Cheetah Python Cameras User Manual | CoaXPress (CXP) Interface
August 27, 2019
Page
75
of
86
Rev 1.0
Image Sensor Technology
General Information
A CMOS camera is an electronic device for converting light into an electrical signal. The
C5180, C4181, C4180, C5190, C4191, and C4190 Python cameras contain ON
Semiconductor CMOS (Complementary Metal-Oxide Semiconductor) image sensors.
The sensor consists of a two-dimensional array of silicon photodiodes, also known as pixels.
The photons falling on the CMOS surface create photoelectrons within the pixels, and 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 intensity and
exposure time, the number of electrons varies with the wavelength of the incident light.
In general operation, when the desired exposure time is reached, the photo-electrons
collected within each photodiode move onto a storage register within the pixel. The pixels
are then read out one row at a time, processed in the analog domain, and digitized to 10
bits. Frame time, or read-out time, is the time interval required for all the pixels from the
entire imager to read out of the image sensor. While reading out the image from the
storage registers within each pixel, the camera captures the next image overlapping the
exposure of the next image with the readout of the current image. The exposure is timed
to end just as the readout of the previous frame ends and the readout of the next frame
begins.
Unlike traditional CCD image sensors, the 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 dynamic range of 59 dB.
A set of color filters (red, green, and blue) arranged in a Bayer pattern over the pixels
generates color images.
A/D Architecture and Frame Rate Controls
The C5180, C4181, C4180, C5190, C4191, and C4190 image sensors multiplex 80 (C5180,
C5190) and 64 (C4181, C4180, C4191, and C4190) columns respectively into an array of 64
A/D converters. The camera takes care of all the details of re-ordering the lines so they are
sequentially deposited in computer memory. Unlike a CCD sensor where digitization occurs
within one pixel-time, these cameras perform digitization at 1/64
th
the pixel rate (64 A/D
converters), and the digitization has a depth of 10 bits.
The image sensor provides up to 32 Low-Voltage Differential Signaling (LVDS) outputs so
the time to read out one line from the image sensor is far less than the time necessary to
capture the data using the CXP interface. The camera compensates for this mismatch in
data output rate versus data capture rate using two methods: a variable pixel clock for line
readout and the ability to add 1 microsecond of delay (row overhead) time at the end of
each line output by the camera. Slowing down the pixel line clock and adding delay (dead
time) at the end of each line allow the CoaXPress frame grabber to keep pace with the
camera output.
The following figure shows a typical CMOS image sensor architecture.
