Hot pixels

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Hot pixels in the night sky at 30 seconds exposure time

Hot pixels are individual pixels in a digital camera that output a significantly brighter image value than should be based on the light received. These pixel errors appear in the image as single, pin-sharp bright points where darkness should be. Since the pixels in the Bayer sensor are monochrome, hot pixels are often red, green or blue in full color. If two neighboring pixels are affected, hot pixels with mixed colors occur.

root cause

Hot pixels are mainly caused by manufacturing inaccuracies: Instead of incident light, an unwanted current flow , the so-called dark current , creates an electrical charge in the affected pixel. The image processor cannot initially tell whether this charge comes from an actual point of light or a dark current.

How far the (low) dark current can charge the pixel - and thus the brightness of the hot pixel - depends on the exposure time of the image, the set ISO value and the temperature of the sensor. Therefore, hot pixels are particularly evident with long exposure times and high sensitivities, as well as with a warm sensor. In simpler cameras, the exposure time is often limited to a few seconds in order to reduce the number of hot pixels. Digital cameras specializing in long exposures (e.g. for astrophotography ) actively cool their image sensors .

Correction in camera production

Every common image sensor already has a proportion of around 1 ⁄ 1000 defective pixels (light or dark) after production . B. 8,000 defective pixels are considered normal. These pixels are individually determined during assembly and saved in the camera. In use, they are automatically interpolated ("masked out") and only appear in the image result as a (very slight) loss of resolution.

Subsequent correction in the camera

Hot pixels that arise after production, for example due to aging or other external influences such as high temperatures or long exposure times , have a particularly disruptive effect on night photos. Some cameras allow the end user to determine current hot pixels and add them to the internal list of defective pixels (with Olympus cameras the menu item is called "Pixel Mapping"), which are not taken into account when developing the sensor image. Usually only customer service can do this.

In some cases, alternative camera software ( firmware ), such as CHDK , can also help . A correction option on the camera side is based on finding and correcting hot pixels based on the high contrast to adjacent pixels.

Another method for hot pixel elimination is the so-called dark image subtraction. In the case of long exposures, another picture is taken with the shutter closed. The same hot pixels are shown on the second image, so that they can easily be calculated from the first.

Subsequent correction in the picture

If removal by the camera is not possible or is no longer possible (because the photo has already been created and saved), hot pixels can also be removed later. Software is offered for this, for example the “HotPixels Eliminator” or “PixelZap”. The function “Remove scratches and dust” can be used in the image processing program Photoshop . The most thorough method is manual retouching, but it takes a long time.

additional

The arrangement of the hot pixels in the image is individual for each image sensor and thus for each camera. It can therefore be used as a camera identification feature in digital image forensics .

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