Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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72359-4
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
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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.
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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
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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.
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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
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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
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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
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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
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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
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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~ .
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