A study of image contrast restriction in displays using diffractive spatial light modulators

The peculiarities of 1D projection displays using spatial light modulators (SLM1) are considered. The conditions for high contrast images in SLM are investigated. It is shown that an amplitude imbalance between the light reflected from the passive and active parts of SLM gratings imposes a limitation on the achievable contrast ratio. This imbalance may come from errors in SLM structure sizes and from diffraction effects inside SLM layers. The key point is the relationship between ridge and hole sizes in SLM diffraction structures. It is shown that odd diffraction orders (especially 1st order) have a significantly decreased sensitivity to ridge/hole tolerances. This phenomenon affects the SLM’s contrast ratio. The optimal ridge/hole ratio for different SLM diffraction orders has been found. To test the theory, experimental samples were manufactured and contrast ratio measurements were carried out. The experimental data are in good agreement with the theoretical results.