Divergent light halo

An image by Lars-Gunnar Jarl in Sweden using a wide angle lens.

Diamond dust ice crystals floated in the -11C air. Lamp glints from them generated the tall thin light pillar. The surprise was the fainter roughly parabolic arc on each side of the pillar and passing through the lamp.

This is a ‘superparhelion’, one of a class of ice halos generated by divergent light. The name comes from comparison with the more familiar subparhelion.

Ordinary halos around the sun or moon come from parallel light. Divergent light halos are more exotic. Find them around street lamps when the air is frigid and diamond dust drifts. When the lamp is nearby the halos take on a weirdly 3D aspect.

Image ©Lars-Gunnar Jarl, shown with permission

More divergent light halos

These light pillars photographed in Latvia have trumpet like flares on their tops. They are divergent light halos from singly oriented column crystals. the ray paths are equivalent to the upper tangent arcs of parallel light halos.

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A superparhelion ray path through a horizontal plate crystal. Rays enter and leave through prism side faces inclined 60° to each other. The rays are reflected internally from the upper horizontal hexagonal face.

Divergent light “22° halo” - This is perhaps the easiest divergent light halo to understand. Imagine that you are near a point source lamp. A randomly oriented column crystal at “X” deviates the lamp light through 22° (or to be pedantic, a minimum deviation angle of 22°). The red lines show the ray’s path. The crystal appears as a bright glint. Crystals anywhere on the red ring will glint to form a circular halo.

BUT, its radius is smaller than the 22° of the parallel light halo we see around the sun or moon. The closer is “X” to the lamp, the smaller is the ring.

The red ring is not the only position where crystals will glint light after a 22° deviation. Crystals anywhere on the surface of the pointed airship will do so. Crystals inside the airship do not glint 22° halo light. Crystals outside form the fainter halo extremities. The eyes see individual crystals glinting and the halo airship takes on a 3D character. The halo definitely appears to be closer than the lamp.

The superparhelion halo surface is more complicated!