Subject: 8i.10  Polaroid Glasses
From: Jobst Brandt <jbrandt@sonic.net>
Date: Tue, 2 May 2006 10:25:45 -0700

Polaroid sun glasses present a problem for bicycling because they
block road glare that is important in assessing traction.  Polaroid
glasses are horizontally polarized to block reflected glare from roads
(horizontal surfaces) a feature that makes distinguishing wet from dry
pavement difficult.  Granularity of dry pavement reflects unpolarized
light that is unaffected by these glasses, while wet pavement reflects
horizontally polarized light that is blocked.  Therefore, dry pavement
can appear more reflective (slick) than wet pavement, while wet
pavement may have essentially no glare depending on how it is
illuminated by incident sun or skylight.

The inability to distinguish wet from dry is especially difficult when
riding on partially shaded roads.  A shaded road is illuminated by
skylight rather than direct sunlight, but the sky itself is usually
polarized.  Polarization of the sky or road can be observed by
rotating Polaroid glasses while looking through one lens.  An
additional annoyance is that LCD displays are generally blank when
viewed through a polarizer.  When sun glasses are needed, for UV
protection for instance, non-polarized ones work best.