METHOD OF OPERATING A SPECTROPHOTOMETER

Abstract: A spectrophotometer of the type using a pulse motor (P) to rotate a diffraction grating (G) through a speed reducing mechanism (B) has a capability of constructing a conversion table according to a program, by which table a rotation amount of the motor (P) is converted to a corresponding wavelength of output light from a monochromator (MC). The spectrophotometer is capable of designating a calibration mode in which an emission peak at a known wavelength of light from a light source is detected, a coefficient in a conversion equation is determined on the basis of the known wavelength of the emission peak and a rotation amount of the motor (P) at a time when the emission peak is detected, and the determined coefficient is used to construct the conversion table which is then stored in a nonvolatile memory. In an analysis mode, a rotation amount of the motor (P) is converted to a corresponding wavelength in accordance with the conversion table in the nonvolatile memory.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method of operating a spectrophotometer which uses a pulse motor to rotate a dispersive element of a monochromator through a speed reduction meachanism as it is mentioned in the preamble of the patent claim 1. Such a method is known from the EP-A-0 207 498. A sine bar has previously been used as a mechanism for performing scanning over a range of wavelengths on spectrophometers equipped with a diffraction grating. In this mechanism, a nut is moved by means of a feed screw to push the sign bar projecting from the axis of the grating. Although the amount of rotation of the feed screw has a linear relationship with the wavelength, the use of a feed screw prevents fast feeding over a wavelength range. The recent advances in computer technology have made it relatively easy to convert the amount of rotation of a pulse motor quickly to a value of a desired function. Under this circumstance, the advantage of linear relationship between the amount of rotation of a feed screw and wavelength has gradually lost its appeal, and the difficulty involved in fast feeding over a wavelength range has become a major concern to the users of spectrophotometers adopting the sine bar mechanism. On the other hand, spectrophometers of a type that allows a dispersive element to be directly rotated via a speed reducing mechanism have gained increasing popularity for several reasons including simple construction and low cost. In this type of spectrophotometer which causes the dispersive element to be directly rotated via a speed reducing mechanism, the amount of rotation of the motor does not have a linear relationship with the wavelength even if a grating is used as the dispersive element. If a grating is used as the dispersive element, the relationship between the angle of rotation, ϑ, of the grating from a reference position and wavelength λ of output light is expressed by: λ = 2d n cos sinϑ where d is the grating constant of the grating and n is the order of diffraction. As shown in Fig. 4, the angle  is one half the angle formed between incident light on the grating G and diffracted light, that is, one half the angle subtended by the entrance and exit slits in the monochromator with respect to the center of the grating. Given equation (1) which describes the relationship between the angle of rotation of the grating and wavelength, the spectrophotometer of the type under consideration (which allows the dispersive element to be directly rotated via a speed reducing mechanism) converts the amount of rotation, x, of the driving motor to a corresponding wavelength using a ROM stored with a table that correlates the amount of x to wavelength by the following equation: λ = K·sinpx (ϑ = px). In equation (2), p is a constant determined by the speed reducing mechanism, but K includes not only the grating constant of the grating [see equation (1)) but also quantities associated with the positions of optical elements such as the entrance and exit slits in the monochromator. These quantities contain errors that will occur during the working and assembly of monochromator components and hence will differ slightly among individual units of spectrophotometer even if they are fabricated of the same design. Furthermore, it is difficult from a practical viewpoint to equip individual units of the device with a ROM stored with a table that was constructed by actual measurements of K in accordance with equation (2). Instead, the following procedure is usually taken: a ROM is preliminarily provided that is stored with a plurality of tables constructed for several values of K according to equation (2), and is mounted on each of the fabricated spectrophotometers; prior to shipment from the factory, a calibration test is conducted for each unit of the device to select an optimum table from those stored in the ROM; and when using a particular unit, the amount of rotation of the drive motor is converted to a corresponding wavelength using the selected table. As described above, the spectrophotometer of the type that allows a dispersive element to be directly rotated with a motor via a speed reducing mechanism uses a conversion table to convert the amount of motor rotation to a corresponding value of wavelength. Ideally, each unit of the device must be equipped with a different conversion table due to the limited precision of working of monochromator components but, in practice, an optimum table is selected from among several prepared tables by a calibration test and used for subsequent spectrophotometric measurements. For this purpose, it is desirable to prepare the largest possible number of tables with slightly different contests. However, the number of tables that can be prepared is limited by the capacity of ROM and it sometimes occurs that an optimum table is not available for a certain spectrophotometer and that the value intermediate between two values in two different tables has to be used as the best one. With another device, the relationship between the amount of motor rotation and wavelength value may no! fit any of the prepared tables. However, there has been no simple prior art method that is capable of; dealing with these problems. SUMMARY OF THE INVENTION With the known method according to the preamble of the patent claim 1 the coefficient determined in the calibration step is compared with a preset nominal value and a correction amount is set using switches in order to more closely approximate the actual value. During the analysing mode each time the rotation amount of said motor is converted to the wavelength of the output light of the monochromator according to the conversion equation wherein a coefficient value is used corrected by adding or subtracting the correction value from the nominal value. If it is intended to improve the approximation it would be necessary however, to provide a large number of more finally stepped switches in order to closely fit the coefficient value used in the conversion equation to the actual value of the specific spectrophotometer. It is the object of the present invention to provide a method of operating a spectrophotometer according to the preamble of the patent claim 1 with which the rotation amount of the motor can be exactly converted to the wavelength of the output light of the monochrometer without excessive expense. For this purpose the method according to the present invention is characterized by the features of the characterizing part of the patent claim 1. With the method according to the present invention a conversion table is constructed by which the amount of rotation of a motor for driving the dispersive element is converted to a corresponding value of wavelength of output light form a monochromator. In a calibration step an emission peak at a known wavelength of light from a light source is detected and a coefficient for use in the construction of a conversion table is determined both from the amount of rotation of the drive motor at a time when the emission peak is detected and from the known wavelength of said emission peak. The thus determined coefficient is used to construct a conversion table which is then stored in a nonvolatile memory. In the analysing mode the amount of rotation of the drive motor is converted to a corresponding value of wavelength according to the conversion table in the memory. Specific embodiments of the method according to the present invention are subject matter of the patent claims 2 to 8. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of the monochromator used in a spectrophotometer according to the present invention; Fig. 2 is a block diagram showing the overall construction of said spectrophotometer; Fig. 3 is a graph showing the spectral energy characteristic of a deuterium lamp to be used in a calibration test; and Fig. 4 is a diagram showing the relationship between incident and exit angles of light on a diffraction grating and a rotation angle of the diffraction grating. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows the monochromator used in a spectrophotometer of the present invention, and Fig. 2 shows the overall construction of this spectrophotometer. Shown by G in Fig. 1 is a planar diffraction grating, and A is the axis of rotation of the grating G which serves as the output shaft of a speed reducing mechanism B. Shown by P is a pulse motor the rotation of which is transmitted to the output shaft A via the mechanism B. Shown by S1 is an entrance slit in the monochromator; S2 is an exit slit; M1 is a collimator mirror which collimates the incident light from S1 to be launched onto the grating G; and M2 is a camera mirror which condenses the diffracted collimated light at a point on the exit slit S2. One half the angle Ψ shown in Fig. 1 is equal to the angle  in equation (1). The orientation of the grating G at which the line normal to the grating G at its center coincides with the line dividing the angle Ψ into two equal parts is the reference position for G. The angle of rotation ϑ of the grating is measured from this reference position. The monochromator shown in Fig. 1 is indicated by MC in Fig. 2. Light sources L1 and L2 are built in the spectrophotometer; L1 is a deuterium lamp for the shorter wavelength range and L2 is a tungsten lamp for the longer wavelength range. A switch mirror m1 is moved in such a way that if it is inserted into the optical path of light from L1, light from L2 is launched into the monochromator MC and that if it is removed from said optical path, light from L1 is launched into MC. Shown by C is a sample cell through which the light emerging from MC will pass. A sample is set within the optical path of light issuing from MC. Shown by PD is a photodetector receiving the light from MC that has passed through the sample cell C. The output signal of the detector PD is amplified by an amplifier AP and fed into a control unit K through an interface IF. The control unit K comprises a central processing unit CPU, a ROM into which an operational program and a conversion table or a program for constructing a conversion table, etc. have been written, a nonvolatile memory EEPROM, and a RAM. The control unit K controls the overall operation of the system and processes the photometric data that has been supplied via IF. Shown by P is a display such as a CRT display for indicating the results of analysis and any other necessary information. Shown by F is an operating section by means of which the operator enters various kinds of data or operational commands into the control unit K. The first embodiment of the present invention is described hereinafter. The ROM in the control unit K has in store a program for constructing a conversion table according to equation (2) which expresses the relationship between the wavelength λ and the amount of rotation, x, of the motor P. A calibration test on the spectrophotometer is performed by the following procedure using the internal light source L1. L1 is a deuterium lamp having a spectral characteristic which, as shown in Fig. 3, has a sharp peak at 486.0 nm and 656.1 nm. Calibration is performed with these peaks. When the grating G is rotated in the direction toward zero wavelength, diffracted light of zeroth order is detected at a wavelength of zero. Search is conducted for the peak of the output of the photodetector PD at about zero wavelength. When the peak center is detected, the orientation of the diffraction grating G corresponding to the detected peak center is used as the reference position of the grating, and the amount of rotation of the pulse motor P from that reference position is substituted as x into equation (2). This amount x is obtained by counting the number of drive pulses fed to the pulse motor P starting at the time when the grating G is on its reference position. The drive pulses are supplied and counted by the CPU in the control unit K. When the amount of rotation of the pulse motor P is counted with the grating G being driven from its reference position toward the longer wavelength region, emission peaks at 486.0 nm and 656.1 nm in the spectrum of light from L1 are successively launched into the photodetector PD. Search is then conducted for the emmission peaks at 486.0 nm and 656.1 nm. When the peak center is detected, the amount of rotation, x, of the pulse motor P corresponding to the detected peak center is made associated with the wavelength of 486.0 nm or 656.1 nm. The value of K in equation (2) can be determined on the basis of the above-obtained values of x and the wavelengths of those two emission peaks preliminarily stored in the ROM. The value of K is determined as the average of the values calculated for the two emission peaks. Thereafter, a conversion table is constructed in accordance with the program in the ROM and written into the nonvolatile memory EEPROM. The program in the ROM for constructing a conversion table calculates wavelength λ for each value of x determined upon successive steps of the rotation of the pulse motor P. As is clear from equation (2), the calculation of λ involves computing the sine of px. In order to shorten the computing time, the values of sinpx are preliminarily calculated for a certain number of x values and stored in the ROM, and values of sinpx for intermediate values of x are computed by a suitable method of interpolation. The thus computed sinpx is multiplied by the value of K and the result of this multiplication is written into the nonvolatile memory EEPROM together with the value of x for each step of motor rotation. When conducting an analysis in practice, the value of wavelength corresponding to x, i.e., the amount of rotation of the pulse motor P from the reference position of the grating G, may be extracted from the conversion table in the nonvolatile memory EEPROM. In the first embodiment described above, the calibration test is conducted using only two emission peaks and hence the value of K to be substituted into equation (2) is calculated by averaging the values of K determined for these two emission peaks. Theoretically, only one emission peak will suffice for calibration purposes if a diffraction gating is used as the dispersive element. In practice, however, the value of K in equation (2) is also a function of wavelength because it is influenced by such factors as variations in grating constant depending on a site on the grating. The calibration can be performed in a very precise manner if the values of K are determined on the basis of several emission peaks and processed by the method of least squares or otherwise to obtain a function K(x) and a conversion table is constructed in the form of K(x)sinpx. The second embodiment of the present invention is described hereinafter with reference to the case where a calibration test is conducted using only one emission peak. The ROM in the control unit K has in store a conversion table according to equation (2) expressing the relationship between wavelength λ and the amount of rotation, x, of the motor P, which table is associated with the design of the spectrophotometer. A calibration test on the spectrophotometer is performed by the following procedure using the internal light source L1. L1 is a deuterium lamp having a wavelength characteristic which, as shown in Fig. 3, has a sharp peak at 656.1 nm. Calibration is performed with this peak. When the grating G is rotated in the direction toward zero wavelength, diffracted light of zeroth order is detected at a wavelength of zero. Search is then conducted for the peak of the output of the photodetector PD at about zero wavelength. When the peak center is detected, the orientation of the diffraction grating G corresponding to the detected peak center is used as the reference position of the grating G, and the amount of rotation of the pulse motor P from that reference position is substituted as x into equation (2). This amount of x is obtained by counting the number of drive pulses fed to the pulse motor P starting at the time when the grating G is on its reference position. The drive pulses are supplied and counted by the CPU in the control unit K. When the amount of rotation of the pulse motor P is counted with the grating G being driven from its reference position toward the longer wavelength range, emission peak at 656.1 nm in the spectrum of light from L1 is launched into the photodetector PD. Search is then conducted for the emission peak at 656.1 nm. When the peak center is detected, the amount of rotation, x, of the pulse motor P corresponding to the detected peak center is made associated with the wavelength of 656.1 nm. On the other hand, if the value of wavelength λ is determined from said value of x according to the conversion table stored in the ROM, the result λ' is usually different from 656.1 nm. Thus, the correct value of wavelength on the spectrophotometer under consideration can be obtained by multiplying the thus obtained wavelength λ' by the correction coefficient, α = 656.1/λ'. This correction coefficient α is written into the nonvolatile memory EEPROM to conclude the steps of the calibration test. The operations involved in the practical use of the spectrophotometer according to the second embodiment of the present invention are described below. First, values of λ corresponding to those of x are read from the conversion table in the ROM. These values are then multiplied by the correction coefficient α stored in the nonvolatile memory EEPROM and the products are written into the RAM in such a way that they are correlated to the values of x. As a result, a table of conversion that expresses the relationship between x (the amount of rotation of the pulse motor P) and λ (wavelength), which is a characteristic of the specific spectrophotometer under consideration, is formed in the RAM. In actual measurements, this conversion table is used to determine the wavelength from the amount of rotation of the pulse motor P. In the second embodiment described above, the calibration test is conducted using only one emission peak. If desired, more than one emission peaks may be used in the calibration test to determine a correction coefficient for each wavelength region. In the second embodiment, the conversion table associated with the design of the spectrophotometer is written into the ROM and the data in it is multiplied by the correction coefficient when the spectrophotometer is actually used. Entirely the same result will be obtained if a table of conversion from px to sinpx is stored in the ROM and if the values of K in equation (2) as determined by actual measurements in a calibration test are stored in the nonvolatile memory EEPROM. In the two embodiments described above, emission peaks in the spectrum of light from an internal light source in the spectrophotometer are used in the calibration test and this offers the advantage that not only the manufacturer but also the user of the device is capable of performing the calibration as he finds it necessary. It should, however, be noted that the calibration test can also be conducted using an external light source. In the embodiments described above, the reference position of the diffraction grating is detected by sensing the peak center of diffracted light of zeroth order. Alternatively, a pin on the rotating axis of the grating may be detected at a specified position by either photoelectric or mechanical means, with the grating being adapted to reach the reference position when this pin is detected. According to the present invention, a suitable table of conversion from x (the amount of rotation of the dispersive element) to λ (wavelength) can be used for each spectrophotometer in spite of working errors that may exist in individual devices. The spectrophotometer of the present invention is capable of very precise measurements without incorporating ROMs having an especially large capacity.

Claims: A method of operating a spectrophotometer which uses a pulse motor to rotate a dispersive element of a monochromator through a speed reduction mechanism comprising an analysing mode in which the rotation amount of said motor is converted to the wavelength of the output light of the monochromator according to a corresponding conversion equation and a calibration step in which a coefficient of said conversion equation is determined on the basis of the known wavelength of at least one emission peak of the light of the light source means and the rotation amount value of said motor at the time said emission peak is detected in a search operation characterized in that in the calibration step under the control of control means a conversion table of rotation amount values to wavelenght values of said output light of said monochromator for the specific spectrophotometer is constructed in accordance with said conversion equation using the determined coefficient and stored in a nonvolatile memory and during the analysing mode under the control of the control means the amount of rotation of said motor is converted to said wavelength using the conversion table stored in said nonvolatile memory. A method according to claim 1, characterized in that the conversion table is constructed in accordance with a programm preliminarily written into a ROM, said programm including said conversion equation. A method according to claim 1, characterized in that said coefficient is determined as an average of two coefficient values obtained in association with two emission peaks. A method according to claim 1, characterized in that said coefficient is determined as a function of said rotation amount of said motor on the basis of a known wavelength value of a plurality of emission peaks and corresponding rotation amount values of said motor. A method according to claim 1, characterized in that a first conversion table is preliminarily written into a ROM, in the calibration step under the control of the control means a correction coefficient by which said wavelength extracted from the first conversion table is to be multiplied to obtain a corrected wavelength is determined and a determined correction coefficient is stored in a nonvolatile memory and in the analysing mode under the control of the control means a second conversion table for the specific spectrophotometer is constructed on the basis of said first conversion table and said determined correction coefficient in said nonvolatile memory and stored in a RAM and said rotation amount of said motor is converted to the corrected wavelength using the second conversion table in said RAM. A method according to claim 1 or 5, characterized in that the conversion equation is: λ = K x sinpx wherein λ represents the wavelength of the output light, K said coefficient, P a constant depending on the speed reducing mechanism and x said rotation amount of said motor. A method according to claim 5, characterized in that said first conversion table is a one associated with the design of said spectrophotometer. A method according to claim 5, characterized in that a plurality of correction coefficients are determined in association with respective emission peaks for every, and used in constructing said second conversion table which has a plurality of regions corresponding to the respective divided wavelength regions.

Inventors: FUKUMA, TOSHIAKI, NISHIGAKI, HIDEHISA, SATO, TATSUMI, ENMEIJI FURAWAHAITSU 302-C
Applicants: SHIMADZU CORPORATION
Application year: 1990
Main classification: G01J3
IPC codes: G01J3