How Camera Sensors Work?
CCD image sensors contain hundreds of thousands (or millions, in the case of megapixel cameras) of individual picture elements, called pixels. Each pixel contains a light sensitive element and one capacitor. The capacitor stores a charge that is proportional to the amount of light incident on the pixel’s surface, which is then transferred to a circuit that converts the charge to a voltage and digitizes it.
A CMOS sensor is constructed from a similar array of pixels, but does not use a capacitor to store the charge for each pixel. The rows of pixels are activated sequentially, and the amount of light incident on the pixel’s surface is converted to a voltage and read directly, at the time of exposure.
Of particular concern for video surveillance is picture quality across a broad spectrum of lighting conditions. In this aspect, CMOS has some significant weaknesses compared to CCD, as CMOS technology tends to have inferior low light ability, does not compensate well in strong back-lighting conditions and is prone to excessive noise and shadows in low light conditions.
To deal with these problems, a newer technology, called ‘Wide Dynamic Range’ (WDR), has evolved and shows great promise. A WDR camera scans the same frame twice, once by a slow sensor, and then again by a high speed sensor. The two frames are then processed, pixel by pixel, and result in a single frame output. WDR technology compensates for bright background and low light conditions, producing clear, low noise images with good contrast.
To date, WDR technology has not been applied to any IP high definition megapixel cameras.
Another area of concern to video surveillance applications are video distortions known as motion artifacts. Again, CCD performs better than CMOS in high motion conditions due to the different type of shuttering used.
Shuttering refers to the manner in which a video camera presents light to the sensor. A CCD sensor uses a “global shutter”, which means that the entire sensor is enabled at once, taking a snapshot of the entire frame. Each pixel’s output is stored in its capacitor and is read by the circuitry prior to taking a snapshot of the next frame.
A CMOS sensor uses a “rolling shutter”. Due to the lack of charge storage in a CMOS sensor, the information from each pixel is read sequentially in small strips of pixels, starting from the top and moving down to the bottom of the array exposing only a portion of the array at a time, hence the name, “rolling shutter”.
Because a CMOS sensor reads different portions of a frame at different times, motion within the frame while it is being captured creates such artifacts as skew, wobble, and in some cases, partial exposure.
You may ask, what does all of this have to do with IP cameras vs. analog cameras? The point is that the two types of cameras are more similar in image capture technology than they are different. However, when it comes to the method of video transmission the differences are significant.
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