XXXX - OPOD

Opposition Effect Streak, Latvia

Imaged by Ivo Dinsbergs from a helicopter during a water bird counting survey around Lake Engure. The helicopter shadow casts onto a reed bed ~90m below. The predominantly vertical reeds produce a vertical opposition effect streak rather than the customary round glow. The effect changes as the aircraft moves (see below) to different terrain.

All images ©Ivo Dinsbergs, shown with permission

Trees are much less elongated shadow casters - except in winter or after a forest fire..

Here the opposition glow is more rounded although there remains some vertical streaking.

..or the opposition effect takes on a different form when the shadow casters are closely spaced and very elongated.

At left is a cross section through a reed bed. The section is in the vertical plane of the sun's rays. The reeds are close enough that - in the plane - they shadow one another with only their tips unshaded.

Lines from the eye - in red - are not parallel (from an eye in the helicopter they will be almost parallel) and over a wide angle they see only the sunlit parts of reeds. This creates the opposition streak. Outside of the cross section the reed shadows are skewed sideways and they no longer quite hide reeds in the line of site behind each other.