As one of important technical specifications of diffraction gratings

stray light directly affects SNR; especially it is harmful to spectrum analysis in UV spectrum. In order to search stray light arising from applications of ruled gratings in the spectrometer

basing on classical Fresnel-Kirchhoff diffraction equation

we deduce equations of relative intensity of stray light and analyze forming causes of stray light in theory. The numerical simulation indicates that random errors in the spacing of the grating grooves and random errors in the depth of the grooves for UV ruled gratings are the main source of stray light; moreover

small-scale random surface roughness affects stray light slightly. Otherwise

we establish the concept of relative width of exit slit and find the relation between height and relative width of exit slit and intensity of stray light. This analysis method

as a theoretical reference

is used to reduce the stray light in the process of fabricating grating essentially and can be applied to reduce the stray light in practice. Finally

in order to compare theory results with experimental results measured by filter method

we present equation which solves the total stray light and measure stray light of four wavelengths by U.V.-visible spectrophotometer. It approves that the maximal relative error between theory results and experimental results is about 13%.