CHEETAH 3G-SDI Camera
November 16, 2017
Page
42
of
53
Rev 1.1
5 Image Sensor Technology
5.1 General Information
A CMOS camera is an electronic device for converting light into an electrical
signal. The 3G-SDI camera contains the latest Sony Pregius CMOS
(Complementary Metal-Oxide Semiconductor) image sensor. The Sony Pregius
image sensor has groundbreaking performance with sensitivity better than
traditional Charge Coupled Device (CCD) image sensors. The sensor has
extremely low dark current and virtually no fixed pattern noise, which has been
the bane of traditional CMOS image sensors.
The Sony CMOS image sensor consists of a two-dimensional array of sensitive
elements called silicon photodiodes, also known as pixels. The photons falling on
the CMOS surface create photoelectrons within the pixels. The number of
photoelectrons is linearly proportional to the light level. Although the number of
electrons collected in each pixel is linearly proportional to the light level and
exposure time, the number of electrons varies with the wavelength of the
incident light.
When the camera reaches the desired exposure time, it shifts the charges from
each pixel photodiode onto a storage register within the pixel, reads out one row
at a time, and then digitizes each pixel to 12 bits. Frame time, or read-out time, is
the time interval required for all the pixels to be read out of the image sensor.
While reading out the image from the storage registers within each pixel, the
camera captures the next image. The exposure ends just as the readout of the
previous frame ends and the next frame begins.
Unlike traditional CCD image sensors, the Sony CMOS image sensor digitizes each
pixel within a row simultaneously. This allows for more settling time, which
lowers the overall noise floor and provides improved sensitivity. The low noise
floor, combined with a reasonably large pixel charge capacity, translates into a
large dynamic range.
The camera allows you to apply up to 48 dB of gain to the image. The first 24 dB
of gain is analog gain and some improvement in noise performance may result.
The camera applies the last 24 dB of gain digitally, which affects both signal and
noise equally.
A set of color filters (red, green, and blue) arranged in a Bayer pattern over the
pixels generates color images. The starting color is Red for SONY Pregius image
sensors. The following figures
show the sensor’s color response (
Figure 32).
