JPS6032814B2 - Planar grating spectrometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a plane diffraction grating spectrometer constituted by a reflecting mirror.

In cases such as emission analysis using a laser microprobe, the brightness of the spectrometer is an important condition, and the spectrometer for this purpose generally has a structure called the Ernitana type, which is effective in obtaining clear spectral images with good resolution. It is suitable for

However, with this structure, since the concave reflector is used off-axis, there is astigmatism, and it is difficult to correct this aberration. Attempts have been made to reduce astigmatism by adjusting the light beam to diverge, but sufficient correction has not been achieved. Particularly in emission analysis, it is necessary to measure the concentration of a spectral line, but there is a problem in that the concentration varies depending on the measurement site of the spectral line. In other words, the spread of the spectral lines in the width direction differs along the height direction of the spectral lines, and the width is the narrowest.
The exact height of the spectral lines differs depending on the wavelength.
Therefore, when the side light element is moved in the wavelength direction of the spectrum, the brightness differs for each spectral line, making it impossible to perform accurate density comparisons. Also, of course, something like a step filter cannot be used. It is thought that the main cause of the above-mentioned problems is astigmatism caused by the reflecting mirror being used in an off-axis state.

The present invention attempts to solve the above-mentioned problems by correcting astigmatism in the optical system of a Czerny-Turner spectrometer. FIG. 1 is a plan view of the Ernitana spectrometer.

S is an entrance slit, G is a plane reflection grating, MI is a collimator mirror and is a concave mirror, M2 is an imaging mirror which is also a concave mirror, and P is a spectral image plane. In the concave mirrors M1 and M2, dashed lines C and O are the optical axes of the concave mirrors, respectively, and 8 and a' are off-axis angles. The optical paths of one concave mirror M1 and M2 are all in the same plane, and off-axis is also done within that plane, so M1
, M2 are weighted without canceling each other out. In order to correct the above-mentioned astigmatism, the present invention provides a collimator mirror MI with a radius of curvature (the plane of the paper in FIG. 1) of the horizontal section.
A barrel-shaped concave mirror in which H is larger than the radius of curvature RV of the vertical section is used.

FIG. 2 is a perspective view of such a concave mirror MI. Of the light reflected from such a concave mirror MI toward the diffraction grating G, if we consider the light that comes from the center of the slit S, the light is parallel in the horizontal cross section, and the light is parallel in the vertical cross section. Since the curvature of the MI is stronger than that of the horizontal section, the curvature of the MI is greater than that of the horizontal section. The curvature of the vertical axis of the collimator mirror MI is thus determined. The table on a separate page shows the coordinates, frame angle, radius of curvature, etc. of each center of M1, M2, grating G, slit S, light-receiving surface (rear surface of spectrum) P in one embodiment of the present invention.

As shown in Figure 1, the coordinates are the center of the diffraction grating G as the origin, the X and Z axes in the direction of arrows X and Z in the plane of the paper, and the Y axis perpendicular to the plane of the paper, and the unit of length is However, in an optical system, if the shapes are similar, the optical paths will be similar, so the coordinates and radii of curvature in this table can be considered to be numerical values representing their mutual ratios. It goes without saying that the present invention can be sufficiently implemented even if the values shown in this table are slightly changed (approximately 5%). FIG. 3 shows aberrations in the above embodiment.

In this figure, the vertical axis △X indicates the deviation in the horizontal direction (within the paper plane of Figure 1) from the ideal point of incidence of a certain ray on the spectral plane P, and the axis of gravity indicates the deviation between the considered ray and the diffraction grating G. It shows the position of the intersection in the grating width direction, and in the two curves shown by a solid line and a dotted line, H=0 indicates the aberration for the ray that passes through a point on the horizontal line passing through the center of the grating G, and similarly 51 is an aberration for the light ray passing through a point on the line along the upper and lower lines of the grating G. Although it is difficult to understand with a verbal explanation, when looking for the coordinate date horizontally to the grid G and the coordinate date perpendicular to the grid G as shown in Figure 3, point A in Figure 3 is located at the grid G.
It shows the lateral aberration of the ray A that passed through the upper point A, and the mouth point shows the aberration of the ray exit that passed through the mouth point on the grating G. Also, in FIG. 3, the words "in = 23 Mm, etc." indicate wavelengths. It can be seen from this figure that the aberrations are within 0.1 range over each wavelength. The present invention determines the specifications of a normal Gerniterna type spectrometer so that the aberration A× (the aberration indicated by the solid line in FIG. 4) is minimized for the light ray in the plane of the diagram in FIG. Since the radius of curvature RH of MI is determined,
Starting from there, RV (starting from the same value as RH)
Calculating the amount of aberration △× by changing it little by little, in Figure 3, the aberration shown by the dotted line of H = 51 is overall 4.
The RV is determined by repeating the calculation until the value becomes smaller.

Accordingly, the present invention provides a collimating mirror which is also preferred for use in a spectrometer of the Germanic turner type. table

[Brief explanation of the drawing]

Fig. 1 is a plan view of the Ernitana spectrometer, Fig. 2 is a perspective view of the concave collimator mirror used in the present invention, Fig. 3 is a graph showing aberrations of the spectrometer of the present invention, and Fig. 4 is a graph showing the aberrations of the spectrometer of the present invention.
FIG. 3 is a perspective view of a spectrometer for explaining light rays that exhibit the aberrations shown in the figure. S...Incidence slit, G...Plane diffraction grating, P...Spectral image plane, MI...Collimator concave mirror,
M2... Concave mirror for imaging. Figure 1 Figure 2 Figure 4 Figure 3

Claims (1)

[Claims] 1. An input slit is arranged on one side of a plane diffraction grating, and a collimator concave mirror is placed opposite the input slit and the diffraction grating so as to form a spectral imaging plane on the opposite side. In addition, in a Zzernyturna spectrometer in which an imaging concave mirror is arranged to face the diffraction grating and the spectral imaging surface, the collimator concave mirror is used to calculate the curvature of the cross section by a plane containing the concave mirrors and the diffraction grating, and the cross section by a plane perpendicular to the same plane. It has a barrel-shaped concave surface with a curvature smaller than X=-
114.712±5% Z=-218.227±5% Light receiving surface: X=213.786±5% Z=-209.490±5% Collimator mirror: X=-316.561±5% Z=772
．． 667±5% Ratio of radius of curvature (in the coordinate plane) R_H to radius of curvature (perpendicular to the coordinate plane) R_V = 1.0324±0.0015R_H,
Average of R_V = 1985-2015 Off-axis angle θ = 5.3
83°±5% imaging concave mirror: X=321.334±5% Z=784.317±5% Radius of curvature=2000.0 Off-axis angle θ': 8.051°±5% A flat diffraction grating spectrometer.