GB2087554A - A refractometer - Google Patents

Abstract

A refractometer, whose method of use is also claimed, comprises; a reference body (1) having a plane interface surface (3) with which a test material is placed in contact and a semicircular cross-section in a measurement plane perpendicular to the interface surface; means (4) for directing a beam of electromagnetic radiation radially into the reference body through its curved surface; means (16, 17, 18, 15, 14) for scanning the beam through a range of angles in the measurement plane; means (8) for detecting radiation reflecting from the boundary between the reference body and the test material; and means 26, 27 for determining the angle through which the beam is scanned from a predetermined starting position at which there is a transition between reflection of the beam from the boundary and transmission. In a preferred optical refractometer embodying the invention, the reference body is a hemisphere made of transparent material, such as glass. <IMAGE>

Description

SPECIFICATION A refractometer This invention relates to a refractometer.

Known refractometers are used to determine the refreactive index, or a related parameter, of a test material by detection and measurement of the critical angle at which visible light travelling through a reference body, having a plane interface surface in contact with a sample of the test material and having a refractive index higher than that of the test material for the wavelength of the light used, is no longer totally internally reflected at the boundary between the reference body and the test material. Usually the reference body is made of glass, a material having a high refractive index.

In known commercial refractometers, it is usual to illuminate the boundary between the reference body and the test material simultaneously over a whole range of angles and to observe the field of reflected light through a telescope. Thus, an operator looking through the telescope at the light reflected from the boundary between the reference body and the sample sees a field of view divided into a light and a dark area, the demarcation line between the light and dark areas marking the critical angle. In order to measure the critical angle, the telescope operator adjusts the telescope to align a cross-piece of the telescope with the demarcation line and reads off the angle from an associated scale.

Clearly, such manually operated refreactometers are not conductive to the rapid or continuous testing of materials and the accuracy of the results obtained depends largely on the skill and experience of the operator.

In particular, the demarcation line between the light and dark areas of the field of view may be difficult to distinguish because of the effects of, for example: uneven illumination across the field of view; high background light levels, a problem which occurs particulary when the test material is a material from which a large amount of spurious scattering is obtained, for example an emulsion such as milk; and random scattering caused by deterioration of the surfaces of the reference body.

Such effects are of little consequence to a human telescope operator who, with experience, encounters relatively little difficulty in correctly aligning the cross-piece of the telescope with the demarcation line. However, it has proved extremely difficult to automate refreactometers operating on this principle in a manner which allows these defects to be ignored and which also gives an accurate reading.

It is an object of the present invention to provide a refractometer which may readily be embodied in a reliable automatic form.

Accordingly, in one aspect, the invention provides a refractometer comprising: a reference body having a plane interface surface with which a test material is placed in contact and having a semicircular cross-section in a measurement plane perpendicular to the interface surface; means for directing a beam of electromagnetic radiation radially into the reference body through its curved surface; means for scanning the beam through a range of angles in the measurement plane; means for detecting radiation reflecting from the boundary between the reference body and the test material; and means for determining the angle through which the beam is scanned from a predetermined starting position to a position at which there is a transition between reflection of the beam from the boundary and transmission of the beam through the boundary.

In a preferred optical refractometer embodying the invention, the reference body is a hemisphere made of transparent material, such as glass. The circular plane face of the hemisphere serves as the interface surface and the measurement plane comprises any of the diametral planes perpendicular to the interface surface. A movable light source providing a narrow beam of light radially of the hemisphere is movable along an arcuate path centred on the geometric centre of the hemisphere so as to scan the light beam in the measurement plane through a range of angles with respect to the interface surface. An elongate light detector is positioned to receive any light reflected from the boundary between the interface surface and test material over the scanning range of the light beam.Advantageously, the angle through which the light beam is scanned to reach the transition between reflection and transmission of the light beam is determined by a counter providing a digital count related to the angle in question.

In accordance with another aspect of the invention, there is provided a method of determining the refractive index, or a related parameter, of a test material, comprising: placing the test material in contact with a plane interface surface of a reference body having a semicircular cross-section in a measurement plane perpendicular to the interface surface; directing a beam of electromagnetic radiation radially into the reference body through its curved surface; scanning the beam through a range of angles in the measurement plane; detecting radiation reflected from the boundary between the reference body and the test material; and determining the angle through which the beam is scanned from a predetermined starting position to a position at which there is a transition between reflection of the beam from the boundary and transmission of the beam through the boundary.

In order that the invention may be readily understood, an embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which FIGURE 1 shows schematically the optical arrangement of an optical refractometer in accordance with the invention; FIGURE 2 illustrates diagrammatically the principle of a preferred drive mechanism for the optical arrangement of Figure 1; FIGURE 3 is a plan view of the basic components of an automated optical refractometer constructed in accordance with the principles of Figures 1 and 2; FIGURE 4 is a side elevation of the refractometer shown in Figure 3; FIGURE 5 is a block diagram of electronic circuitry associated with the refractometer of Figures 3 and 4; and FIGURES 6A, 6B and 6C constitute a flow chart which illustrates the operating cycle of the refractometer of Figures 3 to 5.

Referring to Figure 1, the optical arrangement of an optical refractometer in accordance with the invention comprises a transparent reference body 1 having a refractive index which is higher than the expected refractive index of a test material 2 whose refractive index, or related parameter, is to be determined. The reference body 1 has a semicircular cross-section in a measurement plane, the plane of the drawing in Figure 1, and has a plane interface surface 3 which iies perpendicular to the measurement plane and in contact with which the test material 2 is placed.

The optical arrangement includes a light source 4 for directing a beam 5 of light radially into the reference body 1 through its curved surface 6. A mechanical drive arrangement (not shown in Figure 1) is provided to move the light source 4 along an arcuate path in the measurement plane centred on the geometrical centre 7 of the semicircular cross-section of the body 1, so as to scan the light beam through a range of angles. A stationary elongate light sensitive detector 8 is disposed on the opposite side of the central normal plane 9 from the light source 4 to detect light reflected from the boundary between the reference body 1 and the test material 2 and provide a corresponding output signal.

The light source 4 projects a divergent beam 11 from its outlet aperture 10 such that, upon entering the reference body, the light forms a substantially parallel radially directed beam 12 of narrow width in relation to the dimensions of the reference body. For example, the outlet aperture 10 may be a 0.5 mm aperture located at the focus of the curved surface 6. On emergence from the reference body, the reflected beam 13 is focussed by the curved surface onto the light detector 8. It will be appreciated that the width of the beam has been exaggerated in the drawings in the interests of clarity.

Figure 2 illustrates one form of drive arrangement for moving the light source 4, in which the source 4 is supported at one end of a scanning arm 14 whose other end is fixed to a driven shaft 1 5 for rotation therewith. The shaft 1 5 is mounted in bearings (not shown) so that it is rotatable about an axis extending perpendicular to the measurement plane and passing through the centre 7 of the body 1.

A drive arm 1 6 is attached to the shaft 1 5 and serves to impart a rotational movement to the shaft, the arm 1 6 itself being driven by a stud 17 carried by a drive disc 1 8 rotatable about an axis parallel to the axis of the shaft 1 5. The arm 16 is biased into engagement with the stud 1 7 by suitable means, such as a counterweight (not shown) attached to the shaft 1 5. Upon rotation of the drive disc 18, the arm 1 6 performs a reciprocal rocking motion of predetermined amplitude, resulting in a reciprocal rotation of the shaft 1 5 and a corresponding reciprocal angular motion of the scanning arm 14.A scan detector 1 9 is provided to indicate a predetermined start of scan position, for example by detecting a flag 20 on the drive arm. A similar detector, schematically shown at 21, may be positioned to indicate a predetermined end of scan position.

In operation of the refractometer, the disc 1 8 is rotated to move the scanning arm 14 so as to scan the radial light beam through a range of angles. The light detector 8 detects light reflected from the boundary between the reference body 1 and the test material and thus indicates by a change in its output signal the point in the scan, the so-called critical angle, at which there is a transition between total internal reflection of the beam 5 at the boundary and transmission of the beam through the boundary. By using suitable means to measure the angle through which the light beam is scanned from the predetermined start position in reaching the transition between reflection and transition, the refractive index or a related parameter of the test material may be deduced.

Figures 3 and 4 schematically illustrate the construction of an automatic optical refractometer embodying the principle elucidated with reference to Figures 1 and 2, some parts having been omitted for clarity. In the embodiment of Figures 3 and 4, components corresponding to elements already shown in Figures 1 and 2 are indicated by the same reference numerals.

The reference body of the refractometer illustrated in Figures 3 and 4 is in the form of a glass hemisphere supported by a suitable holder (not shown) so that the flat circular interface surface of the hemisphere is disposed horizontally with the convex surface of the hemisphere directed downwardly. The surface 3 of the hemisphere may be masked by an opaque layer which reduces extraneous effects and leaves a narrow diametral strip 22 of the surface clear.

As can be seen from Figure 3, the scanning arm 1 4 is cranked so as to position the source 4 for scanning in a measurement plane perpendicular to the axis of rotation of the shaft 1 5. The source 4 may be adjustably mounted on the arms 1 4 for radial adjustment to dispose its outlet aperture at the correct distance from the convex surface of the hemisphere.

The drive disc 1 8 is rotated by means of an electric motor 23 via reduction gearing 24, the start of scan detector 1 9 producing an electrical start signal on line 29 when the predetermined start position is reached. The motor also serves to drive means for providing a digital count related to the scanning movement of the light source 4.

These means comprise a slotted code disc 25 mounted on the end of the motor shaft 26. The disc 25 preferably comprises at least a hundred slots equally spaced around the circumference of the disc. The slotted circumference intersects the path of a beam of electromagnetic radiation projected by a slot counter 27 which provides an electrical pulse on an output 28 each time a slot passes the device. The slot counter 27 may be in the form of a photocouple comprising a photodiode and a phototransistor.

The light detector 8 monitors the reflected light from the boundary between the interface surface 3 and a test material placed thereon and provides a corresponding electrical signal on output line 30.

An electrical temperature sensor 31 is provided to detect the temperature of the test material placed on the surface 3 of the hemisphere 1 and to deliver an electrical signal on line32 for the purpose hereinafter described.

Operation of the refractometer is controlled by a microprocessor system which is shown in block diagram form in Figure 5, a flow chart illustrating the operation of the system being given in Figures 6A to 6C. The described refractometer is particularly adapted for use in the sugar industry for measurement of the so-called "Brix" value of a sugar solution.

As shown in Figure 5, the microprocessor system comprises a central processor unit (CPU) 40 connected to an address bus 41, a data bus 42 and a control bus 43. The address bus is connected to a random access memory (RAM) 44 serving as a working store, a programmable readonly memory (PROM) 45 serving as a store for the operating program of the system, a display unit 46 for displaying data fed thereto on the data bus and to a control and decode unit 47 which also receives external control signals from a control panel of the refractometer which is provided with seven control switches 48 to 54.

Operating switch 48 initiates operation of the refractometer, temperature selector switch 49 instructs display of the current temperature of the test material, "Brix" selector switch 50 instructs calculation of the "Brix" value of the test material at the prevailing temperature, "Corrected Brix" selector switch 51 instructs calculation of the "Brix" value corrected to a standard temperature, refractive index selector switch 52 instructs calculation of the refractive index of the test material, normal mode selector switch 53 instructs a normal cycle of operation of the refractometer for a single sample of test material and continuous mode selector switch 54 instructs periodic operating cycles of the refractometer for a flowing or changing sample.

The control and decode unit 47 also decodes address and control signals from the buses 41 and 43 for a data input unit 55 including an analog to digital converter and having respective inputs connected to the detector lines 28, 29, 30, 32 and 58 of the detectors 27, 19, 8, 31 and 21. The unit 47 communicates addresses and instructions to input unit 55 over a bus 56. Enabling instructions for the drive motor 23 and light source 4 are decoded by unit 47 and delivered to the components in question at an output 57.

The microprocessor system controls the operation of the mechanical and optical arrangement of the refractometer shown in Figures 3 and 4 in accordance with the program in PROM 45 and the input instructions from the control panel, the latter also determining the use to which the input data on lines 28, 29, 30 and 32 is put.

Referring to the flow chart of Figures 6A to 6C, the operation of the refractometer of Figures 3 to 5 is as follows: Having placed the test material in contact with the interface surface 3, with the light source in the initial position shown in Figure 4, the microprocessor system is energised using operating switch 48, thereby enabling the motor 23 to perform a single scan without energisation of the light source 4 to ensure that the light source is then correctly positioned slightly upstream of the predetermined start of scan position for the start of a measurement scan. The display 46 and working memory 14 are also cleared.

The desired operating mode is selected by actuation of the panel selector switches 49 to 54.

The microprocessor polls the selector switches to establish their condition and the condition of switches 50-52 is stored in the working memory as operating instructions, a delay being introduced to take account of possible switch bounce.

If temperature switch 49 has been actuated, the signal from the temperature sensor line 32 is input into the microprocessor system, processed by the CPU 40 and a corresponding temperature value supplied to the display unit for display. The temperature signal is repeated processed and displayed until the temperature selector switch is released, whereupon a measurement scan is initiated, after first checking that the parameter to be determined has been selected. If not, the operator is warned by a flashing indicator and the system loops back to poll the selector switches again.

Assuming the parameter to be determined has been selected, the light source and motor are switched on and a start of scan position signal from the detector 1 9 is awaited on line 29.

Receipt of a signal on line 19 enables the input 28 from slot counter 27 and the pulses from the counter are stored in a register of the working memory. The output from the light detector 8 is also enabled.

The rate of change of the detector output is monitored until a maximum rate is observed, the resulting data being stored. The count in the counter register is monitored during this time and if a maximum count is achieved before a maximum rate of change of light detector output is observed, the refractive index of the test material is outside the measurement range of the refractometer. This is indicated (Figure 6C) by illuminating a corresponding indicator lamp and blanking the display, whereafter the end of scan signal is awaited and the motor and lamp shut off.

The operation then cycles back to position A on Figure 6A.

Assuming a maximum rate of change of the light detector output is found, indicating the presence of a critical angle for the boundary between the hemisphere and the particular test material, the count achieved at that time is stored and the input from the slot counter continues to be counted until the maximum count in the counter register is achieved. Thereupon the selected parameter is ascertained from the stored condition of the selector switches and the desired parameter computed from the count achieved at the occurence of the maximum rate of change of the light detector output. The end of scan signal is awaited and the motor and lamp then deenergised, whereafter the operation loops back to position A on Figure 6A.If normal operating mode has been selected, this terminates the operation but, if continuous operation has been selected, the cycle is repeated until stopped by actuation of the operating switch 48.

It is noted that the particular drive system shown in Figure 3 and 4 for performing the scan has a non-linear characteristic such that the number of counts from the slot counter, which is driven at constant speed, per degree of movement of the light source along its arcuate path is greatest where the critical angle varies at the greatest rate with the refractive index of the test material, so that the scale is expanded to give a more accurate reading. The stud 1 7 may be adjustable on the drive disc 18 to optimise this compensating effect.

Although it is preferred to use the above described slotted disc counter to indicate the angle through which the scanning arm moves, alternative counting means such as a stepper motor arrangement may be employed.

Claims (37)

1. A refractometer comprising: a reference body having a plane interface surface with which a test material is placed in contact and having a semicircular cross-section in a measurement plane perpendicular to the interface surface; means for directing a beam of electromagnetic radiation radially into the reference body through its curved surface; means for scanning the beam through a range of angles in the measurement plane; means for detecting radiation reflecting from the boundary between the reference body and the test material; and means for determining the angle through which the beam is scanned from a predetermined starting position to a position at which there is a transition between reflection of the beam from the boundary and transmission of the beam through the boundary.

2. A refractometer according to claim 1, wherein the reference body is a hemisphere made of transparent material, the interface surface of the reference body being formed by the circular plane face of the hemisphere and the measurement plane by any diametral plane of the hemisphere perpendicular to the circular plane face.

3. A refractometer according to claim 2, wherein the hemisphere is made of glass.

4. A refractometer according to claim 1, 2 or 3, wherein the beam of electromagnetic radiation is produced by a movable electromagnetic radiation source movable along an arcuate path in the measurement plane centred on the geometrical centre of the semi-circular cross-section of the reference body.

5. A refractometer according to claim 4, wherein the scanning means comprises a mechanical drive for the electromagnetic radiation source.

6. A refractometer according to claim 5, wherein the electromagnetic radiation source is supported on a scanning arm fixed to a driven shaft for rotation therewith.

7. A refractometer according to claim 6, wherein a drive arm is attached to the driven shaft to impart rotational movement thereto and the drive arm is driven by a projection carried on a drive disc rotatable about an axis parallel to the axis of the driven shaft.

8. A refractometer according to any preceding claim, wherein a counting means is provided to provide a digital count to determine the angle through which the beam is scanned to reach the transition between reflection and transmission of the beam.

9. A refractometer according to claim 8 when dependent on claim 7, wherein the counting means comprises a slotted disc which is movable in synchronism with the drive disc and which intersects a further beam of electromagnetic radiation, and means for converting the radiation passing through the slotted disc into electrical pulses which are then counted.

10. A refractometer according to claim 9, wherein the drive disc constitutes the slotted disc.

11. A refreactometer according to claim 8, wherein the counting means comprises a stepper motor.

12. A refractometer according to claim 8, 9, 10 or 11 , wherein means are provided for detecting the start and end of a scan.

13. A refractometer according to any preceding claim, wherein temperature sensing means are provided to sense and indicate the temperature of the test material.

14. A refractometer according to claim 13, wherein means are provided for determining the value of the refractive index or a related parameter of the test material at a standard temperature from the value obtained at the actual temperature of the test material.

1 5. A refractometer according to any preceding claim, wherein the test material is a sugar solution and the parameter to be measured is the "Brix" value of the solution.

1 6. A refractometer according to any preceding claim, wherein the electromagnetic radiation is visible light.

17. A refractometer according to any preceding claim, wherein operation of the refractometer is controlled by a microprocessor system.

1 8. A method of determining the refractive index, or a related parameter, of a test material, comprising: placing the test material in contact with a plane interface surface of a reference body having a semicircular cross-section in a measurenient prane perpendicular to the interface surface; directing a beam of electromagnetic radiation radially into the reference body through its curved surface; scanning the beam through a range of angles in the measurement plane; detecting radiation reflected from the boundary between the reference body and the test material; and determining the angle through which the beam is scanned from a predetermined starting position to a position at which there is a transition between reflection of the beam from the boundary and transmission of the beam through the boundary.

1 9. A method according to claim 18, wherein the reference body is a hemisphere made of transparent material, the inferface surface of the reference body being formed by the circular plane face of the hemisphere and the measurement plane by any diametral plane of the hemisphere perpendicular to the circular plane face.

20. A method according to claim 19, wherein the hemisphere is made of glass.

21. A method according to claim 18, 19 or 20, wherein the beam of electromagnetic radiation is produced by movable source movable along an arcuate path in the measurement plane centred on the geometrical centre of the semi-circular crosssection of the reference body.

22. A method according to claim 21, wherein the beam is scanned through a range of angles by a mechanical drive for the electromagnetic radiation source.

23. A method according to claim 22, wherein the electromagnetic radiation source is supported on a scanning arm fixed to a driven shaft for rotation therewith.

24. A method according to claim 23, wherein a drive arm is attached to the driven shaft to impart rotational movement thereto and the drive arm is driven by a projection carried on a drive disc rotatable about an axis parallel to the axis of the driven shaft.

25. A method according to any one of claims 1 8 to 24, wherein a counting means is provided to provide a digital count to determine the angle through which the beam is scanned to reach the transition between reflection and transmission of the beam.

26. A method according to claim 25 when dependent on claim 24, wherein the counting means comprises a slotted disc which is movable in synchronism with the drive disc and which intersects a further beam of electromagnetic radiation, and means for converting the radiation passing through the slotted disc into electrical pulses which are then counted.

27. A method according to claim 26, wherein the drive disc constitutes the slotted disc.

28. A method according to claim 25, wherein the counting means comprises a stepper motor.

29. A method according to claim 25, 26, 27 or 28, including detecting the start and end of a scan.

30. A method according to any one of claims 1 8 to 29, wherein temperature sensing means are provided to sense and indicate the temperature of the test material.

31. A method according to claim 30, wherein means are provided for determining the value of the refractive index or a related parameter of the test material at a standard temperature from the value obtained at the actual temperature of the test material.

32. A method according to any one of claims 18 to 31, wherein the test material is a sugar solution and the parameter to be measured is the "Brix" value of the solution.

33. A method according to any one of claims 1 8 to 32, wherein the electromagnetic radiation is visible light.

34. A method according to any one of claims 1 8 to 33, wherein operation of the refractometer is controlled by a microprocessor system.

35. A refractometer substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

36. A method of determining the refractive index, or a related parameter, of a test material substantially as hereinbefore described with reference to the accompanying drawings.

37. Any novel feature or combination of features described herein.