In the June 9, 1994 issue of Nature, Lindsay Matthews wrote the following short article about some recent work of Dr. Alistair B. Fraser (Department of Meteorology).

Alistair B. Fraser of the Pennsylvania State University evidently has a fatigue-defying appreciation for meteorological curiosities. The glistening tree shown here is a fine example of a phenomenon that he first spotted in the small hours of a summer's morning, whilst driving along a forested road in British Columbia. Fraser describes his experience in an article to appear shortly in a special issue of Applied Optics (vol. 33, 20 July 1994): in his words, he noticed that in the car's headlights "the trees of the normally Stygian forest began to glow as if snow-covered", a startling observation on a warm August night. Evidently some form of retroflection from dew-covered leaves, the effect seemed oddly species-specific, even on a first inspection. Later nocturnal expeditions with a powerful flashlight (a proceeding that aroused dark suspicions in at least one local gamekeeper) showed that it favoured only certain types of conifer and a few shrubs such as the yew and rhododendron.

The explanation lies in the contact angle of droplets on the leaves: as this rises above 90 degrees or so, the proportion of light from the car's head lamps that is reflected back towards the occupant increases, and for angles above about 140 degrees, the retroreflection becomes spectacular. Blue spruces show the glow particularly well, because the tangled rod-like morphology of the epicuticular wax responsible for their blue bloom also increases the contact angle.

Fraser dubs the glow sylvanshine, in deference to the heiligenschein, "familiar to those who wander grassy fields in the early morning." But whereas the sylvanshine on a tree is visible to anyone in the car or holding a torch, heiligenschein, the result of sunlight focused by dewdrops held above a leaf's surface by fine hairs, appears to each observer as a halo around the head of just his own shadow on the grass. In his Memoirs of 1562, Benvenuto Cellini, no expert on the laws of optics, interpreted this instead as a sign of divine grace towards himself; and says Fraser wryly, evidently none of those to whom Cellini vouchsafed his secret dared mention the halo about his own head.

Nature Volume 369, 9 June 1994, Page 441