Ruled Grating vs Holographic Grating
Ruled diffraction gratings by the nature of the manufacturing process cannot be produced without defects, which may include periodic errors, spacing errors and surface irregularities. All of these contribute to increased stray light and ghosting (false spectral lines caused by periodic errors).
The optical technique used to manufacture holographic diffraction gratings does not produce periodic errors, spacing errors or surface irregularities. This means that holographic gratings have significantly reduced stray light (typically 10x lower stray light compared to ruled gratings) and no ghosts.
Historically, ruled diffraction gratings offered higher efficiency than holographic diffraction gratings, but with the introduction of blazed holographic diffraction gratings this is no longer the case.
In addition, concave gratings present specific problems for ruled gratings compared to holographic gratings. Ruled concave gratings cannot be utilized for flat field imaging applications as the projected groove pattern of the grating always results in straight, equidistant lines and therefore additional optics are required to correct for aberration. Holographic concave gratings, however, can be designed and produced with curved grooves that produce aberration corrected images. Holographic concave gratings can also be produced with lower f-numbers than ruled concave gratings.
For nearly all applications a blazed holographic diffraction grating will offer significantly better overall performance when compared to a ruled diffraction grating. A ruled diffraction grating should only be used where groove density or spectral range requirements preclude the use of a blazed holographic diffraction grating.