Reflections and glare shift the usable viewing angle
Aliases: specular hotspot · sun-path viewing cone · time-varying glare · mirror wash
What it is
The same outdoor board is readable from the pavement in the morning; by afternoon the sun has moved, and people on that same path see a mirror. Glare relocates the usable viewing angle means specular reflection confines “can read” to a narrow cone that travels with the sun. Outside the cone the content is not a bit worse—it is replaced by an image of the environment. This is not overall contrast flattened by sunlight, which is mush even head-on, and it is not a coating fix. A public screen's audience arrives on a fixed path while the sun moves, so who the panel is for changes through the day.
Why it happens
Glossy glass sends the sun or bright sky into a particular set of eyes along angle of incidence equals angle of reflection. Viewing positions for a public install are pinned by the sidewalk, the platform line, the queue; people cannot sidestep the way a desktop user can. As solar azimuth changes, the hotspot sweeps that path: a stance still inside the cone at 9 a.m. faces a mirror at 2 p.m. Passers-by do not detour into the cone and then start reading; they treat the washed face as off. Glare here reallocates who can see, rather than lowering everyone's contrast by the same amount. Content cannot cancel a physical mirror, but it can keep unloadable fields out of the corner that sits in the hotspot all day, or use motion that still pops in peripheral vision to say the screen is on. Measuring one head-on angle once a day misses the hours when the path is already blind.
Studying it
Log solar azimuth, viewing azimuth, and readability together on site, not only luminance at screen centre. Take several stances along the real walkway at morning, noon, and afternoon, and mark readable / local hotspot / full-face mirror.
Independent variables: solar azimuth and elevation, viewing azimuth relative to the screen normal, which region of the frame holds the content. Dependent variables: whether each stance can read the critical fields, whether the hotspot lands on the main path, share who can read only after detouring.
A lab that lets participants find a comfortable angle hides the rigidity of a public path. Season and facing must be in the plan: winter and summer sun heights shift the blind hours on the same elevation. “Someone can read it” is not a successful install—the question is whether the main traffic stance can read it during hotspot hours.
Where it stops holding
A fully overcast day has almost no hotspot you can step out of; the problem falls back to overall contrast. A lobby vitrine facing one fixed window has a hotspot that barely moves by the hour; design for one worst angle. In a gallery where the audience can walk anywhere, people step out of the mirror; a departure board on a platform does not. At night the sources become streetlights or headlamps—the geometry remains, the path is no longer the sun. If eaves or side fins already keep direct sun off the face, the “cone that travels with daylight” weakens, leaving secondary bounce from pavement or opposite glass.
Applying it
- During siting, draw morning, noon, and afternoon reflection geometry along the main walk and mark stances the hotspot will sweep; do not ship a single head-on beauty frame.
- Move unloadable fields (service, direction, price) off the corner that sits in the hotspot all day; put attracting motion or highlight where the path can still see it.
- Do not treat “please step closer and to the side” as a remedy—the public path does not grant that step.
- Verify in the worst solar hour for that facing, standing where traffic actually stops; if the critical line is readable only after leaving the yellow line or stepping into planting, count that hour as unreadable for the primary user.