We discuss the impact of spatial coherence on the defocusing properties of a retinal projection display. The paper demonstrates that there is an optimum degree of spatial coherence that significantly increases the depth of focus as compared with a fully incoherent or a fully coherent illumination. To this end, a theoretical analysis of the effect of light coherence on image contrast is presented and the influence of coherence effects on image quality is discussed. Measurements of both the contrast function (CF) and the text readability when defocusing are presented using a bench model of the display system. The text readability is evaluated using a combination of objective image quality criteria and human experiments. The results reveal that for reading text targets the best depth of focus can be achieved not by fully coherent illumination, but by partially coherent light of a spatial coherence level between about 0.35 and 0.5. Further, it is shown that the optimum level depends slightly on the targeted text size.
This paper discusses the impact of light coherence on the defocusing
properties of a novel retinal projection display. The display is based
on a liquid crystal display (LCD) illuminated by partially coherent
light from a LED which projects the LCD-image directly onto the eye's retina. It is shown that the increase of the coherence level of the
illumination light enhances on the one hand the contrast of a defocused image. On the other hand, however, the perceived image quality is affected by the occurrence of intermediate defocusing peaks as well as by coherence effects, such as edge-ringing.
Measurements of both the modulation transfer function (MTF) and
of the text readability when defocusing are presented using a bench model of the display system. The experimental results reveal
that for text readability the best DOF can be achieved not for fully coherent illumination, but for partially coherent light of a coherence level σ ≈ 0.35-0.5. Further, it is shown that the optimum σ-level depends slightly on the targeted text size.