Anyone understand what 'bias frames' are?

I was reading about night photography and someone had brought up 'bias frames'. I picked up that these are created when you take a number of photos with your lens cap on to get a base line of the minimum pixel bias. Honestly this is new for me, so although I read what is happening, I don't understand what it means. Can someone break this down to easier understand please?

I'll give it a try.

When photons of light from the subject fall on the camera sensor, the sensor generates electrons, the sum of which carries the image information. Brighter sources generate more electrons and so on. Theoretically, then, if no light falls on the sensor, no electrons should be generated.

In reality, though, this is not the case. Various fluctuations and irregularities in the circuitry cause random electrons to be generated by the sensor independently of the image source. These random electrons are essentially noise in the image. The amount of this type of noise generated is very small. For typical pictures, where the subject is relatively bright and exposure times short this type of noise is essentially meaningless, being overwhelmed by the amount of information generated by the subject.

In astronomical imaging, however, that is another matter. Astronomers photograph very faint objects with exposure times that can be measured in hours. The sensor noise can combine with image data in a way that interferes with the desired signal. Given that such images are often used to measure brightness of astronomical objects any external noise must be eliminated.

Astronomers make use of bias frames, dark frames and flat frames to determine the noise characteristics of the sensor being used. The noise can then be subtracted from the captured image data.

Bias frames are essentially images taken with almost zero exposure time. They are intended to determine the unevenness of the sensor. In the same way that a white sheet of paper is not perfectly evenly white across its surface a sensor with no light falling on it does not read a perfectly even values across its surface. The data from bias frames can be used to cancel out this unevenness in images.

Dark frames and flat frames are similarly used to cancel out random noise generated by hot pixels and similar and by distortions introduced by other sources, e.g. vignetting from the telescope tube.

In scientific and high quality astronomical images the use of these calibration frames is essential. For ordinary photography I don’t see any point. Even in regular night photography the subjects are relatively bright compared with faint astronomical objects. We are usually photographing lit up objects in the night, not the darkness itself. I’ve never heard of anyone worrying about this for regular photography.