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10-2 READOUT METHOD OF CMOS IMAGE SENSOR
The CMOS camera adopts a rolling shutter readout for high speed imaging. In the camera, the odd and
even lines are read out in pairs, therefore the exposure and readout timing is slightly delayed as the
readout moves down the sensor (Figure 9-2). The affect of the rolling shutter timing is very small for most
exposure times.
Rolling shutter
10-3 PRECAUTION WHEN USING CMOS IMAGE SENSOR
This camera uses scientific CMOS image sensor. Careful attention must be paid to the following points
when using CMOS image sensor.
(1) White spot
CMOS image sensor has some high dark current pixels caused by the defect of silicon wafer.
Those high dark current pixels appear as higher intensity and brighter pixels than around pixels
when the exposure time is set long. Those pixels are called as "White spot" ("hot pixel").
This camera has real time defect pixel correction function which can replace the defect pixels
registered in advance with the data of surrounding pixels.
Hitting of cosmic ray or radiation ray (X-ray, gamma ray, UV light, etc.) on the sensor generates
many electrons and they may appear as a white spot, but this white spot is temporary and
disappear in the next frame.
In addition, although the probability is very low, the impact of cosmic rays and radiation (X-rays,
gamma rays, ultraviolet rays, etc.) is large, and it may cause permanent defects in silicon wafers
and defective pixels with large dark current. In current technology, there is no way to avoid
generating high dark current defect pixels. It means there is a possibility to generate new white
spots after the factory shipment.
Even if the white spot occurs, dark offset subtraction* with software can reduce the effect of white
spots because intensities of white spots are proportional to the exposure time and have
reproducibility with a constant sensor temperature.
*
Dark subtraction: After acquiring an image using a certain exposure time is loaded, CMOS image sensor is exposed to darkness for
the same amount of time, and another image is obtained. After this, the difference between the images is determined,
and the data for the dark portion of the original image is nullified.
(2) Folding distortion
A rough-edged flicker may be visible when imaging striped patterns, lines, and similar subject
matter.
Digital CMOS Camera C16240-20UP / C16240-20UP01 Instruction manual_Ver.1.3
Exposure length is same for all lines
Exposure timing is different for every two lines
Figure 10-2 Readout timing of Rolling shutter
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