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

Optical Performance 121
In discrete displays such as FPDs, resolution is given as the number of
horizontal by vertical pixels. These numbers depend on the size of the
display, pixel size, spacing between pixels, and pixel shape (Snyder, 1985).
Typical resolution values are 640 (H) x 480 (V), 1024 (V) x 768 (H), and
1280 (H) x 1024 (V). This expression for pixelated resolution can be
converted into other formats using a number of equations given in Table
5.5 (Task, 1997). Some complications can arise when dealing with color
FPDs. In such displays, a color pixel may consist of several (sub)pixels
(red, green and blue). Depending on the subpixel arrangement, the color
pixel count can be different for the horizontal and vertical directions. In the
example in Figure 5.6 (Task, 1997), where each color pixel consists of
elongated red, green, and blue subpixels positioned in rows of triads, the
color pixel count in the horizontal direction would be one-third of the
(sub)pixel count in that direction, but the color pixel count would be the
same as the (sub)pixel count in the vertical direction.
The pixel output for current FLIR sensors suggest a FP pixel resolution
of greater than 1355 (H) x 960 (V) (Belt et al., 1997). While some research
and development programs are developing miniature FPDs with resolutions
as high as 2560 (H) x 2048 (V) (Girolamo, Rash, and Gilroy, 1997), current
availability appears to be limited to 1280 (H) x 1024 (V).
In any optical imaging system, we want the eye to be the limiting
resolution factor. At an adaptation level of 100 fL, the eye can detect
approximately 1.72 cy/mr (which equates to 20/20 vision). Ideally, the
HMD should match or exceed this value. A more realistic, but still
optimistic, goal for HMD resolution in the central area of vision is 0.91
cy/mr, with values between 0.39 and 0.77 cy/mr being acceptable (Seeman
et al., 1992). Rash et al. (1996) cite monocular vertical and horizontal
resolution specifications for a display background luminance of less than
10 fL as greater than or equal to 0.7 cy/mr (20/50 Snellen equivalent) for
high contrast targets in the center of the monocular FOV and greater than
or equal to 0.57 cy/mr (20/60 Snellen equivalent) at 0.75 distances from the
center to the edge of the FOV.
The resolution (resolving power) of ANVIS and other I 2 devices
usually is expressed in angular units (cy/mr). [For the individual I 2 tubes,
a linear unit of “line pairs per millimeter (lp/mm)” is used to separate the
optical characteristics of the objective and eyepiece lenses from the
resolution of the intensifier tubes themselves. A minimum ANVIS value
is 36 lp/mm.] Optimal I 2 resolution is obtained under high light level
conditions with high contrast targets. Resolution decreases with light level
because of the proportional decrease in luminance output below the