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Elusive rainbows caught on camera

Photonics Spectra
Dec 2011

Diligent camera work and a new meteorological model have captured what may be the first images of ephemeral triple and quadruple rainbows. These natural optical events are difficult to observe, partly because they are dim and appear in the sunward part of the sky.

But new math-based guidelines have given rainbow chasers leads on the weather conditions that bring about triple and even higher-order wonders: First, there must be dark thunderclouds and either a heavy downpour or a rainstorm with droplets all of about the same size. If the sun breaks through the clouds, it could project a triple rainbow against the dark clouds nearby, and the contrasting colors would make the dim triple bow visible. These meteorological signposts come from research conducted by Raymond Lee at the US Naval Academy with his colleague Philip Laven.

Researchers say that this is the first picture ever of a natural quadruple rainbow (right) and the second picture ever of a triple rainbow (left). The bows appear on the sunward side of the sky, at approximately 40° and 45° from the sun, respectively. Courtesy of Michael Theusner/Applied Optics.

Armed with Lee and Laven’s tips, two amateur astronomers in Germany have photographed higher-order bows: One caught a triple rainbow, and one caught the even more elusive quadruple rainbow.

The first was taken May 15, 2011, in Kämpfelbach, Germany, when Michael Grossmann used a rather ordinary digital camera and some image processing to produce the first known photograph of a triple rainbow. The images were captured in RAW mode to prevent compression artifacts. Upon first inspection, Grossman and his colleagues noted that none of the images showed the triple rainbow – but unsharp masking and contrast enhancement made a colored bowlike pattern with a red rim on the outside clearly visible in one picture.

A month later, on June 11 in Schiffdorf, Germany, fellow amateur astronomer Michael Theusner captured the first confirmed photograph of a quadruple rainbow in nature. He concluded that a good strategy for detecting the optical event is collecting several images in short succession to enable “stacking” or superimposition of the images, which improves the signal-to-background ratio.

(a) A triple rainbow is shown as captured in the original, unprocessed image. Reference positions A and B indicate image orientation. (b) The processed version of the original image shows, after contrast expansion and unsharp masking, a rainbowlike pattern next to the image center, marked by the arrows. Courtesy of Michael Grossmann/Applied Optics.

“It was as exciting as finding a new species,” Lee said. His meteorological model for analyzing the triple rainbow’s visibility involved using a modified geometrical optics paradigm to calculate the rainbow’s chromaticity gamuts, luminance contrasts and color contrasts against a background of dark clouds.

Triple and quadruple rainbows, as with the more commonly found single and double ones, result from the refraction, dispersion and reflection of sunlight inside raindrops. It is in the degree of these processes that they differ. Three series of reflections are needed to create a triple or tertiary rainbow, four to create a quadruple or quaternary rainbow.

The findings were published in a special issue of Applied Optics titled Light and Color in the Open Air.

Americasapplied opticscamerasCaren B. LesEuropehigher-order rainbowsimage contrast enhancementimagingLight and Color in the Open AirLighter Sidemeteorological conditions for rainbowsMichael GrossmanMichael Theusneropticsoptics in naturePhilip Lavenquadruple rainbowsrainbow imagesrainbow photographsrainbow photographyrainbowsstacking imagessuperimpositiontriple rainbowsunsharp maskingweather conditions for rainbows

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