EP0031350A1

EP0031350A1 - A method and an apparatus for determining refracted light

Info

Publication number: EP0031350A1
Application number: EP19800901235
Authority: EP
Inventors: Poul Erik Aegidius
Original assignee: N Foss Electric AS
Current assignee: N Foss Electric AS
Priority date: 1979-06-26
Filing date: 1981-01-26
Publication date: 1981-07-08
Also published as: IN152268B; WO1981000152A1

Abstract

Dans un refractometre, un milieu peut etre teste en determinant la position de la limite d'un rayon de lumiere refracte ayant passe par ce milieu. La position de cette limite est determinee en deplacant un dispositif de detection de lumiere (22, 23) relativement et transversalement par rapport au rayon de lumiere refracte suivant une trajectoire entre une premiere et une seconde positions de chaque cote de cette limite. Le dispositif de detection de lumiere comprend une paire de capteurs de lumiere espaces de maniere rapprochee suivant ladite trajectoire pour detecter l'intensite lumineuse de cette trajectoire et pour produire des signaux de sortie en reponse a l'intensite de la lumiere recue. La position de la limite du rayon de lumiere reflechie est determinee d apres le taux de variation de l'intensite lumineuse le long de cette trajectoire derivee des signaux de sortie.In a refractometer, a medium can be tested by determining the position of the limit of a ray of refracted light having passed through this medium. The position of this limit is determined by moving a light detection device (22, 23) relatively and transversely to the ray of refracted light along a path between a first and a second position on each side of this limit. The light detection device includes a pair of closely spaced light sensors along said path to detect the light intensity of that path and to produce output signals in response to the intensity of the light received. The position of the limit of the reflected ray of light is determined from the rate of change of the light intensity along this path derived from the output signals.

Description

REFRACTEDLIGHT

The present invention relates to refractometers, and more particularly to the type being used for testing a medium on the basis of measurement of the critical angle of total reflection.

In such refractometers the medium to be tested is arran¬ ged in contact with the surface of a prism, and light is passed under various angles of incidence to said surface. The critical angle of total reflection is determined by observing the refracted light beam passing through the ^•prism. In visual refractometers of this type the refrac¬ ted light beam is passed through a field of vision which is thereby divided into bright and dark zones, and the visual refractometer is read by estimating the position of the boundary line between these zones in relation to a fixed scale.

When a refractometer is used for testing a liquid having solid particles dispersed therein, such as milk, the boundary line between the bright and dark zones becomes blurred, and it is very difficult to make a reliable visual reading.

It is also known to use photoelectric cells in refrac- tometers for scanning the bright and dark zones in order to determine the boundary line therebetween. Such refrac¬ tometers are disclosed for example in US Patents Nos. 2,972,926 and 2,966,091. While these known refracto¬ meters are rather complicated they are not able to make an accurate reading of the boundary line between the bright and dark zones or the angle of total reflection when the medium being tested is milk. It has been found that a human eye perceives light impressions in a much more complicated way than that in which light impressions are recorded by a photoelectric cell. Thus, a photoelec¬ tric cell will detect a transition between a bright and a dark zone which is much less distinct than that observed

-BUREAU by the human eye. Even when the medium tested in the refractometer is pure water, the photoelectric cell will detect an unsharp boundary line while that observed by the human eye is very sharp. In the last-mentioned US patent it has been proposed to differentiate the signal provided by the photoelectric cell and to determine the position of the boundary line on the basis of this dif¬ ferentiated signal. It has been found, however that this method does not produce the desired accurate results, i.e. due to difficulties in providing a relative movement between the light sensor and the light beam being scanned with an exactly constant speed.

In a refractometer marketed by ANACON, INC., Ashland, Massachussetts, USA under the trade name "Model 47 Process Analyzer", a second photoelectric cell is positioned within the bright zone as a reference in order to compensate for variations in light intensity. However, this measure does not solve the basic problem in obtaining an accurate and reproducible determination of the position of the boundary line between bright and dark zones in a refractometer by a means of a photoelec¬ tric cell or another light sensing device, when a difficult medium such as milk is tested.

The present invention provides a method and apparatus by means of which an improved determination of such boundary line or boundary between light and dark zones may be obtained.

Thus, the present invention provides a method of deter¬ mining a boundary of a beam of refracted light having passed a medium being tested in a refractometer, said method comprising: moving a light sensing device relatively and trans¬ versely to said refracted light beam along a path between first and second positions on either side of said

O boundary, said light sensing device comprising a pair of light sensors closely spaced along said path for sensing the intensity of light along said path, providing an output signal from each of said sensors in response to the intensity of light received thereby, and determining said boundary on the basis of the rate of variation of said light intensity along said path, said rate of variation being derived from said output signals.

It is understood, that the difference between signals simultaneously provided by the said light sensors plotted as a function of the position along the path of the sensing device may be considered the rate of variation of the light intensity along said path or an approximated first derivative of_ the light intensity function deter¬ mined by a single light sensor. Due to the fixed spacing of the said pair of light sensors the signal difference and, consequently, the rate of variation of the light intensity determined will be substantially unaffected by possible variations in the speed at which said sensing device is moved along its path. The spacing between the light sensors, which may, for example, be photoelectric cells or photo- diodes, should be sufficient to secure a suitable signal difference, but as small as possible in order to obtain an acceptable approximation to the first derivative. The said spacing of the sensors, for example, be in the order of 0.5 mm.

When the medium being tested in the refractometer is a liquid containing particles of a size which is rela¬ tively big in comparison with the wavelenght of the light used in the refractometer, such particles will not substantially contribute to the refraction of the light, but tend to reduce the intensity of the light received by the light sensing device and also to scattering of

OMPI_ . IPO the light whereby the boundary between the bright and dark zones scanned by the light sensing device will be even more blurred. When milk is tested in the refracto¬ meter and infra-red light having a wavelength of about 950 nanometers, the content in the milk of solids not being fat (in the following referred to as "SNF" or "solids non-fat") will contribute to refraction of the light, while the fat globules will influence the refrac¬ tion only to a small degree.

It has been found that when a refractometer of the type described is used for determining "solids non-fat" in a milk sample and this determination is based on the position along the path of the light sensing device at which the said rate of variation or the said first derivative of the light intensity function is maximum, the result obtained will to some extent vary with the time elapsed from placing the sample in the refractometer This is probably due to the fact that fat globules in the milk sample will eventually rise upwardly away from the interface between the sample and the supporting transparent body or prism, so that the character of the part of the milk sample positioned at that interface will change.

It has surprisingly been found that if consecutive measurements are being made on one and the same milk sample which has just been placed in the refractometer, a third position along the path of the light sensing device at which said rate of variation of the light intensity is approximately 3/4 of the maximum rate of variation along said path, will be substantially the same for all of the measurements, when a short period of time (for example about 15 seconds) has elapsed. Accord- ing to the invention this third position may be considere the boundary between the bright and dark zones, and determination of the position of this boundary along the path of the light sensing device may then be successfully used for determining "solids non-fat" in liquids contai¬ ning fat globules, such as milk. It is believed that a similar procedure may advantageously be used in testing other kinds of liquids containing rising particles of a size which is relatively big compared to the wavelength of the light used in the refractometer.

According to the invention it is preferred to determine the said boundary by differentiating said rate of vari- ation of the light intensity as a function of a position of said light sensing device along said path by means of and R-C network providing a signal representative of said differentiation. The R-C network may then have a time constant causing.a time delay of said differentia¬ tion signal corresponding to the distance along said path between said third position and a fourth position corresponding to the maximum rate of variation of the light intensity.

A liquid medium to be tested in a refractometer is nor¬ mally positioned between a fixedly mounted prism and a hinged movable prism which may be swing from a closed position, in which adjacent surfaces of the prisms are positioned in closely spaced parallel relationship, to an open position, in which the said surfaces of the prisms are easily accessible.

According to the present invention the said refractometer may comprise a transparent light refracting body defining therein a narrow space communicating with an inlet for introducing the liquid medium to be tested into said space, and with an outlet for discharging said liquid medium therefrom, said method comprising injecting an amount of liquid medium into said space through said inlet at a high speed, said amount of liquid substan¬ tially exceeding the volume of said space. It has been found that when an excessive amount of the liquid medium to be tested, such as milk, is injected into the said space or cuvette at a high speed, a pre¬ ceding sample contained in the said space may effecti- vely be flushed out from the space or cuvette and repla¬ ced by a new sample. Thus, successive measurements may be made without any special cleaning operations between the introduction of successive samples.

The liquid medium to be tested must be injected at such a pressure that the liquid medium to be tested will flow through the sample receiving space or the flow cuvette at a speed which is sufficiently high to remove any traces of the preceding sample. It has been found that this may be obtained when the liquid in the said space attains a speed of about 3 to 6 m/sec, preferably about 5 m/sec. The thickness of the sample receiving space is preferably about 30 to 40 m, for example 35m.

The liquid medium to be tested may be injected into the sample receiving space in any suitable manner generating a pressure which is sufficient to obtain the necessary high flow speed through the sample receiving space or the cuvette. It is preferred, however, to inject the said liquid medium into said space through the said inlet by means of a syringe.

When the liquid medium being tested is a milk sample, rising of the fat globules in the milk sample received in the sample receiving space or cuvette is promoted when the temperature of the sample is increased. Further¬ more, the refractive index of the sample being measured is to some extent dependent on the temperature of the sample. Therefore, the necessary measuring time may be reduced and the measuring accuracy may be increased if the transparent body defining the sample receiving space is heated and kept at a substantially constant temperature.

The present invention also provides an apparatus for use in carrying out the method described above, said appara- tus comprising: a first body of transparent material having a surface for receiving a medium to be tested, a light source for emitting light through said medium and through said body so as to provide a refracted beam of light, a light sensing device comprising a pair of light sensors, means for moving said sensing device relatively and transversely to said refracted beam of light along a path extending between a first position within said light beam and a second position outside said light beam, said light sensors closely spaced along said path and each sensor being adapted to provide output signals in response to the intensity of light received, and means for determining a boundary of said refracted light beam on the basis of the difference between signals provided simultaneously by said pair of sensors along said path.

The invention will now be further described with reference to the drawings showing preferred embodiments of the apparatus according to the invention, and wherein Fig. 1 is a perspective view of a first embodiment of the apparatus with certain wall parts cut away,

Fig. 2 is an enlarged perspective view showing the driving mechanism for moving the light sensing device of the apparatus, Fig. 3 has side view and partially sectional view of the sensing device and adjacent parts of the driving mechanism therefor,

Fig. 4 is a plan view of the light sensing device shown in an enlarged scale, Fig. 5 is a diagrammatic illustration of the path of light rays through the apparatus,

Fig. 6 are graphs illustrating the basic principles of determining light refraction in the apparatus according to the invention,

Fig. 7 is a block diagram showing the electrical system of the apparatus, Figs. 8 and 9 are perspective views of a second embodimen of the sample receiving part of a refractometer according to the invention shown in a closed and an open position, respectively, and

Fig. 10 is a block diagram showing a second embodiment of the electrical, system of the apparatus or refracto¬ meter.

A modified embodiment of the apparatus or refractometer according to the invention will now be described with reference to Figs. 8 - 10, and corresponding parts of the various embodiments shown in the drawings have been provided with the same reference numerals.

The apparatus shown in the drawings comprises a measuring prism 10 which is fixedly embedded in an aluminum block at the top of the apparatus so that the upper surface 11, which is a sample receiving surface, may be exposed. In the preferred embodiment the prism 10 is a right angled, equilateral prism of the type used in binoculars. A similar second prism 12 is mounted in an aluminum block 13 hinged to the upper edge of the housing 14 of the apparatus so that the block 13 with the prism 12 may be swung from an open position, shown in Fig. 1, in which the sample receiving surface 11 of the prism 10 is exposed, to a closed position shown in Figs. 2 and 5, in which the hypotenuse surfaces of the prisms 10 and 12 are arranged in closely spaced parallel relationship, the spacing between the hypotenuse surfaces being de¬ termined by a thin spacer 15 or by countersinking one of said prisms in its block. A too close spacing of the hypotenuse surfaces of the prisms may cause formation of lines of interference, and a wide spacing may cause a

OM significant decrease in light intensity. Therefore, the said spacing must be carefully selected in consideration of these facts. In the embodiment shown on the drawings the spacing may, for example, be 0.06 mm. The hypotenuse surfaces of the prisms are preferably finely ground, and the outer surfaces of the prisms not being passed by light (Fig. 5) are preferably matted and covered by black optical, light absorbing lacquer. In its closed position the block 13 may be latched by means of a latching device 16.operable by a handle 17.

The block 13 also contains a light source, such as.a light emitting diode, adjustably mounted within a bore in the block. This diode 18 is preferably of the type emitting infra-red light. When the liquid sample being measured is milk, the wavelength of the infra-red light may, for example, be approximately 950 nanometers. The use of this relatively long wavelength ensures that also the bigger particles of protein in the sample will con- tribute to refraction of the light instead of scattering the light. Light emitted by the diode 18 is passed through the prisms 10 and 12 and through a liquid sample 19 arranged in the spacing therebetween, Fig. 5. The re¬ fracted light beam passes the opening of an aperture 20 and a lens 21, and a board or panel 22 which is movably mounted at the focus point of the lens carries a pre¬ amplifier 23 comprising a printed circuit 23a. A pair of light sensors 24, such as photodiodes, arranged between the panel 22 and the preamplifier 23 in positions so as to be aligned with holes 25 in the panel 22. The use of photodiodes as light sensors is especially advantageous when a light emitting diode is used as the light source. The light sensors 24 are covered by an opaque film 26 having two transparent windows 27 formed as narrow slots aligned with either one of the light sensors 24. This film may advantageously be made by producing a reduced photographic negative of a drawing with two black lines

OMPI _/. WIPO made on a white background. The width of the slot-like windows may, for example, be in the order of 100 μ, and the distance "a" (Fig. 4) between the slots may, for example, be of the order 0.5 mm (Fig. 4).

The assembly constituted by the panel 22, the pream¬ plifier 23, and the light sensors 24 are mounted on a U-shaped connecting member 28 interconnecting a pair of arms 29 swingably mounted by means of studs 30 about an axis substantially passing through the optical center of the lens 21. The connecting member 28 is connected to the arms 29 by means of screws 31 extending through elongated slots formed in the connecting member allowing translatory and rotational adjustment of the connecting member 28 in relation to the arms 29 and, consequently, of the light sensors 24 in relation to the lens 21, whereby possible variations in the optical character¬ istics of the lens 21 and the prisms 10 and 12 may be compensated for. In order to determine the position of the boundary line 32 between the bright zone 33 within the refracted light beam 34 having passed the lens 21, and the dark zone 35 outside the light beam (Fig. 5) the arms 29 and the light sensors 24 mounted thereon are moved transversely to the light beam by means of a driving mechanism. This driving mechanism comprises a synchroneous motor 36 to which power may be supplied from a suitable AC power source. The motor 36 is driving a cam 37 through a wormgearing 38. The cam surface of the cam 37 shaped as the involute of a circle is in engagement with a cam follower 39 which is mounted on the free end of one arm 40 of a lever 41 which is rotatable about a shaft 42. The other arm 43 of the lever is formed as a screw the free end of which is cooperating with an adjusting nut 44 (Fig. 3) bearing against an angle bracket 45 which is adjustably mounted on the connecting member 28 by means of screws 46 exten¬ ding through elongated slots in the bracket 45. The

Q... . Wii- lever arm 41 is fastened to the shaft 42 by means of a pointed screw 47 so that the angular position of the lever arm 40 on the shaft 42 and, consequently, the relative angular position between the lever arms 40 and 43 may be adjusted so as to optimize the linearity of the movement of the light sensors 24. Further adjustment may be made by means of the nut 44 in order to compensate for possible variations in the refracting index of the prisms. This possibility of adjustment allows the use of rather cheap prisms of the type used in binoculars.

A pin 48 extending from a brake drum 55 described below may cooperate with an actuating arm of a trigger switch 49 which is mounted on a swingable adjusting arm 50 the free end of which extends outwardly through a slot in the housing 14 (Fig. 1). The apparatus also comprises a microswitch 51 for on and off switching of the diode 18 and a digital display 52, and a microswitch 53 for star¬ ting and stopping the motor 36 when the block 13 with the prism 12 is closed and opened, respectively.

The cam 37 is mounted on a rotatable shaft 54 also car¬ rying the brake drum 55. A brake block 56 which is sup¬ ported by a swingably mounted brake arm 57 is in fric- tional engagement with the drum 55 under the bias of a spring 58. This braking system eliminates possible back¬ lash in the driving mechanism of the apparatus.

When a light sensor, such as one of the photodiodes 24, is moved transversely to the refracted light beam 34 from a position in the bright zone within that beam to a position within the dark zone outside the beam, the light sensor will generate an electrical signal "y" in response to the light intensity received. Fig. 6a shows graphs where this electrical signal "y" is plotted versus the position "x" along the path of movement of the light sensor in measuring the contents of "solids non-fat" for two consecutive measurements of a milk sample. The graph shown in solid line may, for example, be plotted imme¬ diately after placing the milk sample 19 on the sample receiving surface 11 of the prism 10 while the graph shown in dotted line may be plotted after a certain pe¬ riod of time when a substantial part of the fat globules within the milk sample has risen to the upper part of the sample receiv-ing space between the prisms 10 and 12 so that the part of the milk sample which is located closely adjacent to the prism surface 11 and on which the actual measurement is being made has a' smaller fat content than immediately after placing of the sample on the surface 11. From Fig. 6a it is noted that a decrease in the fat content causes an increase in the light inten- sity of the refracted light beam 34 and a decreased scattering of the refracted light so that the boun-dary line between the bright and dark zones becomes more marked than when the fat content is higher. P_Q indicates the point on the solid graph where the rate of variation of the signal "y"^" is maximum, i.e. where the slope of the tangent to the curve is maximum. This point _Q could be used for defining the boundary line between the bright and dark zones. However, as appears from Fig. 6a the location of this point along the path of movement of the light sensor is dependent on the period of time passed from the moment of placing the sample on the surface 11, i.e. of the number of fat globules having risen upwardly from that surface.

The preferred embodiment of the apparatus according to the invention is intended for measuring the contents of "solids non-fat" in milk samples. Consequently, the position of the boundary line on which the measurement is based should preferably be substantially independent of the fat rising tendency in milk samples.

Fig. 6b shows graphs representing the first derivatives

O of the graphs shown in Fig. 6a, the dotted line curve having been adjusted to the same peak value U_Q as the solid line graph. From Fig. 6b it is noted that the graphs plotted for consecutive measurements of the same milk sample intersect each other at a point P., located after the point P_Q along the path of movement of the light sensor. It has surprisingly been found that similar graphs plotted for further measurements of the sample intersect each other in substantially the same point P-, which has been found to have an ordinate of about 3/4 of the peak value, i.e. 3/4 U_Q. As the position of the point P₁ along the path of movement of the light sensor is substantially independent of the fat rise in the milk sample it has, according to the invention, been found convenient to base the measurement of SNF on the location of that point, or, in other words, to consider the boun¬ dary line between the bright and dark zones positioned at that location.

Fig. 6c shows the second derivative of the solid graph of Fig. 6a. The differentiation of the function illu¬ strated in Fig. 6b may suitably be made in the electronic circuitry of the apparatus. The second differentiation illustrated in Fig. 6c may conveniently be made in an R-C network having a time constant causing a time delay corresponding to the distance "d" on the abscissa. Fig. 6d illustrates the second derivative having been derived from and delayed in such an R-C network. As appears from Fig. 6d the location of the point P, in Fig. 6b may now ^• be determined as point P₂ in Fig. 6d. In practice the point P₂ is not selected as the intersecting point with the abscissa axis, but as the abscissa of a point P'₂ on the second derivative graph having an^' ordinate 1/n x U_p which is a predetermined small fraction of the maximum value U_p as illustrated in dotted lines. The said small fraction may, for example, be 1/16. The operation of the apparatus described above will now be described with further reference to Fig. 7 showing a block diagram of the electrical system of the apparatus. The apparatus may, for example, be powered from DC power supply 59, such as a battery or a rectified main supply, connected to an oscillator 60 which generates an AC voltage of a specific constant frequency whereby the apparatus may be powered by a battery, if desired. This AC voltage is supplied to a frequency dividing network 61 supplying AC power to the motor 36 and also genera¬ ting counting pulses .62 as further described below. When the aluminum block 13 has been opened and the exposed surfaces of the prisms 10 and 12 have been cleaned, a few drops of a liquid sample, such as milk, to be tested is placed on the sample receiving surface 11 of the prism 10. The block 13 is now moved to its closed position and locked in that position by means of the latching device 16. Movement of the block 13 to its closed position actuates the microswitches 51 and 53 whereby power is supplied from the frequency dividing network 61 to the motor 36 and the digital display 52 as well as the light emitting diode 18 are switched on. The motor will now rotate the cam 37 through the wormgearing 38, and rotation of the cam 37 will cause the cam followe 39 to move downwardly, so that the adjusting nut 44, the connecting member 28, and the parts mounted thereon will be moved upwardly whereby the light sensors 24 is caused to make a scanning movement transversely through the refracted light beam 34. In the apparatus described the position of the boundary line between the bright and dark zones is in fact determined as the period of time passing from the moment at which the pin 48 actuates the trigger switch 49 till the said boundary line (P'₂ in Fig. 6d) has been reached by the light sensors 24 during their scanning movement. Therefore, when the switch 49 is actuated by the pin 48 an output pulse is passed to a start pulse generating gate 63 which provides a start

"g T /.. pulse 64. The light sensors or photodiodes 24 are differentially connected to the preamplifier 23, and the amplified differential output signal - representing an approximation to the first derivative as illustrated in Fig. 6b - is passed from the preamplifier 23 to a low-pass filter 65 for eliminating noise from said output signal. The filtered amplified differential signal from the sensors 24 representing the rate of variation of the light intensity along the path of movement of the light sensors is passed to an R-C network 66 in which the input signal supplied thereto is further differentiated and time delayed in the manner described above with reference to Fig. 6d. The output signal from the R-C network is passed to a peak value detector 67 comprising amplifiers 68 and 69, a rectifier 70, a capacitor 71, and a reset switch 72. The output signal from the peak value detector 67 is transmitted to a comparator 73 which is also connected to the output of the RrC network 66. The comparator 73 which includes a voltage dividing network 74 and an amplifier 75 compares the output signal from the R-C network 66 with a predetermined small frac¬ tion 1/n of the peak value U_p (Fig. 6d) detected by the peak value detector in order to determine when the said output signal has decreased to a level corresponding to the said small peak value fraction. The output signal from the peak value detector 67 is also transmitted to a discriminator 76 having an input 77 to which a predeter¬ mined reference voltage U_R (Fig. 6d) is supplied. When the discriminator 76 has decided that the peak value received exceeds the reference voltage supplied to the input 77 and the comparator 73 has decided that the signal received from the R-C network 66 has decreased to a level corresponding to the said small peak value frac¬ tion, a stop pulse generating gate 78 is caused to gene- rate a stop pulse 79 which is supplied to a flip-flop 80 to which the start pulse 64 generated by the gate 63 is also supplied. The purpose of the discriminator 76 is to ensure that the gate 78 does not generate a stop pulse 79 unless a sufficiently high peak value U_R (Fig. 6d) has been reached, in order to ensure that a measurement is made only when a sufficiently distinct signal is 5 present. The output of the flip-flop is connected to an AND gate 81 having a second input to which the counting pulses 62 are supplied. It should be understood that the flip-flop 80 controls the AND gate 81 in such a manner that the counting pulses 62 from the frequency dividing

10 network 61 are supplied to a counter 82 only from the time at which the start pulse 64 is generated due to activation of the trigger switch 49 to the time at which the stop pulse 79 is generated by the gate 78. Consequent¬ ly, the number of pulses counted by the counter 82 will

15 represent the period of time elapsed from the time of activating the trigger switch 49 to the time at which the conditions laid down by the comparator 73 and the discriminator 76 have been fulfilled, which means that the position (P^f ₂ in Fig 6d) of the desired boundary

20 line between bright and dark zones has been reached by the light sensors 24 scanning the refracted light beam 34. The number of pulses counted by the counter 82 may, of course, be converted into any desired measuring units of the desired characteristic of the liquid sample 19.

25 Thus, if the liquid sample is a milk sample the counted number of pulses may be directly converted into a value indicating the contents of "solids non-fat" in the sample.

When the motor 36 has been started by closing the block 30. 13 as mentioned above the motor continues rotating till the block 13 is opened. Every revolution of the cam 37 represents a measuring cycle starting when the start pulse 64 is generated. The start pulse 64 is not only supplied to the flip-flop 80, but also to the peak value 35 detector 67 causing the reset switch 72 thereof to close so as to shortcircuit the capacitor 71 and thereby reset the peak value detector at the start of every measuring cycle. The start pulse 64 is also directly supplied to the counter 82 causing transfer of the count from the previous measuring cycle to a memory 83 and resetting of the counter. The count stored in the memory 83 may be read out on the digital display 52 in the desired measuring units, such as the percentage of "solids non-fat" in the liquid sample 19. The counter 82 is advantageously adapted to transfer the count of a measuring cycle only in case a stop pulse 64 has been generated in the respective cycle so as to ensure that a measuring result is not read out until the conditions laid down by the comparator 73 and the discriminator 76 have been fulfilled which indicates that the measurement made has been found correct.

The refractive index of the sample being measured is to some extent dependent on the temperature of the sample. Since the sample 19 being arranged in the space 15 be¬ tween the prisms 10 and 12 is very small - the spacing being in the order of 0.06 mm - the temperature of the sample will attain the temperature of the prisms within a very short period of time (approximately 15 seconds). The temperature of the prisms and, consequently, the final temperature of the sample may be detected by a temperature detecting device (not shown) such as a ther¬ mistor which may be electrically connected to the coun¬ ting circuitry which may comprise suitable means for compensating for variations in temperature of the prisms and the sample. When the sample being measured is a milk sample it may be shown that, a change of 0.1% SNF corresponds a temperature change of 1.8°C. Therefore, if, for example, an accuracy of measurement in the order of 0.05% SNF is desired, the temperature compensating means must correct for temperature changes within + 1°C.

Before the apparatus described above is used for measuring samples, the operator should adjust the appara- tus by placing a reference sample of known content of SNF on the sample receiving surface 11. After having locked the block 13 in its closed position and waited a short period of time (approximately 15 seconds) the operator should move the adjusting arm 50 to a position so that the correct read-out of SNF on the digital dis¬ play 52 is obtained. The trigger switch 49 which is mounted on the arm 50 is moved together therewith. Con¬ sequently, movement of the arm 50 causes a change of the time at which counting of pulses by the counter 82 is started.

As explained above, the apparatus or refractometer described is substantially insensitiv to the rise of fat globules in a milk sample before and during testing of the sample. It has been found, however that the measuring result obtained for "solids non-fat" is in fact dependent on the fat content of the sample. The reason for this fat dependency is believed to be the following: When the content of "solids non-fat" in a milk sample is determined by means of the conventional standard method, this content is calculated as percentage by weight based on the total weight of the sample. How¬ ever, when the content of "solids non-fat" is determined by means of a refractometer, this content is determined as percentage by volume based on the total volume of the sample minus the volume of the fat content therein. This seems to be the reason why the measuring results obtained by the refractometer must be adjusted in dependency of the fat content of the sample in order to obtain results comparable to the results obtained by using conventional standard methods for determining contents of "solids non-fat". As stated on the front panel of the apparatus shown in Fig. 1, the true content of SNF may be calcula- ted from the following equation:

SNF = readout - 0.1 x percentage of fat. Correspondingly, the total content of solids TS may be calculated by the equation:

TS = readout + 0.9 x percentage of fat.

Example.

in a preferred embodiment of the apparatus described above further details may be as follows:

The lens 21 may be a cheep non-achromatic lens having a focal lenght of about 120 mm, and the prisms 10 and 12 may be right angled equilateral prisms of the type used in binoculars and made from BK-7 glass which has a rela¬ tively low index of refraction of about 1.518. The light emitting diode 18 may be of the type LD 271 having an angle of radiation of about 30°, a power of radiation of about 16 mW, and a frequency of radiation of about 950 nanometers (infra-red). The light sensors 24 may be photodiodes of the type BPW 32 SIEMENS, and the pre¬ amplifier 23 may be of the type AD 545 J. The photodiodes 24 each has a light sensitive area of about 1 x 1 square millimeters, and the said diodes are arranged with a spacing of about 0.5 mm in the direction of movement.

The slots 27 in the film 26 each has a width of about 100 μ. The R-C network 66 includes a resistor of about 100 k ohms and a capacitor of about 4.5 μF. The R-C network has a time constant of 0.5 seconds causing a time delay corresponding to 0.3% SNF.

The cam 37 has a pitch of 30 mm, and the ratio between the length of the lever arm 40 and the length of the arm 43 is approximately 2.5:1. The wormgearing 38 has a gear ratio of 1:40. The synchroneous motor 36 may be a Crouzet motor rotating at a speed of about 600 r.p.m. The motor is powered from the frequency dividing network 61 at a frequency of 50 Hz. As the motor rotates 10 revolutions per second and the gear ratio of the wormgearing 38 is 1:40, the cam 37 will rotate one revolution every four seconds. The frequency of the counting pulses 62 is 100 Hz and, consequently, the frequency dividing network 61 will generate 400 pulses for every revolution of the cam 37. The maximum measuring interval of the apparatus is approximately 20% SNF which implies an accuracy in measurement of 0.05% SNF provided that errors originating from the mechanical and optical parts of the apparatus have been adjusted or compensated for.

Figs. 8 and 9 show the top portion of the apparatus comprising a measuring prism 10 which is fixedly embedded in an aluminium block at the top of the apparatus so that the upper surface 11 of the prism may be exposed. A similar second prism 12 is mounted in an aluminium block 13 hinged to the upper edge of the housing 14 of the apparatus so that the block 13 with the prism 12 may be swung from an open position shown in Fig. 9 to a closed position shown in Fig. 8. In this closed position the hypotenuse surfaces of the prisms 10 and 12 are positioned in a closely spaced parallel relationship so as to define a sample receiving narrow space there¬ between. This sample receiving space may be peripherally sealed by means of an annular gasket or sealing ring 84 mounted on the block 13 as shown in Fig. 9. Inlet and outlet bores 85 and 86, respectively, in the block 13 communicate with the sample receiving space. At its outer end the inlet bore 85 is provided with a nipple 87 adapted to receive the spout 88 of a syringe 89 as illu¬ strated in Fig. 8. The outer end of the outlet bore 86 is closed by means of a screw 90 and a sealing gasket

91. An outlet conduit 92 including a one-way or back-pres sure valve 93 is connected to the outlet bore 86 through

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Q..f Λ. WIF a transversely extending bore 94 in the block 13. The block 13 may be maintained in its closed position shown in Fig. 8 by means of a latching device comprising a pair of spaced parallel posts 95 extending from the upper surface of the housing 14 and an angular resilient latch member 96 pivotally mounted at the upper end of the posts 95.

As indicated in Fig. 8 the prisms 10 and 12 may be pro- vided with heating means, for example a power transistor 97, and thermostatic control means which may comprise a thermistor 98, for maintaining the temperature of the prisms at a substantially constant, slightly elevated temperature.

The apparatus also comprises a microswitch 51 which may be operated by means of a manually operatable actuating member 99 and an actuating rod 100 connected thereto. The function of the microswitches 51 will be described more in detail below.

In the apparatus shown in Figs. 8 and 9 a liquid sample, such as a milk sample, to be measured is injected into the sample-receiving space defined between the prisms 10 and 12 while the prisms are in their closed position shown in Fig. 8. An excessive amount of the sample to be measured is injected into the sample receiving space through the inlet bore 85 by means of a syringe 89. The sample is preferably injected at such a pressure that the liquid sample will flow through the inlet bore_ 85 and the sample receiving space at a relatively high speed, and the excessive amount of the liquid sample will flow out through the outlet bore 86, the transverse bore 94, and the outlet conduit 92 to waste. The valve 93 secures that the sample receiving space defined between the prisms 10 and 12 remains filled with liquid. It has been found that injection of an amount of liquid sample substantially exceeding the volume of the sample receiving space in the manner just described causes the preceding liquid sample to be flushed out and completely removed from the sample receiving space so. that any further cleaning operations between succeeding measure¬ ments are normally superfluous. After use of the appara¬ tus the sample receiving space may be cleaned by injec¬ ting destilled water or another rinsing liquid into the space. If a milk sample or a sample of another liquid to be measured has been left in the sample receiving space for some time, it may be necessary to swing the block 13 to its open position and clean the surfaces of the prisms manually.

As an example the space defined between the prisms 10 and 12 may have a thickness or height of about 30. to 40 m, especially about 35 m, and-the width of the space may, for example, be about 20 mm. It has been found, that a satisfactory flushing and cleaning of the sample receiving space is obtained when an amount of about 1.3 ml liquid sample is injected into the inlet bore in a period of time not exceeding 0.5 second. The average speed of the liquid within the sample receiving space will then be at least about 3.25 to 4.33 m/sec. However, during injection of a sample the speed will vary and preferably attain a value of 5 m/sec.

The operation of the apparatus will now be further described with reference to Fig. 10 of the drawings showing a embodiment of the electrical system shown in Fig. 7. The apparatus may, for example, be powered from a DC-power supply 59, such a battery or a rectified main supply, connected to an oscillator 60 which generates an AC-voltage of a specific constant frequency. This AC-voltage is supplied to a frequency dividing network 61 supplying AC-power to an electric motor 36 for moving the light sensors (not shown) of the apparatus. The

Q. W motor 36 is preferably a synchronous motor of the type comprising two windings and a capacitor. The motor is preferably not provided with a braking device as that described above.

When a liquid sample has been injected into the sample receiving space as described above the operator depres¬ ses the actuating member 99 whereby the microswitch 51 is actuated. Actuation of the microswitch 51 causes a solid state relay 101 to energize the motor 36. A pin 48 mounted on a cam (not shown) driven by the motor then actuates a trigger switch 49 whereby the operational program of the apparatus as described above is initiated. Thus, actuation of the trigger switch 49 causes genera- tion of an output pulse which is passed to a start pulse generating gate which .provides a start pulse 64. A signal generated by the light sensors 24 of the apparatus is amplified by a preamplifier 23 and passed to a circuitry designated 102 in Fig. 10 and corresponding to the compo- nents designated by the reference numerals 65 to 67, 73 and 76 in Fig. 7. The said preamplified signal from the light sensors is also supplied to one input of a pro¬ grammable counter 103 having its other input connected to the circuitry 102. The counter 103 comprises a pro- gramming device 104 by means of which a desired number of measuring cycles for each measurement may be set. If the sample measured is milk the number of cycles is chosen so as to provide sufficient time for the fat rise in the sample for obtaining a reliable measuring result. As indicated in Fig. 10, for example five cycles may be chosen. When the input signal supplied to the counter 103 from the preamplifier 23 exceeds a predetermined value, this input signal enables the counter which will then start counting of the pulses which are received from the circuitry 102 and which represent the number of cycles. When the counter 103 has received the number of pulses set on the programming device 104 the counter 103 supplies an output pulse to the relay 101 whereby the motor 36 is stopped. If desired, the counter 103 may also be adapted to generate a signal which is supplied to a memory 83 as indicated by a broken line. The display 52 will then indicate only the measuring result obtained during the last cycle. The output signals from the cir¬ cuitry 102 are passed to a gate 78 generating a stop pulse 79 which is supplied to a flip-flop 80 to which the start pulse 64 generated by the gate 63 is also supplied. The output of the flip-flop is connected to an AND-gate 81 having a second input to which counting pulses 62 from the frequency dividing network 61 are supplied. The start pulse 64 is also directly supplied to a counter 82 for counting the pulses 62. The start pulse 64 is also directly supplied to the counter 82 causing transfer of the count from the previous mea¬ suring cycle to the memory 83 and resetting of the counter. The count stored in the memory 83 may be read out on the digital display 52. The function of the cir- cuitry 102 and the electrical elements 78 to 83 is described more in detail above.

As mentioned above, the rise of fat in a milk sample may be accelerated by heating the prisms 10 and 12, for example by means of the power transistor 97. Furthermore, as described above the provision of the programmable counter 103 makes it possible to reduce the number of measuring cycles for each measurement to an impirically determined minimum. As a result, the capacity of the refractometer is considerably increased.

While the apparatus according to the present invention has especially been described in relation to the measurement of milk samples, it should be understood that the apparatus could also be used in measuring other kinds of liquid samples. As an example, the apparatus could be used for determining a content of solids in beer and wort. In the latter case it would normally be desirable to obtain a greater accuracy in measurement. This could, for example, be obtained by changing the gear ratio of the wormgearing 38 so as to obtain a gear ratio of for example 1:100 and by changing the counter conversion correspondingly. In order to allow replace¬ ment of the wormgearing 38 the motor 36 may be mounted on a mounting rail 84 having longitudinally extending slots therein for receiving fastening screws 85 so that the position of the mounting rail 84 and the motor 36 mounted thereon may be adjusted.

Claims

Claims .

1. A method of determining a boundary of a beam of re- fracted light having passed a medium being tested in a refractometer, said method comprising: moving a light sensing device relatively and trans¬ versely to said refracted light beam along a path between first and second positions on either side of said boundar said light sensing device comprising a pair of light sen-sors closely spaced along said path for sensing the intensity of light along said path, providing an output signal from each of said sensors in response to the intensity of light received thereby, and determining said boundary on the basis of the rate of variation of said light intensity along said path, said rate of variation being derived from said output signals.

2. A method according to claim 1, wherein said light sensing device is moved along said path at a substan¬ tially constant speed.

3. A method according to claim 1 or 2, wherein said medium being tested is a liquid containing fat globules, said boundary being determined as an intermediate third position along said path at which said rate of variation of the light intensity is approximately 3/4 of the maximum rate of va-riation along said path.

4. A method according to any of the claims 1-3, wherein said medium is milk.

5. A method according to claim 3 or 4, wherein said boundary is determined by differentiating said rate of variation of the light intensity as a function of the position of said light sensing device along said path.

6. A method according to claim 5, wherein said differen¬ tiation is made by means of an R-C network providing a signal representative of said differentiation.

7. A method according to claim 6, said R-C network having a time constant causing a time delay of said differen¬ tiation signal corresponding to the distance along said path between said third position and a fourth position corresponding to the maximum rate of variation of the light intensity.

8. A method according to any of the claims 3 - 7, wherein a signal representative of said differentiation is passed to a peak value detector, a comparator, and a dis- criminator, said discriminator providing output signals only when a differentiation signal above a predetermined level has been detected by said peak value detector.

9. A method according to any of the claims 1 - 8, wherein said light is emitted by a light emitting diode.

10. A method according to claim 9, wherein said emitted light is infra-red light.

11. A method according to any of the claims 1 - 10, wherein said light sensors are photodiodes.

12. A method according to any of the claims 3 - 11, wherein said determination of the beam boundary is used for calculation of the contents of solids non fat in milk.

13. A method according to claim 12, wherein said calcu¬ lation of solids non fat is based also on the fat content of the milk.

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14. A method according to claim 13, wherein said calcu¬ lation of solids non fat based on said determination of^' the beam boundary is corrected by deducting the percent¬ age of fat in the milk multiplied by a constant.

15. A method according to claim 14, wherein said constant is approximately 0.1.

16. A method according to any of the claims 4 - 15, wherein said milk to be tested is arranged in a thin layer.

17. A method according to any of the claims 1 - 16, wherein said refractometer comprises a transparent light refracting body defining therein a narrow space commun¬ icating with an inlet for introducing a liquid medium to be tested into said space, and with an outlet for dis¬ charging said liquid medium therefrom, said method com¬ prising injecting an amount of liquid medium into said space through said inlet at a high speed, said amount of liquid substantially exceeding the volume of said space.

18. A method according to claim 17, wherein said liquid medium is injected so as to obtain liquid speed in said space of about 3 to 6 m/sec, preferably about 5 m/sec.

19. A method according to claim 17 or 18, wherein the thickness of said space is about 30 to 40 μ.

20. A method according to any of the claims 17 - 19, wherein said liquid medium is injected into said space through said inlet by means of a syringe.

21. A method according to any of the claims 17 - 20, wherein said transparent body is heated and kept at a substantially constant temperature.

22. An apparatus for use in carrying out the method according to any of the claim 1 - 21, said apparatus comprising: a first body of transparent material having a sur- face for receiving a medium to be tested, a light source for emitting light through said medium and through said body so as to provide a refracted beam of light, a light sensing device comprising a pair of light sensors, means for moving said sensing device relatively and transversely to said refracted beam of light along a path extending between a first position within said light beam and a second position outside said beam, said light sensors being closely spaced along said path and each sensor being adapted to provide output signals in response to the intensity of light received, and means for determining a boundary of said refracted light beam on the basis of the difference between signals provided simultaneously by said pair of sensors along said path.

23. An apparatus according to claim 22, further com- prising means for moving said light sensing device along said path at a substantially constant speed.

24. An apparatus according to claim 22 or 23, wherein said determining means comprise means for differentia- ting said signal difference as a function of the position of said light sensing device along said path.

25. An apparatus according to any of the claims 22 - 24 adapted to determine the contents of solids non fat in a liquid sample.

26. An apparatus according to claim 25 wherein said liquid is milk.

27. An apparatus according to any of the claims 24 - 26, wherein said differentiating means comprises an R-C network for providing a signal representative of said differentiation.

28. An apparatus according to any of the claims 24 - 27, wherein said determining means further comprises a peak value detector, a comparator, and a discriminator for suppressing output signals from said differentiating means till a predetermined signal level has been reached.

29. An apparatus according to any of the claims 22 - 28, wherein said light source is a diode emitting infra-red light.

30. An apparatus according to any of the claims 22 - 29, wherein said light sensors are photodiodes.

31. An apparatus according to any of the claims 22 - 29, wherein said means for moving the light sensing device comprise mechanical force transmitting means and means for adjusting the transmission ratio of said force transmitting means.

32. An apparatus according to claim 31, wherein said force transmitting means comprise a lever having first and second arms extending in opposite directions from the fulcrum of said lever, and a rotatable driving cam engaging with said first arm, said second arm engaging with a movable mounting member supporting said light sensing device, said adjusting means being adapted to adjust the ratio of the lengths of said first and second lever arms.

33. An apparatus according to claim 32, wherein said cam has a cam surface shaped as the involute of a circle.

34. An apparatus according to claim 32 or 33, wherein the angular position of said second lever arm in relation to said first lever arm is adjustable.

35. An apparatus according to any of the claims 22 - 34, further comprising a preamplifier for receiving the output signals from said light sensing device, said preamplifier comprising a printed circuit, said light sensing device being mounted on the board of said printed circuit.

36. An apparatus according to claim 35, wherein said board is adjustably mounted.

37. An apparatus according to any of the claims 22 - 36, wherein said light sensors are covered by an opaque masking sheet defining light penetrable zones therein shaped as narrow parallel slots extending transversely to said path, each of said zones being aligned with a respective one of said light sensors.

38. An apparatus according to claim 37, wherein said mask is a photographic film.

39. An apparatus according to any of the claims 26 - 38, further comprising means for determining said contents ^" of solids non fat on the basis of the time used for moving said light sensing device from said first position to the position along said path at which the boundary of the refracted light beam is determined.

40. An apparatus according to claim 39, wherein said means for moving the sensing device comprises a syn¬ chronous motor, said means for determining the solid content comprising counting means for counting voltage pulses of the power supplied to said motor.

41. An apparatus according to any of the claims 22 - 40, further comprising electronic temperature compensating means for temperature compensating said determining means.

42. An apparatus according to any of- the claims 22 - 41, further comprising a second body of transparent material having a medium contacting surface, the medium contacting surface of said second body being movable between a po¬ sition remote from the medium receiving surface of said first body to a position in which said medium receiving and contacting surfaces extend substantially parallel in a closely spaced relationship so as to define a narrow slot or space therebetween for receiving the medium to be tested.

43. An apparatus according to any of the claims 22 - 42, said apparatus comprising a body of transparent material defining therein a narrow space defined by said medium receiving surface, said space communicating with an inlet for introducing a liquid medium to be tested into said space, and with an outlet for discharging said liquid medium therefrom.

44. An apparatus according to claim 43, wherein the width of said space is about 30 to 40 μ.

45. An apparatus according to any of the claims 22 - 44, further comprising heating means for heating said trans¬ parent body, and thermostatic control means for con¬ trolling said heating means so as to maintain said transparent body at a substantially constant temperature.