Grazing incidence spectrometer

A grazing incidence spectrometer is provided with an entrance slit and a diffraction grating for diffracting the light rays incident thereon from the entrance slit and for imaging the light rays as a plurality of spectral lines. The diffraction grating is a curved diffraction grating having a predetermined principal radius of curvature and the groove patterns thereof are formed at unequal intervals so as to make the image plane thereof substantially planar. The entrance slit is disposed within a Rowland circle so as to satisfy EQU 0.7.ltoreq.r/R cos .alpha..ltoreq.0.9, where R is the principal radius of curvature of the curved diffraction grating, r is the distance between the entrance slit and the curved diffraction grating, and .alpha. is the angle of incidence of the principal ray incident from the entrance slit onto the curved diffraction grating.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a grazing incidence spectrometer using a 
diffraction grating. 
2. Description of the Prior Art 
In the grazing incidence spectrometer according to the prior art, as shown 
in FIG. 1 of the accompanying drawings, an entrance slit 2 and a concave 
(spherical) diffraction grating 3 are disposed on a Rowland circle 1. At 
this time, the image plane is formed arcuately on the Rowland circle 1. 
Therefore, an exit slit movable on the image plne has been provided and a 
photoelectric detector has been integrally provided rearwardly of the exit 
slit and by moving the exit slit and the photoelectric detector on the 
image plane, scanning in wavelength has been effected to derive 
information for each wavelength. For the same reason, a photographic 
dry-plate has been disposed while being bent along the image plane (the 
Rowland circle) and by sensitizing this, the wavelength information of the 
entire image plane has been obtained. 
Accordingly, with the above-described technique, it has been impossible to 
treat a plurality of types of wavelength information in real time. 
Also, the angle of incidence of the diffracted light onto the image plane 
has been great, so that the quantity of light impinging on the 
photoelectric detector has been small with a result that a sufficient 
photoelectric output has not been obtained. On the other hand, when a 
photographic dry-plate has been used, the expanse of the image by the 
thickness of the senitive layer has been large to reduce the detection 
accuracy. 
Further, the prior art device has suffered from a disadvantage that the 
angle of incidence .beta.1 of the diffracted light onto the image plane 4 
is a grazing incidence of 80.degree. or more and the length of the image 
plane is very great. That is, the entrance slit has been disposed on the 
Rowland circle 1 (a circle whose diameter is a principal radius of 
curvature R) so as to satisfy r/R cos .alpha.=1, where R is the principal 
radius of curvature of the concave diffraction grating 3, r is the 
distance between the entrance slit 2 and the center of the diffraction 
grating, and .alpha. is the angle of incidence of the principal ray onto 
the diffraction grating (see FIG. 1). 
SUMMARY OF THE INVENTION 
The present invention has for its object to eliminate the above-noted 
disadvantages peculiar to the prior art and to provide a grazing incidence 
spectrometer which is capable of measuring a plurality of types of 
wavelength information at a time with the image plane as being planar and 
which can obtain a good imaging property in a wide wavelength range. 
Such object is achieved by using a diffraction grating having an 
unequal-interval groove pattern that will make the image plane 
substantially planar and whose principal radius of curvature is R and by 
disposing the entrance slit within the Rowland circle so as to satisfy 
0.7.ltoreq.r/R cos .alpha..ltoreq.0.9. More specifically, it has been 
empirically confirmed that where a curved diffraction grating having the 
unequal-interval grooves is used, if the entrance slit is placed in the 
Rowland circle, the angle of incidence of the diffracted light onto the 
image plane becomes smaller. However, if the entrance slit is placed in 
the Rowland circle without limit, the imaging property is aggravated and 
becomes unsuitable for measurement. Therefore, by disposing the entrance 
slit so as to satisfy the above-mentioned condition, the imaging property 
can be maintained good and the angle of incidence of the diffracted light 
onto the image plane can be made sufficiently small. 
The invention will become more fully apparent from the following detailed 
description thereof taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention will hereinafter be described with 
reference to FIG. 2. In the Figure, reference numeral 11 designates a 
Rowland circle, reference numeral 12 denotes an entrance slit, reference 
numeral 13 designates a concave (spherical) or toric holographic 
diffraction grating, and reference numeral 14 denotes the image plane made 
planar by the diffraction grating 13. 
The entrance slit 12 is installed within the Rowland circle 11 in the 
following manner. That is, when R, r and .alpha. are the aforementioned 
radius of curvature, distance and angle, respectively, if a point on the 
Rowland circle 11 that is spaced apart a distance R cos .alpha. from the 
center of the diffraction grating 13 is P and a straight line passing 
through the point P and the center of the diffraction grating 13 is l, the 
entrance slit 12 is disposed at a position on the straight line l which 
satisfies 
EQU 0.7.ltoreq.r/R cos .alpha..ltoreq.0.9. 
By so disposing the entrance slit 12 and the diffraction grating 13, the 
image plane 14 is formed within the Rowland circle 11 and moreover is 
planar over a wide wavelength range. Also, the angle of incidence of the 
diffracted light onto the image plane 14 is, for example, 
20.degree.-50.degree. with respect to the diffraction grating of 
conventional grating constant (1/0.6 .mu.m-1/1.0 .mu.m), this being much 
smaller than conventional. 
Specific numerical values will now be shown. 
EXAMPLE 1 
Conditions are selected such that: .alpha.=87.degree., r/R=0.042, that is 
r/R cos .alpha.=0.803, and use has been made of holographic grating of 
effective grating constant 1/0.9 .mu.m that has been prepared on the 
conditions shown below. The conditions on which the diffraction grating 
are, for example, 
rc=R/9.276 
rd=R/18.67 
.lambda.r=0.4880 .mu.m 
.gamma.=-54.96.degree. 
.delta.=-22.31.degree. 
with the positions of two mutually coherent point light sources C and D 
being C(rc,.gamma.) and D(rd, .delta.), respectively, in the polar 
coordinate indication wherein the center of the diffraction grating is the 
origin and the normaal to the origin is the reference and with the 
wavelength of the light being .lambda.r. As a result, the image plane has 
become planar over a wide wavelength range of 50-200 A. Also, the angle of 
incidence .beta..sub.2 onto the image plane is as small as about 31+. 
Next, the intensity distributions of a plurality of wavelengths (50 A, 100 
A, 150 A, 200 A) on the planar image plane 14 are shown in FIGS. 3A-D, 
respectively. In FIG. 3, the abscissa represents the lateral length (unit 
mm) of the image plane 14 and the ordinate represents the intensity of 
light. The wavelength resolution at this time is 30-80 which is very good. 
EXAMPLE 2 
If .alpha.=89.degree. and r/R=0.014, that is r/R cos .alpha.=0.802, 
then the image plane is planar over a wavelength range of 5-50 A for a 
diffraction grating of effective grating constant 1/1.2 .mu.m and the 
wavelength resolution also is good. The angle of incidence .beta..sub.2 
onto the image plane is as small as about 35.degree.. The conditions on 
which the diffraction grating has been prepared at this time are, for 
example, 
rc=R/34.67 
rd=R/91.98 
.lambda.r=0.4880 .mu.m. 
.gamma.=-67.250 
.delta.=-19.670 
Now, an anode array type photoelectric detector or a semiconductor array 
type photoelectric detector is disposed on the image plane made planar in 
the described manner, and the array of this detector is in a 
one-dimensional or two-dimensional pattern. Accordingly, from this 
detector, a plurality of types of wavelength information can be derived in 
parallel or time-serially and therefore, real time treatment of each type 
of information becomes possible. It should be noted here that the angle of 
incidence of the diffracted light onto the image plane 14 is small. That 
is, the fact that the diffracted light is incident on each photodetector 
on the array at a small angle of incidence when the array type 
photoelectric detector is disposed on the image plane 14 contributes to an 
increased quantity of diffracted light incident on each photodetector and 
improved detection accuracy. 
Specific examples of the holographic grating have been shown in the above 
two examples, but since this holographic grating serves to make the image 
plane planar, it may be replaced with a diffraction grating prepared by 
another method of manufacture if the latter has the same action. That is, 
to make the groove patterns of diffraction gratings similar to one 
another, use may be made, for example, of either the ruling method using 
electronic line scanning or laser beam scanning or the mechanical ruling 
method using a ruling engine. Where the mechanical ruling method is 
employed, complicated groove patterns cannot be formed and therefore, the 
groove intervals may be made identical to one another.