Microprocessor-controlled photodiode array for measurement of diffraction patterns

Microt r es r-co f te¢l pt .,todiode array for measm, ement of diffra¢

patterns Photodiodes solve some of the problems of recording Fourier spectra, but they can be difficult to control accurately. ¥ V Reddy, K Kaipagam and P S Naidu have designed a microsystem for the purpose A linear 512-photodiode array mounted vertically on a horizontal translator is used to scan the optical spectrum under microprocessor control. The signal-to-noise ratio with the processor is 400. The spectrum is sampled in rectangular coordinates with sampling distances of 25.4/gn in the y direction and variable sample intervals, with a minimum 2.5 lgn in the x direction. The maximum size of the pattern that can be scanned is 1.3 cm × 10 cm. Digitization is carried out with a 12-bit A/D converter. A floppy disc and a numeric printer provide storage and printout. microsystems

Fourier transform

process control

Fourier transformation of a two-dimensional signal obtained from a transparency occurs at the back focal plane of a spherical lens when the transparency is placed in the front focal plane and illuminated by a collimated laser beam. Other arrangements are also possible 1. The Fourier spectrum may be recorded on a photographic film or measured with the help of any of the various kinds of light-intensity detector (eg photodiodes, photomultiplier tubes). The technique has been used for pattern recognition 2, determination of sea-surface spectra 3 and particle counting4 . Photographic recording is not convenient from the point of view of computer processing. Furthermore, photographic film has a limited dynamic range and, Department of Electrical Communication Engineering, Indian Institute of Science, Bangalore 560012, India

Y V Reddy graduated in electrical engineering from the College of Engineering, Anantapur, India, in 1972. He obtained a postgraduate degree in control systems engineering from the PSG College of Technology, Coimbatore, India, in 1975. He is n o w an assistant professor of electronics and communication engineering at the JNTU College of Engineering, Anantapur, India, and is working for his PhD under the quality improvement programme of the Indian Institute of Science, Bangalore. His chief research interest is in microprocessor applications and signal processing. K Kalpagam graduated in electronics and communication engineering from the PSG College of Technology, Coimbatore, in 7982. She is n o w working for her MSc (Eng) degree at the Indian Institute of Science, Bangalore. Her research activities are in the fields of optics and signal processing.

0141-9331/85/05226-05 $03.00 © 1985 Butterworth & Co. (Publishers) Ltd 226

microprocessors and microsystems

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P S Naidu has worked for the Geological Survey of Canada and at Dalhousie University, Halifax, Canada, as a postdoctoral fellow. In 1969 he joined the Indian Institute of Science, Bangalore, where he is now a professor in the Department of Electrical Communication Engineering. His research interests are in geophysical signal processing, coherent optical computers and underwater signal processing.

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~ Ground Figure 1. Simplified circuit diagram for the M-series array unless controlled processing is carried out, it is difficult to maintain the photographic constant (1) at a value of - 2 . The use of a photomultiplier tube (PMT) or a photodiode can solve some of these problems. As precision movement of photodiodes and PMTs is difficult to achieve, a fixed photodiode with a large area segmented into convenient parts (eg wedges and circular half rings) has been suggested by George and Kasdan s. In this paper a new method of scanning is described. A linear 512-photodiode array mounted vertically on a linear horizontal translator is used for measuring optical intensities; this process is microprocessor controlled. The output is digitized with a 12-bit A/D converter (ADC80), and storage and printout are provided by a floppy disc and a numeric printer. The software and hardware required for controlling rectangular sampling of Fourier spectra are also described.

the photodiode to the potential of the video output terminal. The amount of charge necessary to restore the photodiode to this potential represents the video signal. Thus a readout of charge is obtained. Output charge is directly proportional to exposure, exposure being defined as light intensity multiplied by integration time. Figure 3 shows the graph of output charge against exposure 6. The video output of the M-series array is a serial train of charge pulses. The M-series processor driver assembly (MPDA) 6 is used to drive and process the output signal from the M-series array. The response of the array extends from the near-ultraviolet to the infrared region of the electromagnetic spectrum. The peak response occurs at a wavelength of about 820 nm. Dark leakage current is temperature dependent and only becomes significant at temperatures above 30°C or integration times in excess of 40 ms. The signal-to-noise ratio with

J SELFSCANNED PHOTODIODE ARRAY The photodiode array is linear and selfscanned, with scanning circuitry integrated on the same chip. Figure 1 shows the circuit diagram for the M-series array 6. The photodiodes, with their associated parallel storage capacitors, are connected through MOS transistor switches to a common video output line and the switches are turned on and off in sequence by a shift register. Each device contains two shift registers, each register accessing alternate diodes. The shift registers are driven by two nonover/apping clock pulse trains, 41 and 42. The array scan is initiated when a scan-start pulse is applied at the scan-start input terminal. The scan pulse is propagated through the registers alternately by 41 and 42. The timing diagram for the clock pulses is shown in Figure 2. By clocking two registers alternately and connecting the video outputs in parallel, a serial stream of information is obtained representing every diode in the array. The photodiodes in the M-array operate in a reversebias light-integration mode. In this mode, the initial scan pulse propagated through the shift register causes each photodiode in turn to be charged to the negative potential applied to the video output terminal. During the period up to the subsequent scan pulse (termed the integration time), the photodiode loses an amount of charge proportional to the total amount of light incident on it. The subsequent scan pulse recharges

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the MPDA 6 is 400 when the signal is above dark-level voltage. In the photodiode array, the photodiodes act only as detectors, and separate shift registers are used to transfer the charge pulses for storage. In chargecoupled photodiode arrays, on the other hand, the sensing elements have to perform both detection and storage functions. The photodiode array is convenient to handle as it is not sensitive to static electric charges.

SYSTEM DESCRIPTION The system was developed around an 8085 microprocessor development board (MDB). It consists of a linear 512-photodiode array, a stepper motor, an 18-column numeric printer and a floppy disc interfaced to the MDB. The array is mounted vertically on a horizontal translator driven by a stepper motor. The data is read into the MDB memory under program control and is then transferred to a floppy disc. If necessary, a printout can also be obtained. The stepper motor and the printer interface are of standard design. The floppy-disc interface has been described elsewhere 7. The complete block diagram for the system is shown in Figure 4. The system is capable of scanning optical intensities in any plane and measuring the light intensity at rectangular sampling points. The array is placed vertically at one edge of the plane to be scanned and the outputs from 512 diodes give the light intensity at 512 points spaced 25.4 pJ~ apart. The array is moved in steps to cover the complete sample area. The data is obtained in matrix form, with 512 rows corresponding to the 512 diodes and a number of columns equal to the number of horizontal steps required to cover the sample area. The specifications for the system are given in Table 1.

ARRAY INTERFACE MODULE The scan-start A pulse (SSA) initiates scanning. The level and width of the SSA pulse are +12 V and 2 ns respectively; unfortunately these values are not acceptable to the MDB 6. The level of the SSA pulse is therefore reduced to +5 V with a CMOS-to-TTL converter (4010). The pulse is then used to trigger monostable 1, giving a pulse of 10/Ls width (Figure 5). The M D B reads this pulse through one of its input ports and identifies the start of scanning. The SSA pulse appears after every 512 ~ clock pulses (q~l and 42) and initiates the nextscan.

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Once the start of scanning is identified, the system begins to acquire the data corresponding to the 512 diodes. The video output from the MPDA is digitized by a 12-bit A/D converter. (The dynamic range of the photodiode array is about 1000. Though a lO-bit A/D converter would have been sufficient to match this dynamic range, a 12-bit converter was used as it was available in the laboratory.) The A/D converter needs a convert-command pulse of +5 V with a width of more than 2 ns for conversion of each diode output. A ~ clock pulse is used for initiating each conversion, but the level and width of these pulses (-t-12 V and 2 ns respectively) are not acceptable to the A/D converter. Each ~ pulse is therefore reduced to +5 V using a CMOS-to-TTL converter (401 O) and is then used to trigger monostable 2, giving a chain of pulses of more than 2 ns width. Each pulse of this chain corresponds to one diode in the array. While the ~ clock pulse initiates A/D conversion, the corresponding diode output is at the analogue input terminal of the A/D converter 8. The end of conversion is indicated by a status signal. The 12-bit data and the status signal of the A/D converter are .read by the MDB through the input ports. The data is read into the MDB memory every time it identifies an end of conversion; this is repeated for the 512 samples. The entire process is then repeated for the next array position. A schematic diagram of the array interface module hardware is given in Figure 5. In operation, the array scans repeatedly. The output is a periodic waveform, with each period corresponding PortA

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Table 1. System specifications

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microprocessors and microsystems

to 512 photodiodes 6. The output of the 512 diodes is read from this waveform under program control. The conversion time of the AiD converter is 22/Js, and another 18/~s is required to read the status and data; hence the system operates at 25 kHz. The speed of operation can be improved with a faster A/D converter and a faster CPU.

TEST SCANS The entire system was tested by scanning the Airy pattern produced when a pinhole is illuminated by a coherent beam of light. The readings obtained are plot-

AID J

READING THE DATA UNDER PROGRAM CONTROL A set of programs was written to acquire data from the array and transfer it onto floppy disc or to a printer. These programs include binary-to-BCD conversion, printer software and floppy-disc software and are of standard design. The flowchart for the array software is shown in Figure 6. The program begins with the identification of the start of scanning and proceeds using the AiD converter subroutine to read the data corresponding to the 512 photodiodes. The flowchart for the AiD converter is given in Figure 7. This subroutine checks the status of the A/D converter. When the end of conversion is identified the 12-bit data corresponding to the diode output is transferred to the memory locations. Once the data corresponding to a position is acquired, control is transferred to stepper software. This can rotate the stepper motor in either direction, resulting in the movement of the array in either direction. The minimum distance that the array can be moved is 2.5/~m. The software can be modified for either uniform or nonuniform sampling. The flowchart for the stepper-motor software is given in Figure 8. After the array has been moved to its next position, control is transferred again to the array software module for data collection.

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Figure 9. Plot of the Airy pattern as observed by the system ted in Figure 9. The resultant Airy pattern has not undergone any smoothing. The width of the main lobe (2r) is given by 2r = (N - 1)s = (88 - 1)25.4/~m = 2209.8/~m where r is the radial distance of the first minimum from the centre of the main lobe; N is the number of diodes at and between the first minima on either side of the main lobe; s is the diode pitch. (The peak of the main lobe is cut off owing to saturation of the diodes.) The pinhole diameter (I) may be estimated using the formula

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where ,~ = 632.8 nm is the wavelength of light used; z = 11 cm is the distance between the diffraction plane and the diodes. The pinhole diameter estimated in this way is 76.8/~m, which compares well with the known value of 75/Lm. The linearity of the array may be tested against PMT (type 9558). The photodiode array and the PMT are placed close to each other in a plane and are exposed to a uniform light source. The source is controlled by a Variac. A plot of single-diode output against PMT current is shown in Figure 10. The maximum deviation

230

Figure 10. Single-diode output versus PMT current from linearity is well within 2% and is due to voltage fluctuations. The MPDA introduces noise irrespective of the operating frequency and signal level, so the signal-tonoise ratio will be small at low signal levels. This may be seen in the Airy pattern graph (Figure 9), on which the outer lobes are affected by the noise.

REFERENCES 1 2 3 4

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Goodman, J W Introduction to Fourier optics McGraw-Hill, New York, USA (1968) UIIman, J R Opto-electronics Vol 6 (1974) pp 319332 Stillwell, D 'Directional energy spectra of the sea from photographs' J. Geophys. Res. Vol 74 No 8 (1969) pp 1974-1986 Slark, H and Shao, G 'Design considerations in counting particles by size with an optical digital training method' Appl. Opt. Vol 16 (1977) pp 1670-1674 George, N and Kasdan, A L Proc. Electronic Optical Systems and Devices Conf., Anaheim, CA, USA (1975) pp 494-503 M series photodiode array and processor driver assembly manual IPML, UK (August 1978) Anantharaman and Kumar, Sunil '8272 controlled 8085 and minifloppy interface' BE Project Department of Electrical Communication Engineering, University of Bangalore, India (April 1984) Burr-Brown ADCSO AG12 data manual (1979)

microprocessors and microsystems