METHOD FOR OPERATING A PHOTODIODE ARRAY SPECTROMETER AND PHOTODIODE ARRAY SPECTROMETER

Abstract: A method for operating a photodiode array spectrometer for determining the spectral composition of a polychromatic beam of radiation which spectrometer comprises an array (11) of photosensitive elements (15-1, 15-2 to 15-n) intercepting different wavelengths of the polychromatic beam of radiation. An output signal representative for the charge accumulated by a charge amplifier circuit (32) is output each time a predetermined number greater than 1 of photosensitive elements was scanned such that the output signal is representative for the integral of a plurality of the amounts of radiation intercepted by a corresponding plurality of the photosensitive elements.

Description: The invention relates to a method for operating a photodiode array spectrometer according to the preamble of claim 1. Such a spectrometer can be used, for example, for measuring the absorption spectrum of a sample substance in order to derive information about the chemical composition of the sample and the quantities of the individual constituents in the sample. A photodiode array spectrometer as mentioned in the preamble of claim 1 is known from "A High-Speed Spectrophotometric LC Detector", Hewlett-Packard Journal, April 1984. This known spectrometer is used in a liquid chromatograph for analysing the substances eluting from the chromatographic column. The known spectrometer comprises a light source emitting a broad spectrum of ultraviolet and visible radiation and an optical system for focusing the beam onto a sample cell through which the sample substances to be analysed flow. Depending on the specific substances flowing through the cell, the sample absorbs certain characteristic spectral portions of the radiation entering the sample cell so that the spectral composition of the radiation leaving the cell is indicative of the sample substances. In the known spectrometer, the spectrum of the radiation leaving the sample cell is derived by means of a diffraction grating arranged in the optical path behind the cell. The diffraction grating directs light rays of different wavelengths into different directions. A linear array of photodiodes is arranged to receive the light diffracted by the grating. Each diode receives light corresponding to a different wavelength range. The electrical signals produced by the impinging light in each photodiode are read out by a read-out circuit and converted to digital data values which are representative of the the intensity of the light impinging on the specific diode. These data values are then displayed as a function of wavelength in any convenient form, for example on a CRT screen. The photodiode array is built on semiconductor material and comprises a plurality of photosensitive elements which are connected via electronic switches to a common ouput line (video line) which in turn is connected to a so-called charge amplifier, more exactly a charge accumulator or a charge-voltage-converter. Each photosensitive element has an associated capacitor which represents the junction capacitance of the photodiode. The combination of photosensitive element and associated capacitor will subsequently also be referred to as "photocell". Light impinging on the photosensitive material generates charge carriers which discharge these capacitors. In operation, the capacitors of the photocells are initially charged to a fixed value, respectively, and then the whole array is scanned in predetermined intervals by sequentially closing the switches such that the photocells are recharged by the charge amplifier to their original charge level. The amount of charge transferred hereby causes a voltage change at the output of the charge amplifier which is proportional to the amount of light which has caused the discharge of the photocell. Three important quantities for characterizing the performance of a spectrometer are spectral resolution, spectral range and sensivity. Spectral resolution indicates how well radiation components with closely adjacent wavelengths are separated so that they can be identified as separate components. Spectral range indicates the interval of wavelengths which can be analysed by the spectrometer. Sensitivity indicates how well weak signals can be distinguished from background noise and corresponds to the signal/noise ratio. In spectrometers using photodiode arrays as light detecting elements, the requirement of a wide spectral range at a given resolution leads to arrays with a large number of individual photodiodes, e.g. 1024 diodes as in the known spectrometer. For each reading out of a photosensitive element during a scan, a new data value is obtained, which is further processed e.g. by a A/D converter. The sensitivity of a spectrometer can generally be improved by increasing the power of the light source of the spectrometer. As a consequence thereof, the light power incident on the photodiodes also increases so that the capacitors associated with the photosensitive elements are discharged to a larger extent than in case of lower light power. Since the capacitors must not be completely discharged in order to avoid nonlinearities and since the size of the capacitors is limited by economic usage of the chip area of the photodiode array, the scan rate with which the capacitors are recharged has to be increased when the light power is increased. Consequently, the number of data values per time unit, i.e. the data rate, is increased unless the number of photocells is reduced which, however, would result in a smaller spectral range and/or resolution. According to the foregoing considerations, known photodiode array spectrometers are not satisfactory in all respects because they require a compromise between spectral resolution, spectral range, sensitivity and data rate so that not all of these parameters can be simultaneously selected to the desired value in a specific application. Known photodiode array spectrometers therefore require comparatively high circuit expense if high performance of the spectrometer is desired. Relative to the prior art, it is an object of the invention to provide a method and a photodiode array spectrometer according to the preambles of claims 1 and 2 respectively which permit to increase sensitivity without substantially increasing the cost and complexity of the signal processing circuitry. According to the invention, this object is solved by the characterizing features of claims 1 and 2 respectively. According to an underlying principle of the invention, the photosensitive elements of the photodiode array are read out with a higher frequency than the charge amplifier produces output signals. Therefore, less data are produced within a certain time interval than in prior art photodiode arrays of comparable performance wherein during each scan each reading out of a photodiode produces an output signal of the charge amplifier. Thus, the data rate of the spectrometer is kept small. The rate at which output signals of the charge amplifier circuit occur is smaller than the scanning rate of the photosensitive elements. This fact makes possible a greater light throughput of the spectrometer compared with conventional spectrometers without increasing the data rate, i.e. the rate of the output signal which has to be further processed, for example by means of an A/D converter. Nevertheless, since the scanning rate of the photosensitive elements can be made greater than with conventional spectrometers without raising the data rate of the output-signal, saturation effects and resulting non-linearities of the photosensitive elements are avoided. The "predetermined number" of switches which have been closed before an ouput signal representing the charge accumulated by the charge amplifier is output is smaller than the total number of the photosensitive elements, preferably much smaller, e.g. 2 or 3 or e.g. up to 8 compared with a total number of 1024 of the photosensitive elements in an embodiment of the invention. Such, costly data processing and data storage circuits as high speed A/D converters, microprocessors, mass storage devices, are not required which would be otherwise necessary if a higher data rate had to be used. The light throughput can be made greater compared with conventional spectrometers by using a light source of higher intensity. In most cases, however, since the light spot is circular, in order to increase the light throughput it is possible to simply enlarge the slit width of the spectrometer with the consequence that the radiation of a specific wavelength covers a plurality of photosensitive elements. Broadening of the rays impinging onto the photosensitive elements simultaneously results in an light-integrating effect. Such broadening may be achieved by other means, e.g. optical means, e.g. a lens. The spectrometer may have a variable width of the entrance slit. Then, if the intensity of radiation impinging onto the photosensitive elements is relatively high, a small slit can be used in order to get a high resolution. When the intensity of the radiation is relatively low, a broader slit may be used in order to increase the light throughput and to reduce the effect of noise. If the intensity of radiation is high, it may be necessary to reduce the intensity e.g. by using a small slit width in order to take into account the sensitivity of the photosensitive elements. By increasing the light throughput by a factor n and accumulating the signals from n photocells, the improvement in signal to noise ratio is approximately a factor of n compared with a conventional spectrometer. Without increasing the light throughput the factor would be SQR (n), SQR being the abbreviation for square root. The features and advantages described for the method of the invention are also valid for the apparatus claimed in claim 2. It is sufficient if the capacitor means of the charge amplifier is great enough for accumulating the charges of the predetermined number of photoelectric elements. It may be of advantage to make the capacitor means variable in capacitance in order to match different conditions. The variability of the capacitance can be achieved advantageously by providing a plurality of capacitors which can be switched in parallel in order to build the required capacitance. If the capacitance of the capacitor means is variable, it is possible to vary the voltage amplification provided by the charge amplifier and thus to provide a further reduction of noise by chosing an appropriate capacitance. Subsequently, an embodiment of the invention is explained in more detail with reference to the drawings. Fig. 1 is a schematic diagram of a photodiode array spectrometer comprising a photodiode array according to the invention. Fig. 2 schematically shows the photodiode array of the invention with a charge amplifier circuit for reading out the array. Fig. 3 shows in more detail an embodiment of the switch control circuit of a photodiode array according to the invention, Fig. 4 shows timing diagrams. Fig. 1 schematically shows a photodiode array spectrometer which allows to measure the absorption of a polychromatic beam of ultraviolet and/or visible radiation by a sample to be analysed. The spectrometer comprises a light source 1, e.g. a deuterium lamp, which emits a beam 2 of polychromatic radiation. The beam 2 is focused by a lens system 3 into a sample cell 5. The lens system 3 preferably is an achromatic system which ensures that rays of different wavelengths substantially have the same focal point. A shutter is provided which permits to interrupt the light beam 2 in order to measure the dark signal at the photodiodes of the photodiode array 11. In the actual measuring process and apparatus wherein the beam 2 passes through the sample cell 5, the dark signal and other electronic offset signals are subtracted from the measuring values to compensate for any measuring errors. The sample cell 5 may comprise an inlet and an outlet through which a sample liquid to be analysed flows continuously. Such a spectrometer can be used, for example, in a liquid chromatograph wherein the inlet is connected to the chromatographic separation column from which sample substances are continuously eluting. The polychromatic radiation entering the sample cell 5 is partially absorbed by the substances in the cell, whereby, depending on the sample substances, rays of certain wavelengths are absorbed more strongly than rays of other wavelengths. As a result thereof, the beam leaving the cell and entering the spectrograph 6 through an entrance slit 7 of variable width has a different spectral composition than the beam entering the cell and the resulting spectrum thus contains information about the kind of substances in the cell and about their quantities. The beam passing the entrance slit 7 impinges on a holographic diffraction grating 10, which disperses the light according to the different wavelengths in the beam impinging on it. The spatially separated light rays from the grating 10 impinge on the photodiode array 11, which consists of a plurality of individual light-sensitive diodes, which are separated by light-insensitive gaps. The photodiode aray 11 is connected to a read-out circuit 20 for periodically reading out electrical signals from the photodiodes, whereby these signals are indicative of the intensity of the light rays impinging on the photodiodes, respectively. Details of the read-out circuit 20 are explained below with reference to Figures 2 and 3. The electrical signals read out from the photodiode array 11 are then further processed in a signal processing circuit 21 which typically comprises an analog-to digital converter 22 and circuitry for storing and further processing these digital values. The operation of the read-out circuit 20 and the signal processing circuit 21 is controlled by a controller 23, typically comprising a microprocessor, which also controls the operation of a display means for displaying the final spectrum of the analyzed sample. The signal processing circuit 21 may also comprise circuitry for correcting the electrical signals from the individual photodiodes regarding the above-mentioned dark currents of the photodiodes and for other effects. Fig. 2 schematically shows the photodiode array 11 comprising a plurality of n individual photodiodes 15-1, ..., 15-n forming part of a semiconductor chip. Each of the photodiodes intercepts a specific spectral portion of the diffracted radiation (if the entrance slit 7 has a small width corresponding to the highest resolution of the spectrometer). Each photodiode has an associated capacitor Cd1, ..., Cdn which represents the junction capacitance of the photodiodes, or, in applications where a separate capacitor is switched in parallel to the photodiodes, the sum of this capacitor and the junction capacitance. The photodiodes and the associated capacitor will also be referred to as photocells Cell 1, Cell 2, ..., Cell n. The cells 1 ... n are connected to a common video line 30. The electrical connections between the individual cells and the video line 30 can be interrupted by means of electronic switches SW1 ... SWn, respectively. The switches are controlled by a switch control circuit 31. The switch control ciruit is explained in more detail below with reference to Fig. 3. The video line 30 is connected to a charge amplifier 32 which is designed as an integrator comprising an operational amplifier 33 with a capacitor means 37 in the feedback loop. The non-inverting input of the operational amplifier 23 is connected to the signal U guard which has a fixed potential of, for example, -5 V. Thus, the inverting input (video line 30) virtually has the same potential. A controllable reset switch 38 across the capacitor means is closed before each charge transfer to reset the integrator. In operation, the capacitors of selected photocells are initially charged to a fixed value. When photons are penetrating the photosensitive material, charge carriers are generated which discharge the capacitors corresponding to the amount of photons received within a given integration period. The discharge level of each individual capacitor is proportional to the incident light intensity during the integration period. These capacitors are periodically recharged in further scan sequences. The amount of charge transferred hereby causes a voltage change at the output of the charge amplifier 32 which is proportional to the integral of the incident light level during the integration period. The output signal of the charge amplifier 32, the "PDA signal", is now further processed by additional circuits, in the example a sample and hold circuit 35 and an A/D converter. Before each charge transfer from the charge amplifier to a cell, the reset switch 38 is closed to reset the charge amplifier 32 in preparation for the next charge transfer. The capacitor means 37 is shown more exactly in Fig. 3 and comprises a plurality of (in the example) seven capacitors 40-1 ... 40-7 having capacitances of 1/8, 1/4, 1/2, 1, 2, 4 and 8 times, respectively, the capacitance of the diode which forms the photosensitive element. The capacitors 40 are each connected in series with a corresponding controllable switch 41 which is formed by a FET the control electrode of which is connected to a respective output of the decoder 44. The purpose of having a variety of selectable capacitors is firstly providing the proper amount of full scale capacitance according to the number of photo cells accumulated, secondly providing the capability of proper gain setting for each individual bunch of photo cells. The ouputs of the decoder are controlling the FET's to switch the desired capacitors into the feed back loop of the integrator. Which combination of capacitors is switched on depends on the pattern on the signal lines Bunch 1 ... Bunch 3 and the pattern on the signal lines Gain 1 and Gain 2. The pattern on the lines Gain 1 and Gain 2 represents the actual gain factor (1, 2, 4 or 8) for an individual bunch of photo cells. The pattern on lines Bunch 1 ... Bunch 3 represents the binary coded number of photo cells to be accumulated. Any number in a range from 1 to 8 is possible in the example. A bunch consists of consecutive photo cells of the photodiode array. The truth table for the decoder is shown in table 1. While the pattern of the signal lines Bunch 1 ... Bunch 3 is kept constant (corresponding to the width of the entrance slit) during a complete analysis, the patterns on the signal lines Gain 1, Gain 2 are controlled for each individual bunch of photo cells. The gain patterns for all individual bunches of photo cells are stored in a memory of the controller at the beginning of an analysis by taking a blank scan with a fixed gain factor and assigning the gain pattern according to the photon density for the individual wavelength increments. During an analysis, before each read-out of a bunch of photo cells, the controller sets the actual gain pattern to signal line Gain 1 and Gain 2 and resets the charge accumulator by activating the reset switch through the reset line. During the "reset"-pulse all capacitors are switched into the feed back loop. This causes that all capacitors are nulled during the reset operation of the charge accumulator. With an amplification of 1, the total capacitance of the capacitor means 37 is the capacitance of Cd multiplied by the number of the photodiodes in a bunch. With amplifications greater than 1, the total capacitance is smaller. The timing diagram of Fig.4 shows the clock signals sent to the photo diode array and the associated reset and output signal (V-out) for different numbers of photocells accumulated. Note, that this timing diagram is not to scale. The clock frequency for the clocks (START-PDA, CLOCK 1, CLOCK 2) is variable and has to be adapted to the photodensity depending on the width of the entrance slit. If the slit width is increased, the spectral resolution is dominated by the entrance slit and bunching of photo cells is possible without significantly affecting the total spectral resolution of the spectrograph. Bunching of photo cells compensates for higher scan rates and the conversion rate of the A/D converter can be kept constant. Fig. 4 shows signals for "no accumulation" (a), that is the known kind of operation of the spectrometer, and for an accumulation of two (b) and three (c) photocells. Though the switch means (SW_1 to SW_n, 38, 41) are all electronic switching elements, they are shown as simple switches in order to simplify the drawings.

Claims: Method for operating a photodiode array spectrometer for determining the spectral composition of a polychromatic beam of radiation which spectrometer comprises an array (11) of photosensitive elements (15-1, 15-2, ..., 15-n) intercepting different wavelengths of the polychromatic beam of radiation, a plurality of transfer switching elements (SW1 to SWn), one transfer switching element being connected to a photosensitive element, respectively, a read-out circuit (20) connected to the transfer switching elements for opening or closing (making conductive) the transfer switching elements and for generating during a read-out cycle signals indicative of the amount of radiation intercepted by the photosensitive elements, and a charge amplifier circuit (32) for transferring an electric charge to a photosensitive element when the associated transfer switching element is closed, thus causing a voltage change at the output of the charge amplifier circuit indicative for the transferred electric charge, characterized in that an output signal representative for the charge accumulated by the charge amplifier circuit is output each time a predetermined number greater than 1 of transfer switching elements was closed such that the output signal is representative for the integral of a plurality of the amounts of radiation intercepted by a corresponding plurality of the photosensitive elements. Photodiode array spectrometer for determining the spectral composition of a polychromatic beam of radiation, comprising: an array of photosensitive elements (15-1, 15-2, ..., 15-n) intercepting different wavelengths of the polychromatic beam of radiation, a plurality of transfer switching elements (SW1 to SWn), one switching element being connected to a photosensive element, respectively, a read-out circuit (20) connected to the transfer switching elements for opening and closing same and for generating during a read-out cycle signals indicative of the amount of radiation intercepted by the photosensitive elements, the output terminals of the transfer switching elements (SW1 to SWn) are connected to a common video line (30) which is connected to a charge amplifier circuit (32) for transferring electric charges to a photosensitive element when the associated transfer switching element is closed, thus causing a voltage change at the output of the charge amplifier circuit, the voltage change being proportional to the amount of light which has impinged on the photosensitive element since the previous opening of the associated transfer switching element, characterized in that the charge amplifier circuit consists of an integrator having capacitor means (37) in a feed-back loop of the integrator, the capacitance of the capacitor means being sufficient to integrate the signals of a plurality of the photosensitive elements. Spectrometer according to claim 2, characterized in that the capacitance of the capacitor means (37) is controllable. Spectrometer according to claim 3, characterized in that the capacitance of the capacitor means (37) is controllable in binary weighted steps. Spectrometer according to any of the claims 2 to 4, characterized in that the capacitor means (37) comprises a plurality of capacitors (40-1 to 40-7) which can be switched in parallel by means of controllable switching elements (41) such that a plurality of different capacitances can be formed. Spectrometer according to any of the claims 2 to 5, characterized in that means for broadening the beam of radiation are provided such that a specific wavelength of the beam impinges on a plurality of adjacent ones of the photosensitive elements (15-1 to 15-n) Spectrometer according to claim 6, characterized in that the broadening means are realized by setting an appropriate width of an entrance slit (7). Spectrometer according to any of the claims 2 to 7, characterized in that the width of an entrance slit (7) of the spectrometer is variable.

Inventors: KUDERER, HUBERT
Applicants: HEWLETT-PACKARD GMBH
Application year: 1989
Main classification: G01J3
IPC codes: G01J3