Efficiency in higher orders usually follows the first order efficiency curve. For a grating blazed in first order the maximum efficiency for each of the subsequent higher orders decreases as the order k increases. Holographic gratings may be designed with groove profiles that discriminate against high orders. This may be particularly effective in the VUV using laminar groove profiles created by ion etching. If the diffraction grating has periodic ruling errors, a ghost, which is not scattered light, will be focused in the dispersion plane.
Ghosts are focused and imaged in the dispersion plane of the monochromator.
Stray light of a holographic grating is usually up to a factor of ten times less than that of a classically ruled grating, typically nonfocused, and when present, radiates through 2pi steradians. Holographic gratings show no ghosts because there are no periodic ruling errors and, therefore, often represent the best solution to ghost problems. When laser light is present, e.g., Raman, laser fluorescence, etc. Whenever an ion etched holographic grating is available. Beware of using ruled gratings in high order or with high groove density.