A Software Implementation of Data Acquisition Control and Management for Czerny Turner Monochromator

Transcription

1 A Software Implementation of Data Acquisition Control and Management for Czerny Turner Monochromator HAI-TRIEU PHAM, JUNG-BAE HWANG, YONGGWAN WON Department of Computer Engineering, Chonnam National University 300 Yongbong-dong, Buk-gu, Gwangju KOREA Abstract This paper introduces structure and functions of our new software associated with the design and process of a Czerny Turner monochromator. The major goal of this new software is to reduce the data acquisition time for the monochromator system. It controls the diffraction grating to change automatically the wavelength of light beam and to achieve the enough intensity of the selected wavelength. Besides the control capability, the software also can provide various functions such as drawing the graph for plotting the acquired data, analyzing the data in various ways in order to investigate the properties of samples based on intensities of different wavelengths. Keywords: spectrometer, monochromator, spectrum, Czerny-Turner, data acquisition, control 1 Introduction In the last few years, optical instruments play an important role in the study of science and engineering, and the monochromator is a fundamental device for the various optical instruments. A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input, which finally produces a monochromatic light. Furthermore, an optical system equipped with the monochromator is used in many fields including biology, chemistry, biochemistry, food science, medicine, plasma science, astronomy, and archaeology, etc [1-9]. Among many types of monochromators, the Czerny Turner configuration is widely used due to its ability to achieve broad spectral range by rotating the planar of grating [10]. Although the current hardware system can acquire intensities of various wavelengths, there is a limitation on the acquisition process. It cannot acquire intensities of different wavelength at the same time. It is inconvenient to acquire data if anyone wants to analyze a large number of single lights. They have to manually change the angle of grating a lot of times to achieve all the intensities of a desired range of spectral bands. To overcome this problem, we developed new software associated with the design and process of a Czerny Turner monochromator system. This software helps analysts do their job faster by providing a convenient way for acquiring data. They only need few steps instead of many steps of handling the system to get the multi-wavelength data. The data acquisition is done automatically with various combinations of options by changing the groove, blaze of grating, or the resolution of scanning. We also developed a tool to draw all data in graphical presentation which helps the analyst have a general view for the spectra of sample. 2 Hardware Overview A monochromator is an optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. A Czerny Turner monochromator system includes: entrance slit, exit slit, mirrors, grating, PMT (photomultiplier tube) detector, and analog-to-digital (A/D) device as shown in figure 1. ISBN:

2 The range of colors (the size and position of the beam) leaving the exit slit is a function of the width of the both slits, thus determined by the simultaneous adjustment of widths of both the entrance and exit slits. It makes sure the light passing through the slits is a parallel light and its size is small enough. There are a lot of settings to change the size of slit for various experiments. Fig. 1: General operation of Czerny Turner Monochromator In the common Czerny-Turner design, the broad band light source (A) is aimed at an entrance slit (B). The amount of light energy available for use depends on the intensity of the source in the space defined by the slit (width*height) and the acceptance angle of the optical system. The slit is placed at the effective focus of a curved mirror (the collimator, C) so that the light from the slit reflected from the mirror is collimated (focused at infinity). The collimated light is refracted by the prism or diffracted from the grating (D) and then is collected by another mirror (E) which refocuses the light, now dispersed, on the exit slit (F). The grating (D), more precisely diffraction grating, is a device that, in optics, is a reflecting or transparent optical component with a periodic structure on which there are many fine, parallel, equally spaced grooves. It splits and diffracts light into several beams travelling in different directions. The directions of these beams depend on the spacing of the grating and the wavelength of the light so that the grating acts as the dispersive element. Different color beams (wavelengths) are reflected in different angles as shown in figure 2. Because of this property, gratings are commonly used in monochromators and spectrometers. At the exit slit (F), the colors of the light reflected by the second mirror (E) are spread out into several beams (the colors of the rainbow). Because each color beam arrives at a separate point in the exit slit plane, there are a series of images of the entrance slit focused on the plane. Because the entrance slit is finite in width, parts of nearby images overlap. The light leaving the exit slit (F) contains the entire image of the entrance slit (B) of the selected color plus parts of the entrance slit images of nearby colors. A rotation of the dispersing element causes the band of colors to move relative to the exit slit, so that the desired entrance slit image is centered on the exit slit. Fig. 2: Grating disperses the wavelengths The light from the exit slit (F) goes through the sample (H) and then enters PMT (I). PMT (I) is a device to convert light intensity to electron, or more exactly, it converts photon to electron. The output of PMT is the form of analog signal which is the voltage or the current, and then forwarded to an A/D conversion device (J) which converts the analog signal to digital signal that will be sent to a computer system. A/D conversion device (J) is equipped inside the computer system via PCI slot. Nowadays, there are a lot of A/D conversion devices using USB interface but they are still expensive and slow. The A/D device has two modes to convert signal: current mode or voltage mode. With Current mode, the user can select one of three gains: 10-6, 10-7, and With Voltage mode, the user can select one of two gains: from 0 to 1 volt or from 0 to 10 volt. The output of A/D device is the intensity of a single light wavelength. By collecting the intensities of different wavelengths, the user can analyze the properties and material constitution of the sample. However, analysis collecting task takes long time for changing the angle of grating because the structure of the hardware allows the user to handle only one wavelength at a time. Therefore, an efficient method to save the data acquisition time and effort for the user is needed. This is one of major requirement for the software for controlling the hardware. ISBN:

3 3 Software Based on the functional properties, the software can be divided into three modules: communication, scan and graphical presentation. This section explains the details of the software modules. 3.1 Communication module The communication module is composed of two parts. The first one is to send the control command signals to the grating through RS-232 serial port. The second one is to send signals through the PCI slot to control the data acquisition process of A/D conversion board. The grating is controlled by the commands sent through the serial port. The commands then create associate signals that the controller board can understand and then, change the status of grating. The controller board can adjust the parameters of grating or make the motor movements. The available parameters for adjustment include those for the groove and the blaze. The software also receives the response from controller board via serial port listener. When a query is sent to the serial port, the controller board sends an appropriate message back to the serial port. If the query is for a motor movement, the controller board only sends the notification after the motor finishes the movement. The common commands are listed in table 1. Table 1: Common commands to control the grating Command Explain Example Response CW Display the current information of grating cw MZ GS# Display the information of Monochromat or Select the Grating to active mz Turret=1/Grating=1/Wave length(nm)=0.000nm/step =50001* MONOCHROMATOR=1 /MODEL=z/INIT_SPEE D= /BACK_LAS H=3000/#TU=1/TURRE T=1/GRATING=1/GRO OVE=1200/BLAZE=500 nm/stp- NM= /0nm=50000 /REGION= /STA RT=1/INIT- WL= (nm)/GRA TING=2/GROOVE=2400 /BLAZE=500nm/STP- NM= /0nm=30000 /REGION= /STA RT= /INIT- WL= (nm)* gs2 * SL# Move the motor to the selected wavelength sl Similar to the controller board, the A/D board also has a system of functions to acquire data. First of all, we develop a device driver to interact with A/D board via PCI interface. The device driver then provides an interface to our main software. This interface is a list of functions including the function for choosing the scan mode and acquiring intensities of the light beams. 3.2 Scan module The scan module manages all parameters and carries out the scanning operation of the software. This module receives the choice of scanning specifics from the user and makes the change on the associated parameters through communication module, and then acquires data matched with the parameters. Based on the device parameters, the software module has to have the specified value for the following parameters before it scans: Start Wavelength, End Wavelength, Number of points, Scan Resolution, Integration Time, Number of Scan, Delay Time, and Number of Accumulation. The Scan Resolution (SR) parameter is calculated by the formula: where SW is Start Wavelength, EW is End Wavelength, and NP is Number of Points. As we described above, the light reaches at the grating first, and then the A/D board. That leads the software to send the command to move the motor first, and then send the command to acquire data. From this major sequence, we form the procedure to scan the intensity of different wavelengths as follows: - Step 1: Set no.scan (No. of Scans) by 1 - Step 2: The software sends command to the controller board to make the motor movement so that the output wavelength equals the Start Wavelength. * ISBN:

4 Yes no.scan = 1 Output wavelength = Start Wavelength Output wavelength Start > End Wavelength No Wait a Delay Time - Step 4: After the motor finishes the movement, the software waits for a time of the Delay Time. - Step 5: Acquire the intensity for light of the selected wavelength from A/D board during Integration Time. - Step 6: Display the intensity by graph. Go to step 3. - Step 7: Increase no.scan by 1. - Step 8: o If no.scan is larger than Number of Scan, end the procedure. o If no.scan is not larger than Number of Scan, go to step 2. The procedure is illustrated in figure 3. Acquire intensity Display intensity on graph no.scan = no.scan + 1 no.scan Fig. 3: The procedure of scan > End Number of Scan Yes - Step 3: o If the output wavelength is not larger than the End Wavelength, the motor moves to the next wavelength by the amount of resolution. Go to step 4. o If the output wavelength is larger than the End Wavelength. Go to step 7. No 3.3 Graphic module The output data of the A/D device is a list of indexed numbers. With this raw data, it is still difficult for the users to find out the relationship between different intensities of the sample. Previous tools generally save the raw data in a file in specific format, and the user has to use a third party graphical tool. Thus, we need a better way for data presentation integrated with the main software, and we chose the graphical visualization with data plots for wavelength (x-axis) vs. intensity (y-axis) as shown in Figure 4. In this graphical user interface (GUI), the right white column shows the pairs of values for the wavelengths and the intensities acquired by A/D device. The area of center column presents these values by plot form: the horizontal axis is for wavelength and the vertical axis for intensity. As described before, a major merit of our software is to easily perform multiple scans as described in Figure 3. The data sets for the multiple scans are saved in the database, and a single scan data can be displayed by selection of scan number as shown in Figure 4. The scan number can be selected from the list box located in the top of the right column in Figure 4. ISBN:

5 Fig. 4: Spectrum graph 4 Conclusion In this paper, we introduce a software for controlling the Czerny Turner Monochromator. We first overview the hardware of the monochromator, and describe the configuration of the software. Our software provides some merits: it can automatically change the wavelength by change the angle of the grating, and perform multiple scans. The feature of our control software can make the monochromator more powerful and convenient in use. The user can save much time for acquiring data. Also, it provides graphical display for better analysis and intuition for the results. For future improvement for the control system, we will develop a tool to automatically recognize the materials composing the test sample by comparing the intensity distribution with the standard intensity pattern in a database. In order to make this progress, we need to build the database of standard intensity distribution for various material samples and develop an intelligent method to determine the material based on the distribution of the obtained intensities. Reference [1] N. Niimura, et al., "A Monochromator for Neutron Crystallography in Biology," Physica B- Condensed Matter, vol. 213, pp , Aug [2] M. Seto, "Studies on nuclear resonant scattering of synchrotron radiation by K-40," Structural Chemistry, vol. 14, pp , Feb [3] A. Di Venere, et al., "Resolution of the heterogeneous fluorescence in multi-tryptophan proteins: ascorbate oxidase," European Journal of Biochemistry, vol. 257, pp , Oct [4] J. H. Lee and M. G. Choung, "Determination of Protein Content in Pea by Near Infrared Spectroscopy," Food Science and Biotechnology, vol. 18, pp , Feb [5] D. Cozzolino, et al., "Usefulness of near infrared spectroscopy to monitor the extent of heat treatment in fish meal," International Journal of Food Science and Technology, vol. 44, pp , Aug [6] F. Prino, et al., "Effect of x-ray energy dispersion in digital subtraction imaging at the iodine K- ISBN:

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