In photography, an orb is a typically circular artifact on an image, created as a result of flash photography illuminating a mote of dust or other particle. Orbs are especially common with modern compact and ultra-compact digital cameras.
Orbs are captured during low-light instances where the camera's flash is used. Cases include night or underwater photography, or where a bright light source is near the camera. Light appears much brighter very near the source due to the Inverse-square_law, which says light intensity is inversely proportional to the square of the distance from the source.
The orb artifact can result from retroreflection of light off solid particles, such as dust or pollen, or liquid particles, especially rain. They can also be caused by foreign material within the camera lens.
The image artifacts usually appear as either white or semi-transparent circles, though may also occur with whole or partial color spectra, purple fringing or other chromatic aberration. With rain droplets, an image may capture light passing through the droplet creating a small rainbow effect. In underwater conditions, particles such as sand or small sea life close to the lens, invisible to the diver, reflect light from the flash causing the orb artifact in the image. A strobe flash, which distances the flash from the lens, eliminates the artifacts.The effect is also seen on infrared video cameras, where superbright infrared LEDs illuminate microscopic particles very close to the lens.
The artifacts are especially common with compact or ultra-compact cameras, where the short distance between the lens and the built-in flash decreases the angle of light reflection to the lens, directly illuminating the aspect of the particles facing the lens and increasing the camera's ability to capture the light reflected off normally sub-visible particles.
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