotropic dispersion pattern over the range of angles used to provide a homogenous beam spot on the measuring surface. The use of a homogenous beam spot provides the necessary linearity for the operation of the invention.
The light emitting diode used in the invention must have a temperature response which is matched to the inverse response of the detectors. Such use of a matched source/detector pair allows the device to compensate for variations in ambient temperature 2 ~ 7 within the range of temperatures occurring in the ocean (ie.
-2 C to 35 C).
Also mounted within the cap unit 3 is light detector 7 for measuring light reflected from the measuring surface 11. The most suitable light detector for this application is a small aper-ture, high sensitivity phototransistor. Such detectors should have thermal properties matched to that of the LED since source/detector sets must track the ambient temperature identical-ly for best compensation. This generally means that the LED's and the detectors are identical in shape, size, and casing material.
The detector used must also show a constant light acceptance as a function of incident angle over the range used for measurement in order to enhance the linearity of the response of the device to refractive index. One suitable LED/dectector pair which is commercially available is the OP660 manufactured by TRW Ltd. This pair is comprised of the OP160 LED 6 and the OP550 7 phototransis-tor.
Referring now to Figures lA and lB, it will be seen that light source 6 is eccentrically mounted in a spherical metal mounting or bushing 5 which is rotatable within the cap 3.
Similarly detector 7 is eccentrically mounted on metal mountings or bushings 8 which are also rotatable within the cap unit 3. The use of metal bushings to hold the light source and detectors permits a rapid and uniform response to changes in ambient temperature suitable operation of the device. The metal bushings also serve as heat sinks for the detector and the LED light source.
Positioned over one end of the prism 1 is cap 3 which holds the light source 6 and the phototransistor 7. The cap unit 3 is suitably machined or moulded metal such as aluminum, brass or stainless steel. The use of metal as the material for the cap ensures that the LED/detector sets will not shift under mechanical strain and that the entire cap will be subjected to a more even temperature distribution. As discussed above this objective is furthered by use of metal mounts 5, 8 and 9 to hold the LED 6 and the detector 7. For proper rapid compensation to ambient tempera-ture changes the LED 6 and detector 7 must be subjected to the same temperature.
In the event that the main cap unit 3 is composed of a non-heat conducting material such as polyvinylchloride, the use of the metal bushings surrounding the detectors and the light source is particularly important for providing even temperature distribution.
The cap 3 comprises an end portion 24 and an outer sleeve portion 23 which surrounds one end of the prism 1. Mounted within the end portion 24 of the cap 3 is spherical mounting 5 holding the LED 6. This mounting is composed of metal such as brass or stainless steel. Also mounted with the end portion 24 of the cap 3 is a large diameter eccentric bushing 9 which holds smaller diameter eccentric bushing 8 which in turn holds photo-detector 7.
Positioned on the surface of the end portion 24 is axial track guide 10. The track guide 10 includes a central slot 25 through which LED and photodetector pins 27 and 28 respectively, project. The pins 27 and 28 are adapted to slide along the slot ~5~887 72359-~
25 as the mountings are rotated and thus maintain LED 6 and photo-transistor 7 in alignment along a predetermined axis by restrict-ing movement of the LED 6 and the phototransistor to a linear path. The pins 27 and 28 also provide the electrical connections to the LED 6 and the photo transistor 7.
The LED 6 and the photodetector 7 are mounted eccentri-cally within the bushings 5, 8 and 9, to enable the positions of the LED 6 and photodetector 7 to be altered as necessary to ensure their proper placement to obtain maximum gain and linearity during operation. Adjustment of the positions of the detectors is accomplished by rotating the bushings. The use of the outer bush-ing or mounting 9 for the photodetector 7 permits the photo-detector to be moved over a relatively large path as is necessary to accomodate various settings of the LED. Use of the inner bush-ing of mounting 8 permits fine adjustment of the position of the photodetector.
The spherical shape of mounting 5 permits LED 6 mounted on it to be tilted about an axis perpendicular to the axis of the slot 26 to thereby vary ~ the angle of the beam emitted by the LED
6. Variation of the angle of the beam serves to vary the angle at which the beam impinges on the measuring surface 11 and hence varies the range of refractive indices to be measured.
Positioned between the prism 1 on the end cap 3, as shown in Figure lA, is an optical band-pass filter 4. The filter is preferably composed of glass to reduce the effects of tempera-ture associated with plastic filters. The filter 4 is fitted in front of LED 6 and photodetector 7 using a light tight seal to ~ 0 ~ 7 prevent ambient light in the visible and ultraviolet end of the spectrum from exciting the photodetector.
Prior to operation the device is calibrated using eccen-tric mountings 8 and 9 to adjust the position of the photodetector 7. The spherical eccentric mounting 5 adjusted to position the LED 6 along the axis of the slot 25 and also to adjust the angle of the beam emitted by the LED 6.
In operation, the device is immersed in the liquid to be measured and power is applied to operate the LED 6 and the photo-detector 7. As illustrated in Figure lA, a beam, the outer edgesof which are defined by dash dot lines 29 and 30, is emitted by the LED 6, and impinges on reflective surface 2 which reflects it to the measuxing surface 11. The beam impinges on the measuring surface 11 at various angles over the face of the beam. The angle at which the beam impinges on the measuring surface is arranged so that the angles of incidence of a portion of the beam with the measuring surface is less than 0CRIT (the critical limit of Snell's Law) which part will be transmitted by the interface into the liquid rather than being reflected. In Figure lA the portion of the beam which is transmitted to the liquid and is hence absor-bed as represented by the shaded area between lines 31 and 30.
The portion of the beam having angles of incidence with the measuring surface, greater than ~CRIT will be reflected at the interface towards the photodetector 7. The photodetector 7 will conduct a current representative of the light received which can be converted to a voltage by the components represented in Figure 5. This voltage may then be measured and may be converted to a digital signal by an analogue to digital converter (not shown) which signal may ~e transmitted to the surface. The signal thus transmitted is representative of the refraction of the liquid being measured.
The incident angle ~ of a given light ra~ impinging on the measuring surface is the linear sum of the divergence angle the LED angle ~, and the prism angle defined by (90-~) or:
0= (so-e) +~ + ot The critical value of 0 is defined through the critical limit of Snell's Law and determines the light/dark front seen by the detec-tor. When e, and ~ are fixed and their sum is equal to thecritical angle of the interface, 0 varies with ~ and the light/dark front varies within the beam spot as in Figure 4. All light rays having angles of incidence of 0 which are greater than the critical angle are reflected at the interface towards the detector 7 while those having angles of incidence which are less are transmitted through the interface and are dissipated in the liquid. As the refractive index of the sample changes, the critical angle of the interface 11 also changes and the fraction of the light rays reflected towards the detector changes as the light/dark front shifts.
The value ~ defines the functional linear range of angles of light emitted by LED 6 which the detector 7 can see (see Figures 3 and 4). The value of ~ is determined both by the dia-meter of the photodetector 16 and the length 15 of the prism 1.
In prior art, the value of r was the same or greater than the maximum value of d. In the refractometer of the present inven-tions, the value of ~ is close to one tenth that of the maximum value oE~ .
This relates to critical angle refractometers for measuring the concentration of solutions.
OBJECT OF INVENTION
It is an object of the invention to provide a compact high resolution opto-electronic refractometer designed specifical-ly for the linear, continuous measurement of the refractive index of liquids within narrow limits or bands. More particularly it is an object of the invention to provide a device which operates on similar principles as an Abbe type refractometer but which uses a specific balance of critical dimensions for measuring small changes in refractive index, linearly. Although the device measures narrow bands of refractive index continuously the positioning of the bands can be manually adjusted. This allows the device to be used to measure small changes in the refractive index of a large number of different liquids.
It is a further object of the invention to provide a device the applications for which range from the on-line monitor-ing of beer and wine sugar concentrations to the remote sensing of oceanic salinity and density. In many of said industrial applica-tions, narrow limits in the concentrations of dissolved sugars or salts must be maintained and accurate monitoring of the solutions is required. The invention is adapted to provide for such monitoring and for easy use through its simple linear response, to control mixing processes for a wide range of applications.
SUMMARY OF INVENTION
In accordance with the present invention there is ~5~87 provided a refractometer comprising (a) a light conducting element comprising a light trans-parent material (i) said light conducting element including a measuring surface adapted to provide an interface between liquid, the re-fractive index of which is to be measured, and said light trans-parent material, (b) light emitting means for emitting a diverging beam of substantially monocromatic light, (c) a photo sensitive means having a small light admitting aperture for measuring light admitted by said aperture, (i) means for providing a signal representative of the level of light received by said photosensitive means, (d) mèans for fixing the position of said light emitting means so that said beam of light emitted from it is transmitted through said light conducting element and impinges on said measur-ing surface at a range of angles over the face of the beaml (i) said range of angles encompassing the critical angle of incline which lies between angles of incidence at which said light is reflected by said interface and angles of incidence at which said light is transmitted through said interface into the liquid, (e) means for fixing the position of said photosensitive means so that a first portion of said aperture receives light reflected from said measuring surface and a second portion of said aperture does not receive reflected light due to the fact that the corresponding portion of the beam has been transmitted through the interface into the liquid.
2~ ~887 BRIEF DF.SCRIPTION OF DRAWI~GS
Figure lA of the drawings i5 a cross sectional view of the assembly containing the optical elements of one embodiment of the invention.
Figure lB of the drawings is an end view of the assembly of Figure lA.
Figure 2 of the drawings is a top, a side and an end view of a prism in accordance with this invention showing suitable dimensions.
Figure 3 is a curve showing level of light emitted by the light emitting diode (LED) over the range of the angle of divergence of the light beam emitted from the LED.
Figure 4 is a curve showing the electrical current conducted by the photodetector verses the range of angles at which the device provides linear response of light beam emitted by the LED.
Figure 5 illustrates circuitry for use in measuring current conducted by the photodetector.
DESCRIPTION OF INVENTION
With reference to Figure lA of the drawings, the assem-bly containing the optical components of the device shown general-ly by drawing reference 32 which includes a prism 1 and a cap unit 3. Since the prism is used as a fixed reference in the measuring process the prism is composed of a material which displays a stable refractive index as a function of ambient temperature and pressure. It has been found that borosilicate glass displays a suitable index of refraction and has a suitably stable refractive index with changes of temperature and pressure. The prism 1 includes an entry/exit window face 22 at one end thereof, which is substantially perpendicular to the longitudinal axis of the prism.
At the end, the prism remote from the window 22, there is a polished planar surface 11 which serves as the measuring surface and is adapted to from an interface with the liquid being measured. The measuring surface 11 is situated at an angle ~ to the major axis of the prism. The angle ~ = 90 ~CRIT (where 0CRIT is the critical angle limit of Snell's law when the prism is immersed in fresh water at room temperature) and is fixed close to 27 degrees.
With reference to window face 22, as illustrated in Figures lA and 2 the vertical dimension A must be greater than that of B, to allow all the light rays reflected by the reflecting surface 2 to be incident upon a region through which the detector 7 can be moved. Such proportionings provides the refractometer with the optimum range for the measurement of refractive index when the angle ~ of the LED 6 is altered. In the embodiment shown in Figures lA and 2, the dimension A is approximately twice tnat of B.
Situated perpendicular to the measuring surface 22 is planar reflecting surface 2 which is adapted to reflect the light beam emitted by the LED 6 onto the measuring surface 11. In order to provide the necessary reflectivity the surface 2 is plated with a reflective material. Gold has been found to be a suitable re-flective material for this purpose since it is least affected by corrosion by seawater and is easily evaporated onto the planar surface 2.
2 ~
At the end of the prism 1 remote from the measuring and reflecting surfaces 11 and 2 a cap unit 3 is mounted over the end of the prism, as shown in Figure lA. Mounted within the cap unit 3 is light source 6 which consists of a gallium arsenide light emitting diode (LED) having an isotropic beam dispersion within the range of angles used for measurements, i.e. plus and minus 10 degrees from the axis of the device. The preferred type of infra-red emitting diode has a narrow spectral emission peak at 930 nm.
and can be considered monochromatic. The use of monochromatic light avoids the non-linear effect resulting in the degradation of the precision of the instrument which would result from use of non-monochromatic light.
In applications which involve immersion of the instru-ment in seawater, solar infra-red light is not present at depths of more than 1 meter and the refractometer is able to function without interference from ambient light through the use of a band-pass or long pass optical filter positioned in front of the light sensing devices.
The diode used as a light source in the present inven-tion provides an semi-isotropic dispersion pattern over the range of angles used to provide a homogenous beam spot on the measuring surface. The use of a homogenous beam spot provides the necessary linearity for the operation of the invention.
The light emitting diode used in the invention must have a temperature response which is matched to the inverse response of the detectors. Such use of a matched source/detector pair allows the device to compensate for variations in ambient temperature 2 ~ 7 within the range of temperatures occurring in the ocean (ie.
-2 C to 35 C).
Also mounted within the cap unit 3 is light detector 7 for measuring light reflected from the measuring surface 11. The most suitable light detector for this application is a small aper-ture, high sensitivity phototransistor. Such detectors should have thermal properties matched to that of the LED since source/detector sets must track the ambient temperature identical-ly for best compensation. This generally means that the LED's and the detectors are identical in shape, size, and casing material.
The detector used must also show a constant light acceptance as a function of incident angle over the range used for measurement in order to enhance the linearity of the response of the device to refractive index. One suitable LED/dectector pair which is commercially available is the OP660 manufactured by TRW Ltd. This pair is comprised of the OP160 LED 6 and the OP550 7 phototransis-tor.
Referring now to Figures lA and lB, it will be seen that light source 6 is eccentrically mounted in a spherical metal mounting or bushing 5 which is rotatable within the cap 3.
Similarly detector 7 is eccentrically mounted on metal mountings or bushings 8 which are also rotatable within the cap unit 3. The use of metal bushings to hold the light source and detectors permits a rapid and uniform response to changes in ambient temperature suitable operation of the device. The metal bushings also serve as heat sinks for the detector and the LED light source.
Positioned over one end of the prism 1 is cap 3 which holds the light source 6 and the phototransistor 7. The cap unit 3 is suitably machined or moulded metal such as aluminum, brass or stainless steel. The use of metal as the material for the cap ensures that the LED/detector sets will not shift under mechanical strain and that the entire cap will be subjected to a more even temperature distribution. As discussed above this objective is furthered by use of metal mounts 5, 8 and 9 to hold the LED 6 and the detector 7. For proper rapid compensation to ambient tempera-ture changes the LED 6 and detector 7 must be subjected to the same temperature.
In the event that the main cap unit 3 is composed of a non-heat conducting material such as polyvinylchloride, the use of the metal bushings surrounding the detectors and the light source is particularly important for providing even temperature distribution.
The cap 3 comprises an end portion 24 and an outer sleeve portion 23 which surrounds one end of the prism 1. Mounted within the end portion 24 of the cap 3 is spherical mounting 5 holding the LED 6. This mounting is composed of metal such as brass or stainless steel. Also mounted with the end portion 24 of the cap 3 is a large diameter eccentric bushing 9 which holds smaller diameter eccentric bushing 8 which in turn holds photo-detector 7.
Positioned on the surface of the end portion 24 is axial track guide 10. The track guide 10 includes a central slot 25 through which LED and photodetector pins 27 and 28 respectively, project. The pins 27 and 28 are adapted to slide along the slot ~5~887 72359-~
25 as the mountings are rotated and thus maintain LED 6 and photo-transistor 7 in alignment along a predetermined axis by restrict-ing movement of the LED 6 and the phototransistor to a linear path. The pins 27 and 28 also provide the electrical connections to the LED 6 and the photo transistor 7.
The LED 6 and the photodetector 7 are mounted eccentri-cally within the bushings 5, 8 and 9, to enable the positions of the LED 6 and photodetector 7 to be altered as necessary to ensure their proper placement to obtain maximum gain and linearity during operation. Adjustment of the positions of the detectors is accomplished by rotating the bushings. The use of the outer bush-ing or mounting 9 for the photodetector 7 permits the photo-detector to be moved over a relatively large path as is necessary to accomodate various settings of the LED. Use of the inner bush-ing of mounting 8 permits fine adjustment of the position of the photodetector.
The spherical shape of mounting 5 permits LED 6 mounted on it to be tilted about an axis perpendicular to the axis of the slot 26 to thereby vary ~ the angle of the beam emitted by the LED
6. Variation of the angle of the beam serves to vary the angle at which the beam impinges on the measuring surface 11 and hence varies the range of refractive indices to be measured.
Positioned between the prism 1 on the end cap 3, as shown in Figure lA, is an optical band-pass filter 4. The filter is preferably composed of glass to reduce the effects of tempera-ture associated with plastic filters. The filter 4 is fitted in front of LED 6 and photodetector 7 using a light tight seal to ~ 0 ~ 7 prevent ambient light in the visible and ultraviolet end of the spectrum from exciting the photodetector.
Prior to operation the device is calibrated using eccen-tric mountings 8 and 9 to adjust the position of the photodetector 7. The spherical eccentric mounting 5 adjusted to position the LED 6 along the axis of the slot 25 and also to adjust the angle of the beam emitted by the LED 6.
In operation, the device is immersed in the liquid to be measured and power is applied to operate the LED 6 and the photo-detector 7. As illustrated in Figure lA, a beam, the outer edgesof which are defined by dash dot lines 29 and 30, is emitted by the LED 6, and impinges on reflective surface 2 which reflects it to the measuxing surface 11. The beam impinges on the measuring surface 11 at various angles over the face of the beam. The angle at which the beam impinges on the measuring surface is arranged so that the angles of incidence of a portion of the beam with the measuring surface is less than 0CRIT (the critical limit of Snell's Law) which part will be transmitted by the interface into the liquid rather than being reflected. In Figure lA the portion of the beam which is transmitted to the liquid and is hence absor-bed as represented by the shaded area between lines 31 and 30.
The portion of the beam having angles of incidence with the measuring surface, greater than ~CRIT will be reflected at the interface towards the photodetector 7. The photodetector 7 will conduct a current representative of the light received which can be converted to a voltage by the components represented in Figure 5. This voltage may then be measured and may be converted to a digital signal by an analogue to digital converter (not shown) which signal may ~e transmitted to the surface. The signal thus transmitted is representative of the refraction of the liquid being measured.
The incident angle ~ of a given light ra~ impinging on the measuring surface is the linear sum of the divergence angle the LED angle ~, and the prism angle defined by (90-~) or:
0= (so-e) +~ + ot The critical value of 0 is defined through the critical limit of Snell's Law and determines the light/dark front seen by the detec-tor. When e, and ~ are fixed and their sum is equal to thecritical angle of the interface, 0 varies with ~ and the light/dark front varies within the beam spot as in Figure 4. All light rays having angles of incidence of 0 which are greater than the critical angle are reflected at the interface towards the detector 7 while those having angles of incidence which are less are transmitted through the interface and are dissipated in the liquid. As the refractive index of the sample changes, the critical angle of the interface 11 also changes and the fraction of the light rays reflected towards the detector changes as the light/dark front shifts.
The value ~ defines the functional linear range of angles of light emitted by LED 6 which the detector 7 can see (see Figures 3 and 4). The value of ~ is determined both by the dia-meter of the photodetector 16 and the length 15 of the prism 1.
In prior art, the value of r was the same or greater than the maximum value of d. In the refractometer of the present inven-tions, the value of ~ is close to one tenth that of the maximum value oE~ .
Claims (11)
1. A refractometer comprising (a) a light conducting element comprising a light trans-parent material (i) said light conducting element including a measuring surface adapted to provide an interface between liquid, the refractive index of which is to be measured, and said light trans-parent material, (b) light emitting means for emitting a diverging beam of substantially monocromatic light, (c) a photo sensitive means having a small light admitting aperture for measuring light admitted by said aperture, (i) means for providing a signal representative of the level of light received by said photosensitive means, (d) means for fixing the position of said light emitting means so that said beam of light emitted from it is transmitted through said light conducting element and impinges on said measur-ing surface at a range of angles over the face of the beam, (i) said range of angles encompassing the critical angle of incidence which lies between angles of incidence at which said light is reflected by said interface and angles of incidence at which said light is transmitted through said interface into the liquid, (e) means for fixing the position of said photosensitive means so that a first portion of said aperture receives light reflected from said measuring surface and a second portion of said aperture does not receive reflected light due to the fact that the corresponding portion of the beam has been transmitted through the interface into the liquid.
2. A refractometer according to claim 1 wherein said light conducting element includes a window means and a reflecting sur-face situated at 90 degrees to the measuring surface, said light emitting means and said photosensitive means being positioned adjacent one another at said window means, said light emitting means being positioned so that the emitted light beam impinges on said reflecting surface, is reflected onto the measuring surface and is partly reflected by said measuring surface to said aperture of said photosensitive means.
3. A refractometer according to claim 1 wherein said light emitting means is eccentrically positioned on a rotatable mounting and means for restricting movement of said light emitting means to a linear path.
4. A refractometer according to claim 3 wherein said photo-sensitive means comprise a phototransistor, said phototransistor being eccentrically mounted on a rotatable mounting and said means for restricting movement of said light emitting means also serving to restrict movement of said phototransistor to said linear path.
5. A refractometer according to claim 4 wherein said mount-ing means for said light emitting means and said phototransistor are composed of metal.
6. A refractometer according to claim 5 wherein said mount-ing means for said light emitting means and said phototransistor, are held by a metal cap positioned over one end of said light conducting element.
7. A refractometer according to claim 1 including an optical light pass filter positioned between said light conducting element and said phototransistor and said light emitting means.
8. A refractometer according to claims 1, 2, 3, 4, 5, 6 or 7 wherein said light emitting means comprises a light emitting diode having a temperature response which is matched to the inverse temperature response of said phototransistor.
9. A refractometer according to claims 1, 2, 3, 4, 5, 6 or 7 wherein said monitoring means for said light emitting means is adapted to permit said light emitting means to pivot to thereby alter the range of angles at which said beam impinges on said measuring surface.
10. A refractometer according to claim 1, 2, 3, 4, 5, 6 or 7 wherein said beam has a maximum angle of divergence .alpha. and the portion of said beam subtended by the diameter of the orifice constitutes an angle .gamma. , wherein .gamma. equals approximately 1/10.alpha..
11. A refractometer according to claims 1, 2, 3, 4, 5, 6 or 7 wherein said light emitting means comprises a light emitting diode having a temperature response which is matched to the inverse temperature response of said phototransistor, said monitoring means for said light emitting means is adapted to permit said light emitting means to pivot to thereby alter the angles at which said beam impinges on said measuring surface, and said beam has a maximum angle of divergence .alpha. and the portion of said beam subtended by the diameter of the orifice constitutes an angle .gamma., wherein .gamma. equals approximately 1/10.alpha..
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2054887 CA2054887A1 (en) | 1991-11-06 | 1991-11-06 | Critical angle refractometer for measuring refractive index of seawater |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2054887 CA2054887A1 (en) | 1991-11-06 | 1991-11-06 | Critical angle refractometer for measuring refractive index of seawater |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2054887A1 true CA2054887A1 (en) | 1993-05-07 |
Family
ID=4148695
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2054887 Abandoned CA2054887A1 (en) | 1991-11-06 | 1991-11-06 | Critical angle refractometer for measuring refractive index of seawater |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2054887A1 (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10078048B2 (en) | 2016-01-27 | 2018-09-18 | Corning Incorporated | Refractometer assemblies, methods of calibrating the same, and methods of determining unknown refractive indices using the same |
-
1991
- 1991-11-06 CA CA 2054887 patent/CA2054887A1/en not_active Abandoned
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US10078048B2 (en) | 2016-01-27 | 2018-09-18 | Corning Incorporated | Refractometer assemblies, methods of calibrating the same, and methods of determining unknown refractive indices using the same |
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