Helmet-Mounted Displays: - USAARL - The - U.S. Army

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Clarence E. Rash and William E. McLean
Luminance uniformity
Variation in luminance across a display image can be distracting
(Farrell and Booth, 1984). Luminance uniformity across an image is best
described by its absence or nonuniformity (Snyder, 1980). Three important
types of nonuniformity are: Large area nonuniformity, small area
nonuniformity, and edge discontinuity. Large area nonuniformity is a
gradual change in luminance from one area of a display to another; e.g.,
center to edge or edge to edge. Small area nonuniformity refers to pixel to
pixel luminance changes over a small portion of the image. Edge
discontinuities occur over an extended boundary.
While uniformity requirements are still lacking in the classical
literature, one such guidance is that the luminance at any two points within
a flat field image shall not vary by more than 20% (Rash et al., 1996).
Farrell and Booth (1984) suggest limiting small and large area
nonuniformities to 10% and 50%, respectively. The HIDSS allows a 20%
variation from the mean image luminance, which should be based on
luminance readings of at least 9 or more equally spaced positions within the
image. [In cases where the entire image area is not useable, variation can
be based on only that portion which provides acceptable image quality.]
CRTs provide uniformity on the order of 37% (i.e., the luminance of
any small area can decrease to 63% of center luminance) (Farrell and
Booth, 1984). FP technology displays also should provide reasonably
acceptable uniformity. In EL displays, uniformity will be a function of
quality control on the deposition of the phosphor. Uniformity in LED
displays will depend on the variation in individual LEDs within and across
production lots. LCD uniformity (typically