JPH0434088B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to a diffraction grating drive device for a diffraction grating spectrophotometer.

(b) Prior Art Conventionally, a sine bar mechanism has been used as a diffraction grating drive mechanism in a diffraction grating spectrophotometer. This mechanism has a configuration as shown in Fig. 1, and the moving distance H of the wavelength feed nut F and the rotation angle of the sine bar L fixed to the rotation axis of the diffraction grating G (therefore, the rotation angle of the grating G) θ
The relationship H=lsinθ (l is the length of the sine bar) holds between the two. The spectrophotometer has a configuration as shown in Figure 2, where S1 is the incident direction, S2 is the exit direction, and each direction is fixed, and the normal line of the grating G is such that both slits extend to the center of the grating G. The grating movement in the direction of the angle bisector is the wavelength point O, and the rotation angle of the grating G is measured from this position and is defined as θ. Assuming that the angle that both slits make with respect to the center of the grating is 2α, the relationship between the wavelength λ of the first-order diffracted light and the rotation angle θ of the grating G is λ=2dcosα·sinθ (a). Here, d is the lattice constant of the lattice G,
cosα is a constant determined by the spectrophotometer.

Therefore, if the length l of the sine bar is made so that l=2dcosα in the sine bar mechanism, the movement of the nut F will immediately give the wavelength λ. However, in reality, the grating constant d of the diffraction grating has some error for each grating, so this error was corrected by finely adjusting the length l of the sine bar.

As mentioned above, using a sine bar mechanism to drive the diffraction grating has the advantage that the wavelength can be determined immediately from the amount of movement of the nut F, and it is also possible to correct errors in the grating constant, but the mechanism itself is complicated. It's expensive. On the other hand, as the use of small computers becomes more common, the advantage that the amount of movement of the nut of the sine bar mechanism immediately represents the wavelength becomes less valuable, and the disadvantage that the mechanism is complicated and expensive becomes greater. Therefore, as shown in FIG. 3, a direct axis drive system has been adopted in which the diffraction grating G is directly driven by a pulse motor M via gears. In this case, the directly available data regarding the position of the grating G is the rotation angle θ.
Since it is not sin θ, a computer calculates the wavelength λ from the value of θ.

The above-mentioned direct axis drive system has the advantage that the mechanism is simpler and cheaper than the sine bar mechanism, and also has the advantage that it can be driven at high speed. However, there is no mechanical means for correcting errors in the grating constant of the diffraction grating. Therefore, an operation was required to obtain the correction coefficient.

(c) Purpose The present invention provides a diffraction grating spectrophotometer that automatically corrects variations in the grating constant of the diffraction grating described above.

(d) Configuration The present invention uses two emission line spectra with known wavelengths to determine the angular position of the diffraction grating from which light of each spectrum is emitted, and calculates the error in the lattice constant of the diffraction grating from the difference Δθ between the two angular positions, and the error in the lattice constant of the spectrometer. This diffraction grating spectrophotometer determines the position of the diffraction grating when the light of wavelength O incident on the input slit exits from the output slit, and determines the correct wavelength from data on the angular position of the diffraction grating.

(e) Embodiment FIG. 4 shows the flow of automatic grid error correction operation in an embodiment of the present invention. Prior to explaining this operation, the principle of operation will be explained. λ1=486.0nm and λ2= obtained from the deuterium lamp as two emission lines
Two emission lines at 656.1 nm are used. Figure 5 shows the spectral characteristics of the deuterium lamp.

λ1=2dcosαsinθo...(1) λ2=2dcosαsinθ(θo+Δθ) (2) Drive the spectrophotometer from the short wavelength side to detect the light of λ1. In this case, since the wavelength origin is undetermined, θo is unknown. After detecting λ1, wavelength driving is continued to detect light of λ2. Here, Δθ is directly determined by counting the input pulses to the pulse motor for driving the diffraction grating from the time when λ1 is detected until the time when λ2 is detected. In the above equations (1) and (2), the lattice constant d contains an error and is an unknown quantity, and θo is also an unknown quantity.
Calculations to obtain d and θo from equations (1) and (2) are extremely troublesome. Therefore, the features of the present invention are as follows. The manufacturing error of the diffraction grating is about ±2 at most in terms of the number of gratings per 1 mm. In the case of the regular number of grids No/mm, Δθ in the above equation (2) can be found by calculation. Similarly, Δθ for No±1 and No±2 is also found. The number of input pulses to the pulse motor corresponding to these Δθ is calculated and stored in the computer memory in advance. If we put this in a table, we get: Number of lattices Number of pulses No−2 n2′ No−1 n1′ No no No+1 n1 No+2 n2. Therefore, by searching to which of the above table the number of pulses for the actually determined Δθ corresponds, the lattice constant can be immediately determined. If the lattice constant is determined in this way, θo can be determined using the above equation (1), and the wavelength origin can be determined. Also, 2dcosα as a constant unique to the device
Since the wavelength λ is also determined, the wavelength λ is immediately calculated from the rotation angle of the diffraction grating.

Automatic error correction is performed as follows. First, switch the light source of the spectrophotometer to a deuterium lamp and start the automatic correction operation. Referring to Figure 4,
The spectrometer is driven to the shorter wavelength side until a part of the mechanism hits the limit switch. This limit switch is designed so that, for example, a sector gear at S shown in FIG. 2 comes into contact with it. Once the limit switch is turned on (this position does not require precision), the spectrometer is driven toward longer wavelengths until the emission line of the deuterium lamp at wavelength λ1 = 486.0 nm (first emission line) is detected. D. When the first bright line is detected, a counter for the pulses supplied to the pulse motor is started, and the spectrometer is further driven to the longer wavelength side until the bright line with wavelength λ2 = 656.1 nm (second bright line) is detected. to. Second
When a bright line is detected, the spectrometer is stopped, and the count number n of pulse motor driving pulses at that time is compared with n2' to n2 in the table to find a match or a similar one. This operation determines the number of gratings in the diffraction grating, so calculate θo using equation (1) above.
Calculate 2dcosα, store it in memory, set n+ (number of pulses corresponding to θo) in the counter,
While driving the spectrometer to the short wavelength side, the contents of the counter are subtracted by the pulse motor drive pulse.
Yo, ta, when the counter reaches 0, stop the spectrometer and end the automatic correction operation. This determines the wavelength origin of the spectrometer. From then on, the count of the counter gives the rotation angle of the diffraction grating, and the wavelength is calculated using equation (a) shown at the beginning, using the coefficients found in step (e). To calculate θ, change the value of θo from the number of grids No. - 2 to No. according to the previous table.
It is also possible to calculate each value up to +2 in advance, and then select from the calculated values based on the search results for .

FIG. 6 is a block diagram showing the configuration of a spectrophotometer according to an embodiment of the present invention. M is a spectrometer, D is a deuterium lamp, W is a tungsten lamp, m is a light source switching mirror, PM is a pulse motor for driving a diffraction grating,
PG is a pulse motor drive circuit, and CPU is a computer that controls the drive of the spectrometer, processes photometric data, and performs the automatic correction operations described above. C is sample cell, PD
is a photodetector, AD is an A/D converter, and R
is a recorder or other display device.

(f) Effect The spectrophotometer of the present invention has the above-described configuration, and correction for variations in the diffraction grating is automatically performed, and the lattice constant can be adjusted using a mechanically simple and inexpensive direct-axis drive type diffraction grating spectrophotometer. It has become possible to obtain a highly accurate spectrophotometer with corrected variations.

[Brief explanation of drawings]

Fig. 1 is a plan view of the sine bar mechanism, Fig. 2 is a plan view showing the optical path of the spectrometer, Fig. 3 is a plan view of an example of a direct-axis drive spectrometer, and Fig. 4 is an embodiment of the present invention. FIG. 5 is a flowchart of the automatic correction operation in FIG. 5, spectral characteristics of a deuterium lamp, and FIG. 6 is a block diagram showing the configuration of an embodiment of the present invention. G... Diffraction grating, D... Deuterium lamp, W...
Tungsten lamp, M...Spectrometer, PM...Pulse motor, PG...Pulse motor drive circuit, C
...sample cell, PD...photodetector, R...display device.

Claims (1)

[Claims] 1. An operation program that detects the lattice constant of the diffraction grating from the difference in the angular position of the diffraction grating when two emission line lights of known wavelengths are incident on the input slit of the diffraction grating spectrometer and exit from the output slit of the spectrometer;
A diffraction grating spectrophotometer equipped with a control device having a function of calculating a wavelength from a detected angular position of a diffraction grating using a detected grating constant.