Smear

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Smear (vertical, white line under the light source). Also seen here: Blooming .

As a smear or smear (engl. Smear "smearing") refers to white stripes with digital cameras in the image that occurs at very bright lights in the image area. If the light source has a sufficient speed, these strips run at an appropriate angle to the vertical. If the light source has no own speed, they are themselves vertical. Based on this "smear angle", conclusions can be drawn about the speed. The reason for this is the type of image evaluation with CCD sensors . The two most common methods are "frame transfer" and "interline transfer".

Smear on frame transfer

With the frame transfer structure, there is another area of ​​the same size below the light-active pixel area that is not exposed. The electrical charges of the individual pixels are quickly transported downwards line by line until the image (i.e. the electrical charges of all individual pixels) is completely within the covered area. From there, the image is read out line by line. When the charges are transported into the covered pixel area, all pixels that “pass” the image of the intense light source are also exposed for a short time. This results in the vertical line.

Smear on interline transfer

In the interline transfer structure , next to each pixel column there is also a column of covered (i.e. not light-active) transport registers (hence also interline ). The charge accumulated in the light-sensitive pixels is pushed into the transport register after exposure and from there it is transported and read out cell by cell. In principle, the transport registers are also sensitive to light, but covered by an opaque material (metal). Light can still get into the transport register due to diffraction and scattering . When the charge packets are pushed past a strongly exposed area of ​​the CCD sensor, additional charge carriers are collected.

Countermeasures

If exposure is prevented during cargo transport, no “smear” will occur. This can e.g. B. can be achieved by a mechanical lock .

This effect does not occur in cameras with a CMOS sensor , since each pixel is read out directly, i.e. the charges are not shifted before reading.

Differentiation from the blooming effect

The smear effect should not be confused with the blooming effect, which also leads to a light, but mostly limited-length stripe. Blooming is based on the outflow of excess charge carriers during the exposure, whereas the cause of smear lies in the reading of the image after the actual exposure (sometimes also in the "emptying" of the memory cells before the exposure).

See also

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