What is the Corona / Black-Sun effect?

The Corona effect, also known as the Black-Sun effect, is an image artifact that can occur with CMOS image sensors when one or more pixels are extremely overexposed or heavily saturated.

Under normal conditions, a very bright light source causes the affected pixels to reach their maximum signal level and appear white. With the Black-Sun effect, however, an extremely strong light source can cause the affected pixels to behave differently: instead of remaining white, they can become dark or even completely black.

The effect is primarily associated with the pixel architecture and readout characteristics of CMOS image sensors.

Why does the Black-Sun effect occur?

When a pixel is exposed to an extremely strong light source, it can accumulate significantly more electrons than it can normally hold.

Once the pixel is heavily saturated, the excess charge can affect the pixel's reference level and neighboring circuitry. As a result, the measured signal can decrease instead of remaining at the sensor's saturation level.

In extreme cases, the signal difference can become very small or effectively disappear. The affected area can then appear black, even though it is being exposed to an extremely bright light source.

This is why the effect is sometimes described as the "Black-Sun" effect: an extremely bright light source can produce a dark spot or dark region in the image.

What does the Black-Sun effect look like?

The effect typically appears around an extremely bright, highly concentrated light source.

Instead of seeing a normally saturated white area, the image may show:

• a dark spot at the center of the light source

• a dark or black region surrounded by a bright halo

• a "donut" or ring-like structure around an extremely bright point

• dark areas that become more pronounced as the exposure to the bright source increases

The exact appearance depends on the sensor architecture, exposure settings and characteristics of the light source.

When can the Black-Sun effect occur?

The effect requires very strong illumination and typically occurs when individual pixels are driven far beyond their normal saturation level.

Potential sources include:

• direct sunlight or very bright reflections

• lasers or laser-based illumination

• high-power LEDs

• welding arcs

• other highly concentrated or intense light sources

The effect is generally not caused by ordinary scene overexposure. A normally overexposed pixel simply reaches its maximum digital value and appears white. The Black-Sun effect occurs under much more extreme illumination conditions.

Is the Black-Sun effect a camera defect?

No. The Black-Sun effect is generally considered a characteristic of the image sensor and its pixel architecture, rather than a defect in the camera.

The exact susceptibility and appearance can vary significantly between different sensors and sensor generations.

Can the Black-Sun effect be prevented?

The most effective approach is to prevent individual pixels from being exposed to excessively high light levels.

Depending on the application, this can be achieved by:

• reducing the exposure time

• reducing the sensor gain

• reducing the intensity of the illumination

• using optical attenuation or suitable filters

• avoiding direct imaging of extremely bright light sources

• adjusting the optical setup to prevent highly concentrated light from reaching individual pixels

For applications involving very intense light sources, the sensor's behavior under extreme illumination should be evaluated during the camera selection and system design process.

Does a high dynamic range prevent the Black-Sun effect?

Not necessarily.

Dynamic range describes the sensor's ability to capture a range of signal levels from dark to bright. The Black-Sun effect is related to the behavior of a pixel when it is driven far beyond its normal saturation level.

Therefore, a sensor can have a high specified dynamic range and still exhibit a Black-Sun effect under sufficiently extreme illumination.

For applications involving very bright, concentrated light sources, it is therefore important to consider not only the sensor's dynamic range and full-well capacity, but also its behavior under extreme overexposure.