Time Varying Lights

Changing Lights

All is not what it seems in this composite of images from a new book by Dan Bennett (site) “A Field Guide To Time-Varying Light Sources".

It explores human and natural fluctuating lights. It gives simple techniques for seeing with the unaided eye and photographing the oddities of automobile tail lights, restaurant signs and the unexpected behaviour of LED indicators and screens.

The convoluted reds are a single line of neon tubes above a restaurant. The camera zoomed and rotated during the exposure to expose the intricate time variation. Each stripe is a half cycle of the alternating current driving the tube. The discharge through the neon gas requires a certain voltage to start. The inset shows one or more separate sharp lines where the discharge strikes and then dims again. The later, more continuous, light shows up as quite complex. The 'grains' are pulses travelling along the tube. Their travel is indicated by the diagonal pattern (blur your eyes) made as the camera swept perpendicular to the tube. The pulses change direction with each half cycle. The humble restaurant sign shows surprising richness.

Audi A8 Left and Right Tail Lights

They are LED clusters. The camera was swept vertically during the exposure to reveal the light variations.

The LEDs are dimmed for normal tail light display by interrupting their power. The outer lights are powered separately because their interruptions are not synchronised with the inner lights.

The inset shows the brakes applied. The outer lights are now continuously DC powered while the inner ones are interrupted but on most of the time. Why does Audi have two separate circuits? No doubt there is a good reason.

The thin lines show – left to right – Sirius, Venus and Arcturus. The camera, on 1.6 second exposures, was jiggled to reveal the twinkling. Arcturus was closer to the horizon and varied most strongly. Venus, a disk rather than point source, has its twinkles smoothed and damped.

Moving pockets of different air density refract the starlight. When strong, the refraction generates caustic sheets that sweep across the eye or camera. The colours arise because shorter wavelength blues refract more strongly than longer wavelengths yellows and reds.