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Anal. Chem. 1985, 57, 2413-2414
Evaluation of a Low-Cost Imaging System for Analytical Applications E. J. Zuck and E. D. Salin* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 Recently a number of articles have appeared in this journal and others (1-4)relating to the applications of robotics or automation in the laboratory. The advantages of automation seem clear while the path to automation does not. Some of the advantages would be increased sample throughput, minimization of operator fatigue and boredom, reduction in per sample cost, higher precision, and superior data retention and storage. While some robotic systems have been developed for the laboratory, they are presently somewhat limited in their capacity and are relatively expensive. Modern instruments are better suited to a role in a highly automated laboratory, and yet many laboratories have procedures and equipment which cannot be economically replaced in the immediate future. Modern moderate and low cost robots have a great deal of flexibility yet still lack the most important sense used by humans in the laboratory, vision. A recent release to the market of a very low cost image sensor (less than $300 U.S.) prompted us to evaluate this device as a replacement for human image sensing in a typical routine chemical analysis application. Image sensors have been widely discussed in the literature as sensors for spectrometry, and yet their mention in the chemical literature as an image sensor for automation of this type is almost nonexistent. Humans usually measure volumes during titration by vision. If a low cost sensor can replace the most critical human element in this system, then the implications are wide-reaching. One can then envision extension to other vision-based experiments without the replacement of equipment originally oriented toward the output of information in a visual format. This would include strip charts and other analog displays, as well as vernier or other more mechanical readouts. Our goal then was to evaluate this sensor as a general purpose device. The titration experiment was selected as an example of an experiment which placed serious quantitative demands on the vision of a human operator.
EXPERIMENTAL SECTION There are several models of the MicronEye digital camera system available from Micron Technology, Boise, ID. The general purpose model is interfaced by an RS-232C serial interface. A number of models are available for popular personal computers including the Apple 11,IBM-PC, Radio Shack TRS-80, and the Commodore 64. The interface for these computers sits directly on the computer bus and provides a relatively high speed information transfer rate (150 kbaud). The RS-232C interface, while very general purpose, suffers from a much lower transmission rate than the versions designed for specific computers. We selected an IBM-PC specific version due to the higher memory capacity of this system. The camera system was provided with both hardware, software, and documentation adequate for an evaluation of both. Generally the MicronEye is accessed from BASIC language programs which call assembler level routines. Source code is provided for the assembler level programs. Our evaluation programs were written in BASIC since operational time was not a consideration. The camera (IS32A Opticam) has been discussed extensively in the popular literature (5-7). It is, in essence, a 64K byte RAM (random access of read/write) memory in a dual inline package (DIP) covered with a transparent layer. It consists of two 128 by 256 addressable elements arranged in a 129 by 514 staggered cell arrangement. Each array is 878.6 pm high and 4420 pm wide. The arrays are separated by a 120-pm dead (insensitive) zone. Each element is 8 by 9 pm with a vertical center-to-centercolumn spacing of 21.5 pm and a row spacing of 8.5 pm. The device
operates in the charge storage mode (8)during the integration time; however the readout is digital since the voltage level of each memory element is compared against a reference level on readout. If the level is higher, then the pixel is said to be black (dark), otherwise it is said to be white (light). Because of the digital nature of the IS32, it is suitably mainly for monitoring high contrast scenes. By variation of the integration time it is possible to obtain a measure of the intensity falling on a given pixel; however this is crude compared to the capabilities of more expensive vidicon and photodiode array sensor systems. For these experiments we used only one of the two arrays. The pixels are arranged in a staggered arrangement which makes it possible to discriminate between 512 discrete column positions in applications where information is presented as a series of lines. Optical Arrangement. Figure 1 illustrates the optical arrangement used. An ordinary 50-mL Kimax laboratory buret with minor gradations every 0.1 mL was used. The f/1.6 16 mm camera lens was replaced with a Switar 25-mm f/1.4 C-mount lens placed approximately 30 cm from the buret. Exact placement was determined by focusing the image for the best picture as presented on the IBM-PC video screen. Illumination was provided by two Carl Zeiss 6-V 15-W lamps.
RESULTS AND DISCUSSION Due to the high contrast nature of the experiment, the meniscus was visible as a clearly defined black band across the image of the buret. A sufficient increase in exposure time (under computer control) resulted in the total exposure of the image (i.e., all pixels were white). By measurement of the exposure time necessary to cause a pixel to be set, it was possible to arrive at an estimate of the definition of the meniscus edge (Figure 2). Each of the peaks in Figure 2 corresponds to an etched line on the buret. The higher the peak, the darker the line. The illumination was focused on the reflector behind the meniscus, so the illumination was more intense near the center (pixel numbers 125 to 400 in Figure 2) than a t either end. This explains the tendency of the buret lines near the center to “wash out” as evidenced by their lower peak heights. The meniscus is the broad band located at pixel 325 (peak A). The small band just to the left of this peak is a major line. Because of the parallax effect, a double peak is observed near pixel 230 (peak B) where a major 10-mL line circumscribes the buret. The width of the meniscus was found to be constant at a given magnification. The location of the meniscus was deemed to be the leftmost pixel location in the meniscus band. Small volumes of water (fractions pf a drop) were delivered from the buret, and new meniscus locations were determied visually and by the camera. The weight of each drop was measured so as to enable a comparison between the readings taken visually and those made by the camera. The data are presented in Figure 3. Least-squares linear regression yielded comparable correlation coefficients (r) of 0.9982 for the camera and 0.9981 for visual readings. The standard error of the estimate (9) gives a measure of the average magnitude of the difference between measured values and values estimated from the curve fitting procedure. The standard error of the estimate of the linear least squares regression was 0.018 mL for visual readings and 2.65 pixels for the camera when used in the 512-pixel readout mode. This corresponds to an uncertainty of 0.5% of the image on the sensor surface in the vertical (buret) direction. The camera standard error value translates to 0.016 mL. These correspond to errors for both methods of less than 2% or 0.2% for volumes of 1 and 10 mL, re-
0003-2700/85/0357-24 13101.50/0 @ 1985 American Chemical Society
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Anal. Chem. 1985, 57, 2414-2417
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was the focusing portion of the setup. Once conditions had been established, the system was easy to use. The camera observed approximately 2 mL of the buret. Demagnification will result in an increased observation range but a loss in resolution. If the range greater than 2 mL (or the equivalent) is essential, then one must either use another camera or provide a movement mechanism for the camera. We consider the second more practical since the camera could be used to determine its own position during and after movement by reading the buret or an externally attached scale. It should be noted that a high precision movement mechanism is not essential because of this “self calibration” capability. With the experimental magnification used, the camera was comparable to human visual performance. The low cost, high performance, and ease of use of this device make it far more than a curiosity or teaching tool. Clearly it offers the possibility for automation in environments which previously could not consider the cost of conversion.
ACKNOWLEDGMENT 400
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The authors wish to thank Micron Technology for the donation of the MicronEye used in this research.
Exposure time study.
LITERATURE CITED
spectively. Obviously this will depend on the type of buret used. Certain limitations should be mentioned. It is essential that the subject be brightly illuminated. Usually this requires light levels above ambient. In some environments this may not be practical. An advantage accrued from high-intensity illumination is shorter exposure times and larger f-stops (smaller apertures) with an attendant increase in the depth of field. An increase in magnification can also lead to a loss of depth of field. This can make focusing a problem if the scale under observation is long. The major problem with the experiment
Dessy, R. E. Anal. Chem. 1983, 55, 1100A. Dessy, R. E. Anal. Chem. 1983, 55, 1232A. Owens, G. D.; Eckstein, R. J. Anal. Chem. 1982, 5 4 , 2347. Geni, G. J . Electroanal. Chem. Interfacial Necrochem. 1982, f40, 137. (5) Ciarca, S. Byte 1983, 8, 20. (6) Ciarca, S. Byte 1983, 8, 67. (7) Wieland, C. Byte 1983, 8, 316. (8) Weckier, G. fop. Nectronics 1967, 13, 75. (9) Parsons, R. “Statistical Analysis: A Decision Making Process”; Harper and Row: New York, 1974; pp 694-696.
(1) (2) (3) (4)
RECEIVED for review February 25, 1985. Accepted Mas 21, 1985.
Modification of an Inductively Coupled Plasma Radio Frequency Supply for Amplitude Modulation with Complex Wave Forms Ronald Withnell, G. D. Rayson, A. F. Parisi, and G . M. Hieftje* Department of Chemistry, Indiana University, Bloomington, Indiana 47405 Amplitude modulation of an inductively coupled plasma (ICP) has potential both as a means of improving detection 0003-2700/85/0357-2414$01.50/0
limits in atomic spectrometry (1)and as an investigative probe for fundamental studies (24).With such a system, detection 0 1985 American Chemical Society
