A semi-quantitative representation of the distribution of the specularly reflected beam intensity as a function of the glancing angle of the X-ray beam to the sample surface. I0: intensity distribution of the convergent X-ray beam incident onto the sample. Blue and red lines are for cases α0 = 0.96° and α0 = 1.20°, where α0 is the glancing angle which the horizontal X-ray beam from the source to the polychromator crystal makes with the sample surface after being reflected downward by the polychromator crystal. R: the specular reflectivity of a sample (silicon single crystal). I0 × R: the product of I0 and R which approximately represents the intensity distribution I of the X-ray beam specularly reflected by the sample.