Grazing incidence spectrometer

In a grazing incidence spectrometer provided with an entrance slit and a diffraction grating for diffracting the light rays incident thereon from said entrance slit and for imaging said light rays as a plurality of spectral lines, the improvement residing in that

• said 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,
• said entrance slit is disposed on a line segment l at a position satisfying a relation:
• 0.7.ltoreq.r/R cos .alpha..ltoreq.0.9,
• where R is said principal radius, .alpha. is angle of incidence of the principal ray incident from said entrance slit onto said curved diffraction grating, l is the line segment connecting between the center of said curved diffraction grating and a point on a Rowland circle of the curved diffraction grating, said point being distant from the center of said curved diffraction grating by R cos .alpha., and r is a distance on the line segment l between said entrance slit and the center of said curved diffraction grating,
• the ray incident through said entrance slit being diffracted by said curved diffraction grating and imaging on said image plane within said Rowland circle.