Abstract:
The present invention provides a system for transmission loss comparison. 
     On embodiment of the present invention compares the transmission losses through optical paths through a first solution and a second solution. This embodiment includes first and second light sources for transmitting light through the first and second solutions. Also provided are first and second light detectors corresponding to the first and second light sources, and comparison means for comparing the outputs of the two detectors in order to determine which solution is darker. 
     In addition, this embodiment includes a common gain balance configuration means for calibrating the relative gain of the two light detectors in order to compensate for signal differences between each source/detector path arising from various mechanical or optical factors. Further included is a dark solution compensation bias to compensate for differences between each source/detector path arising from spectral differences between the two light sources. 
     In order to stabilize the measurement decision, the system also provides source modulation hysteresis, wherein a feedback path is provided between the output of the comparison means and one of the light sources.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of co-pending U.S. application Ser. No. 912,953, filed on Sept. 29, 1986, now U.S. Pat. No. 4,759,631, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to systems for comparing transmission losses, especially through optical transmission paths, although the invention has application to other types of transmission paths. 
     2. Description of Related Art 
     Systems for comparing transmission losses through alternative paths are well known in the art. One prior art optical comparison device employs a split common light source to transmit light along a given path through each of a corresponding pair of samples. The transmitted light is monitored by a corresponding pair of light receivers. The output of the light receivers is then used as the basis for comparing the two samples. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system for transmission loss comparison. 
     One embodiment of the present invention compares the transmission losses through optical paths through a first solution and a second solution. This embodiment includes first and second light sources for transmitting light through the first and second solutions. Also provided are first and second light detectors corresponding to the first and second light sources, and comparison means for comparing the outputs of the two detectors in order to determine which solution is darker. 
     In addition, this embodiment includes a common gain balance configuration means for calibrating the relative gain of the two light detectors in order to compensate for signal differences between each source/detector path arising from various mechanical or optical factors. Further included is a dark solution compensation bias to compensate for differences between each source/detector path arising from spectral differences between the two light sources. 
     In order to stabilize the measurement decision, the system also provides source modulation hysteresis, wherein a feedback path is provided between the output of the comparison means and one of the light sources. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of a preferred embodiment of an electrical circuit according to the present invention; and 
     FIG. 2 is a schematic drawing of a preferred embodiment of a mechanical arrangement of the preferred embodiment of FIG. 1. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention can be better understood by reference to FIG. 1, which shows a preferred embodiment of an electrical circuit according to the present invention. 
     The device shown in FIG. 1 compares the darknesses of two solutions: a reference solution A in one test tube, with a known concentration of solute, and a sample S of the same type of solution, but of unknown concentration. As described more fully below, the device accurately determines whether the sample S is a stronger or weaker solution than the reference solution. 
     In use, the operator actuates pushbutton switch S1, which causes current to flow through the device from battery BT1. Matched light-emitting diodes DS1 and DS2 then transmit light through sample S and reference A, respectively. Light from LED&#39;s DS1 and DS2 not absorbed after passing through the tubes and the liquids is focussed on matched detectors D2 and D3. Each detector produces an output current analogous to the amount of light detected. These currents in turn produce voltages across resistors R6, R4 and balance potentiometer R9. In manufacturing tests, these voltages can be measured at test points TP4 and TP1. 
     The two voltages are compared at op-amp U1B, which is configured as a comparator. Capacitor C1 serves to filter out any signal component in the power supply to op-amp U1. If TP4 is greater than TP1, then comparator U1B will go high, which will provide a positive current through LED DS4 via resistor R3, thus causing LED DS4 to light. It will be seen that TP4 will be greater than TP1 when there is greater current flow through D2 than through D3. This means that more light has reached D2 from DS1 than has reached D3 from DS2, thus indicating that the reference A has absorbed more light than sample S. Thus, the lighting of DS4 indicates that sample S is a weaker concentration of solution than reference A. 
     If TP1 is greater than TP4, then comparator U1B will go low, and op-amp U1A, also configured as a comparator, will go high, thus producing a positive current flow through LED DS3, causing it to light. It will be seen that TP1 will be greater than TP4 when there is greater current flow through D3 than through D2. This means that more light has reached D3 from DS3 than has reached D2 from DS1, thus indicating that sample S has absorbed more light than reference A. Thus, the lighting of DS3 indicates that sample A is a stronger concentration of solution than reference A. 
     It will be seen that the detected S current from photodiode D2 flows through resistor R6 and part of potentiometer R9 (from CCW to the wiper) to ground. Similarly, the detected A current from photodiode D3 flows through resistor R4 and the CW side of potentiometer R9 to ground. The position of the wiper on potentiometre R9 is adjusted during manufacture to compensate for overall gain differences between the A and S measurement paths. Potentiometer R9 compensates for optical (mechanical), electro-optic, as well as electrical gain differences. This type of common gain balance configuration is used in other applications. 
     During manufacture, a variable bi-directional current source is used to inject a periodic triangular waveform into the circuit at TP4. The particular waveform is chosen such that the system just barely flips back and forth between the two indicator states. Potentiometer R9 is adjusted so that the computed average voltage at TP1 equals the computed average voltage at TP4 with clear water in each test tube, i.e., at high signal levels, typically 4 volts at TP1 and TP4. Because the relative gain is adjusted based on the computed average voltage, the result of the calibration is that the A side gain slightly exceeds the S side gain. The difference between the two gains is equal to half of the source modulation hysteresis value, discussed below. 
     In addition to the gain adjustment provided by potentiometer R9, the present invention includes a novel use of offset current to provide dark solution compensation bias. The offset current flows through resistor R5 and potentiometer R8. The amount of offset current flowing through resistors R4 and R6 depends upon the position of the wiper of potentiometer R8, which acts as a current divider. Further, because photodiodes D2 and D3 possess extremely high impedance, it will be seen that the voltage across resistor R5 depends upon the voltages at TP1 and TP4: the lower the voltages at TP1 and TP4, i.e., the lower the signal level, the greater the voltage across resistor R5, with an associated increase in current flow. Thus, the dark solution compensation bias is greatest at lower signal levels and smallest at higher signal levels. This feature permits the dark solution compensation bias to be used in conjunction with the common gain balance configuration described above. 
     During manufacture, potentiometer R8 is adjusted at the other end of the dynamic range from that used to adjust potentiometer R9. A dark solution is used that absorbs 97 percent of the light from LED&#39;s DS1 and DS2, transmitting only 3 percent. This is an absorbance of 1.5 A. [Absorbance≡-log 10  (transmitted light).] With matched dark solutions in each tube, R8 is adjusted so that the voltage measured at TP1 equalts the voltage measured at TP4. Thus, resistor R5 and potentiometer R8 provide the dark solution bias compensation mentioned above. 
     As mentioned above, LED&#39;s DS1 and DS2 may not have identical spectral output, even though they may have matched overall brightness. Reference solution A and sample solution S may have a relatively narrow absorption peak. If the peak wavelengths of DS1 and DS2 do not match, or if they have mismatched out-of-band energy, then the detected currents in photodiodes D2 and D3 will not stay matched for high absorbance solutions. 
     The necessity for dark solution bias compensation can be appreciated from the following example: In a system without dark solution bias compensation, if one source has 1 percent of its energy outside of the absorption band of the solution, then the relative gain between the two light detectors can be easily adjusted for a match using clear solutions. However, if a dark solution with 97 percent absorption of in-band energy is introduced into the system for comparison, it will be appreciated that without dark solution bias compensation, there is no attenuation of out-of-band energy, and the original 1 percent out-of-band energy now represents 33 percent of the energy received at the detector. A similar phenomenon occurs if the sources have different peak wavelengths that coincide with a sloping absorbance curve for the solutions under test. 
     Thus, it will be seen that the dark solution bias compensation possesses several desirable features: 
     First, there is minimum interaction with gain adjustment. At high signal levels, the voltage across R5 and the bias current are reduced, when compared with low signal levels and maximum voltage across R5. 
     Second, there is minimum interaction with detector load impedance levels. The bias impedance stays high, relative to detector loads R4 and R6. 
     This, the gain adjustment will properly adjust dark compensation bias. The use of offset currents permits the adjustment for spectral differences using a different mechanism from that used to adjust for differences arising from miscellaneous mechanical factors. The system behaves as though the sources were exactly matched for spectral content. This greatly simplifies the calibration procedure. 
     Returning now to comparators U1A and U1B: As discussed above, comparator U1B decides which voltage is higher, TP1 or TP4. Comparator U1A is connected as an inverter, using the anode voltage of LED DS2 as a logic threshold voltage. Indicator LED&#39;s DS3 and DS4 are connected back-to-back and are driven differentially. This insures that they will not be on at the same time, and also minimizes the part count, as only one drive resistor is required. 
     An important part of the circuit is feedback resistor R7. The comparator feedback to LED DS1 via feedback resistor R7 is the source modulation hysteresis mentioned above. 
     When TP4 is slightly higher than TP1, output pin 7 of comparator U1B will start to go from low to high. As it does so, current flows through feedback resistor R7, increasing the brightness of LED DS1, which in turn produces positive feedback at TP4 of approximately 31/2 percent, thereby stabilizing the system. The increase in brightness at LED DS1 is approximately linear with current at the operating point established by R1 and R2. The values shown in FIG. 1 provide a current increase of 0.035 percent, which corresponds to an equivalent absorbance change of 0.015 A. 
     In operation, the hysteresis operates as follows: Assume that the sample solution S is darker than reference solution A. TP4 should be less than TP1. However, further assume that, because of some transient state, TP4 is actually greater than TP1. Feedback through resistor R7 causes more current to flow through LED DS1, causing it to become brighter. As the solutions return to their normal state, TP1 will start to drop until it becomes lower than TP4. When that happens, the output of comparator U1B will drop. Because of the feedback path through resistor R7, the current through LED DS1 will also drop, causing it to become dimmer. The net result is to stabilize the circuit by minimizing the number of transitions between indicator states. 
     Source modulation hysteresis provides a significant improvement over normal voltage feedback hysteresis. With source modulation hysteresis, the hysteresis is constant in absorbance units (equivalent to dB), throughout the dynamic range of the instrument. This results in a well-behaved system, that is not twitchy or overly senstive for clear solutions nor insensitive for dark solutions, as is the case using normal voltage feedback. Source modulation hysteresis yields stable readings, even though the tubes may exhibit slight scratches, or move slightly in the sleeves (optical path), or the instrument may be jiggled during measurement. 
     It should be noted that other configurations of the circuit would be within the spirit of the invention. For example, the polarity of the feedback can be changed from positive to negative. This changes the circuit to an oscillator, if the sample solution S and the reference solution A are close to the same concentrations. By adjusting the feedback, the circuit can be used as a window detector. Since the feedback is constant in percentage at the source, the window will be constant in absorbance units over the entire measurement range of the instrument. With slight additional circuitry, such as a ramp generator, the window comparator provides a variable duty cycle indicator, and shows how far apart the solutions are, i.e., through variable duty cycle blink rates on the indicator LED&#39;s. 
     Preferred components for use in the circuit are set forth in Table I. Table II sets forth the specifications required for LED&#39;s DS1 and DS2 and photodiodes D2 and D3: 
     
                                           TABLE I__________________________________________________________________________                        PER#  DESCRIPTION               BOARD                             REF. DESIGNATOR__________________________________________________________________________1  CAP,.01 uF 20% SOV MONO. CERAMIC .1&#34; LS                        1    C1   AVX #SR205E103MAA   NIC #NCM20Z5U103M5002  RES, CARB. FLM. ohm 5% 1/4 W                        1    R1 (spare)3  RES, CARB FLM 240,330/470 ohm 5% 1/4 W                        1    R2 (match w/DS1&amp;2)4  RES, CARB. FLM. 680 ohm 5% 1/4 W                        1    R35  RES, CARB. FLM. 12K ohm 5% 1/4 W                        1    R76  RES, CARB. FLM. 100K ohm 5% 1/4 W                        2    R4,67  RES, CARB. FLM. 5.6 M ohm 5% 1/4 W                        1    R58  POT, 50K ohm 3/8&#34; SQ. CERMET/SEALED                        1    R9   LAYDOWN BOURNS #3386P-1-503   VRN #780-12P-50K7  POT, 1 M ohm 3/8&#34; SQ. CERMET/SEALED                        1    R8   LAYDOWN BOURNS #3386P-1-105   VRN #780-12P-1M10 DIODE, 1N4148             1    D111 PHOTODIODE #TELEFUNKEN #BPW 46 (PIN)                        2    D2,3   SEE MEMO   SCREENED TO CSPDOIT                   1M61021 A12 LED RED #CSRD20T T-1 3/4  100 mc                        2    DS1,DS2   STANLEY #ESBR5501 or #5701   SCREENED TO CSRD20T13 LED RED T-1,3mc LITE ON #LTL-4221                        2    DS3,DS4   LUMEX #SSL-LX3054ID14 IC LM324 (or CS224B)      1    U115 CONN, BATTERY SNAP MOUSER #12BC421                        1    J116 SWITCH, SPST ITT Schadow #KSA-OM-221                        1    SW1   OR #KSA-OA-22117 BATTERY, 9V CARBON        1    BATT18 PCB, CSI #1682 REV.B      1    PCB   CASE COMPONENTS19 CASE, FRONT w/printing    1    SANTIN ENGINEERING20 CASE, REAR                1    SANTIN ENGINEERING21 CASE, TOP                 1    SANTIN ENGINEERING22 SLEEVE-HALF, TUBE         4    SANTIN ENGINEERING23 WINDOW/LENS, MOLDED       4    SANTIN ENGINEERING24 CAP, SWITCH BUTTON        1    SANTIN ENGINEERING25 SPACER, SWITCH BUTTON     1    SANTIN ENGINEERING26 SPACER, LED               1    SANTIN ENGINEERING27 FOAM INSERT, BATTERY      1    GREEN RUBBER28 FOAM INSERT, COVER        1    GREEN RUBBER29 LABEL, INSTRUCTIONS/SERIAL NUMBER                        1    TECHPRINT30 LABEL, &#34;OK&#34;               1    AMHERST LABEL31 SCREW, #6 × .5&#34; PH. PAN HD.TYPE-B BLUNT                        4   PT. STEEL ZINC PLATED32 SCREW, #2 × 1/4&#34; PH. PAN TYPE &#34;25&#34;                        8   STEEL ZINC PLATED    PACKAGING33 FOAM, .125 × 4.625 × 9.125 IN.                        134 BOX, 200# DIE CUT 1PC. FOLDER                        1    HORN CORP.35 TAPE, 2&#34; CLEAR PVC QTY = ROLL                        .005__________________________________________________________________________ 
    
     
                       TABLE II______________________________________SPECIFICATIONS FOR LED&#39;S  DS1, DS2CSRD20T - MATCHED PAIR(SIMILAR TO H.P.HLMP-3750)______________________________________LED ULTRABRIGHT RED T-(13/4)PEAK                 650 ± 20 nmI.sub.V @ 10 ma      80 mcd (min)V.sub.F @ 10 ma      2.3 V (max) @ 25° C.VIEWING ANGLE        ≦25°I.sub.V FLATNESS vs ANGLE                TO BE DETERMINETEMP RANGE - OPERATING                10° TO 35° C.TEMP RANGE - STORAGE -20° TO 70° C.LINEARITY MATCHING1 ma ≦ I.sub.F ≦ 10 ma                I.sub.V1 = KI.sub.V2 ± 2%BRIGHTNESS MATCHING@ 1.sub.F = 5 ma     ±15% maxPEAK WAVELENGTH MATCHING:@ I.sub.F = 5 ma W = wavelengthFOR W.sub.peak 640 TO 660                W.sub.1 = W.sub.2 ± 7 nmFOR W.sub.peak 660 TO 670                W.sub.1 = W.sub.2 ± 5 nmFOR W.sub.peak 670 TO 685                W.sub.1 = W.sub.2 ± 2 nmOUT OF BAND ENERGY MATCHING:% OF TOTAL EMITTED ENERGY OUTSIDE 600 TO 700 nmMUST MATCH WITHIN ±0.2% OF TOTAL ENERGY.SPECIFICATIONS FOR PHOTODIODES D2,D3CSPDOIT - MATCHED PAIRPHOTODIODETO-18, TO-92, OR T-(13/4)CASE(SIMILAR TO SIEMENS#SFH206K)PHOTO SENS.@ 650 nmV.sub.R = 8 VE.sub.σ = 0.5 mw/cm.sup.2                35 μa minRADIANT SENS AREA    5 mm.sup.2 min.VIEWING ANGLE        60° min.DARK CURRENT@8 V 25°      3 na max.LINEARITY MATCHING0.1 μa ≦ I.sub.p ≦ 10 μa                I.sub.p1 = KI.sub.p2 ± 1%SENS. MATCHING       I.sub.p1 = I.sub.p2 ± 15%TEMP RANGE - OPERATING                10° TO 35° C.TEMP RANGE - STORAGE -20° to 70° C.______________________________________ 
    
     FIG. 2 is a schematic drawing of a preferred embodiment of a mechanical arrangement of the preferred embodiment of FIG. 1. Sleeves 21 and 22, shown in cross section, are used to hold test tubes containing respectively sample solution S and reference solution A. It is contemplated that when the two test tubes are inserted that the sleeves are sufficiently opaque to prevent ambient light conditions from adversely affecting the accuracy of the comparison. Adjustment of potentiometers R8 and R9 is accomplished through centered screw slots 23 and 24.