Separate versus integrated visual field display
Interacting with smartwatches — which are worn on the wrist and are therefore out of the main visual field — is essentially a separate, independent experience. The user’s attention needs to actively turn away from the direction he is looking to focus instead on the smartwatch screen (which receives the full attention for the interaction duration). When using smartglasses, however, the display is integrated into the main field of vision. Wherever the person’s attention is and in whichever direction he is looking, the display is right there, overlaid on top of the visual field; it cannot be separated from it. The user’s attention inevitably spans both — an additional UX challenge when designing for these devices. Furthermore, at this point in time (and probably for the next few years), most smartglasses don’t provide a seamless AR integration with the environment; rather, they project a small screen-like display within the field of vision. As a result, this virtual screen covers a portion of the background information. Finding the sweet spot where the overlaid display is integrated effectively in the visual field, visible enough to the user when needed but not in his way, requires careful handling (and continuous testing) in terms of display location, size, shape, colors, opacity, and so on. Currently, different smartglass providers take different approaches along these dimensions. Additional testing with larger populations is required to determine the optimal settings for such wearables.
Level of control on the display
With smartwatches, users can control — at the least — the angle of the screen and how close they are to it; therefore, they can increase legibility and facilitate usage.
With the displays for smartglasses, until they can be digitally manipulated (for example, the ability to zoom in/out), users are constrained to a fixed screen (in terms of size, angle, and location). This further enhances the need, discussed earlier, to keep the design very simple, clean, and focused on the very essence. As you add more visual elements to the display, you will face a more pressing need to use smaller font sizes, add colors/shades, decrease spacing, and so on to accommodate all the elements and establish visual prioritization between them. This in turn increases the cognitive load on the users, cluttering the display and harming legibility.