Sensor glow
The sensor glow is an (undesirable) effect on long exposures in digital photography .
Image sensors are more sensitive to a broader color spectrum than the human eye is; so also in infrared light. Thermal radiation, which the camera emits through its electrical components, is recorded and displayed by the sensor, although the radiation source is located behind the sensor or to the side of it. This lightening appears as a purple-colored spot, often as a segment of a circle and, depending on the camera model, also in several places on the edge of the image, depending on the type and number of heat sources inside the camera.
An infrared filter is installed on the front of the sensor, but it only dampens the infrared light from the direction of the lens; it has no effect on the thermal radiation inside the camera.
Because this near infrared radiation hits the pixels in the affected area equally, it should normally be "white". However, since the color filters in front of the individual subpixels of a Bayer sensor allow different amounts of light to pass through in the respective color range and the infrared filter in front of the sensor often "swallows" part of the red spectrum, the white balance of the camera multiplies the signals of the individual color channels with different factors and thus turns the "white" into a purple hue (the RAW image would look more greenish with Bayer sensors without white balance).
The glow of the sensor is only visible in the image in seconds or minutes with longer exposures, since the thermal radiation from the camera is normally very low. Because the camera electronics are active for a long time with long exposures, the effects accumulate: The camera gradually heats up and the longer the exposure, the more visible the thermal radiation becomes. This problem is circumvented in special cameras, e.g. B. in astrophotography with passive cooling of the back of the sensor by cooling plates or by active cooling with Peltier elements , water cooling or even the use of refrigerants .
In digital astrophotography, sensor glow is a well-known phenomenon that is counteracted by subsequent correction ("dark image print").