Why do bright LED points appear to spread light into the dark areas between them?
Asked 5/26/2017
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In a photo of individual LED chips, the LEDs themselves are clearly visible, but the spaces between them are not completely dark and seem to glow. What causes that light spreading between bright points? Is it flare, focus, or lens aberrations such as coma?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
7
The reason the background is not dark is because there is light shining on it. The same light sources that you are attempting to record directly are also illuminating the areas around the light and those areas are reflecting some of that light to the camera. Additionally, The highlights of the lights themselves are so overexposed that they appear almost pure white rather than the specific color they are emitting. With the highlights white or near white the less intense reflected light will be more color accurate and appear to be more colorful.
There may be some flare in the upper left example, but it is not a veiling type of flare. Veiling is generally caused by off axis light just outside the field of view that reduces contrast over a very large area. The upper left example also seems to demonstrate some blooming. Blooming is not a lens issue, but rather a sensor issue caused by more light than each pixel well can hold falling on parts of the sensor an 'spilling over' into adjacent pixel wells.
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
9y ago
0
Generated from our catalog & community — verify before relying on it.
It’s mostly not simple “out of focus.” Bright point sources often spread beyond a perfect point because real lenses and sensors are not ideal.
From the answers, the main causes are:
- Reflected light from the scene: the LEDs illuminate nearby surfaces, and those surfaces reflect light back to the camera, so the gaps may not truly be black.
- Lens scatter / flare: a small amount of light is scattered inside the lens, lowering local contrast around very bright points.
- Optical aberrations: especially with point light sources, aberrations such as coma can keep rays from focusing to one exact point.
- Diffraction: light passing near the aperture edges can also spread slightly.
- Overexposure / blooming appearance: the LED cores may be so bright that they clip to white, making the surrounding lower-intensity light more visible.
So the effect is generally a combination of scene reflections plus imperfect lens rendering of bright point sources, with possible minor flare. It is usually not veiling flare in the strict sense, since veiling flare more often reduces contrast over a much larger area, often from strong off-axis light.
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