The sylvanshine: retroreflection from dew-covered trees

Some dew-covered plants are strongly retroreflective. The bright glow seen when the antisolar point falls on grass is known as the heiligenschein. Its widely accepted explanation requires that the grass be covered with hair. The discovery of the sylvanshine, a closely related phenomenon best seen at night, revealed that strong retroreflection can occur on hairless dew-covered plants. A simple model shows that below a contact angle of 90°, the enhancement in the backscatter direction is virtually identical to that given by a diffusely reflecting surface, but as the contact angle climbs to 140°, enhancement increases by 2 orders of magnitude. Plants that exhibit such large contact angles are not the norm, but can be found chiefly among coniferous trees, a few of which display the sylvanshine with great brilliance.