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Pixel Pitch

The issue of resolution and FoV is insidiously complicated because so many variables are involved. One discussion that gets attention is pixel pitch.

Pixel pitch (sometimes called dot pitch, line pitch, stripe pitch, or phosphor pitch) directly correlates your display resolution and optimal viewing distance. The smaller the pixel pitch number, the more pixels used to make up the image on your

When the display is close to the viewer’s eyes the minute distances between the pixels becomes visible crating what is known as the “Screen-Door” effect or Fixed Pattern Noise

Fig. 8.9 When the display is close to the viewer’s eyes the minute distances between the pixels becomes visible crating what is known as the “Screen-Door” effect or Fixed Pattern Noise (Image source Wikipedia)

display, thus improving the resolution and optimal viewing distance. Dot pitch describes the distance between dots (sub-pixels) on a display screen. In the case of an RGB color display, the derived unit of pixel pitch is a measure of the size of a triad (RGB dots) plus the distance between triads. Dot or pixel pitch can be measured horizontally, vertically, or diagonally, and can vary depending upon the aspect ratio of the display.

The screen-door effect (SDE) or fixed-pattern noise (FPN) is a visual artifact of displays, where the distance between pixels (or subpixels) become visible in the displayed image to the viewer, usually as a black line or border (Fig. 8.9).

The screen-door effect can be seen in digital projector images and regular displays under magnification or at close range. Increasing display resolution reduces it; however, with displays very close to the eye as in a virtual reality headset the screen door effect has been an issue because the display is a single display across the viewer’s entire field of view. In augmented reality systems, the display occupies a smaller portion of the viewer’s total field of view so the screen-door effect is not as noticeable.

The pixels per inch (PPI) are seldom published by the augmented reality headset manufactures.

Smartphones, for example, with a 5.5-inch screen and 1080 x 1920 resolution have a PPI of 400, and at that resolution, near your eyes, the screen door effect will be noticeable. The PPI should be greater than 500, if close to the eye. A HUD for example is far enough away it can use a smaller resolution. However, as the PPI is increased, within a fixed display space, the FoV will be reduced.

The display area in worn augmented reality devices (glasses and helmets) is typically to the right of the right eye, and either slightly above or below the center of vision. Factory-installed head-up displays in automobiles are typical at the bottom of the windscreen, and in an airplane either near the top or at the bottom of the windscreen. After-market head-up displays can be almost anywhere. Some augmented reality glasses put information on the far edge of both lenses. Advocates and critics argue about the location, the distraction to focus (and taking your eyes off the main object); there is no right answer, and in most cases, it’s a matter of learning to accommodate the augmented reality device. That accommodation is one of the limiting factors in consumer acceptance of augmented reality.

 
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