- Spectral efficiencies in the wavelength range from 1.5 to 14 mum of nonspecular orders reflected by grating-on-silicon structures with periods comparable to the wavelengths have been measured. The measurements were carried out at varying incident and scattered angles while keeping their sum fixed. The spectra consist of broadened peaks at wavelengths close to those predicted by the grating equation. A mechanism of the broadening due to finite incident beam width has been discussed. Simulations using rigorous diffraction theory and wavelet analysis have been performed. The results obtained agree well with the experimental data.