Cheetah Python Camera with USB3 Interface
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User Manual
December 5, 2017
Page
76
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78
Rev 1.0
6 Image Sensor Technology
6.1 General Information
A CMOS camera is an electronic device for converting light into an electrical signal. The
C5180, C4181, and C4180 Python cameras contain ON Semiconductor CMOS
(Complementary Metal-Oxide Semiconductor) image sensors.
The sensor consists of a two-dimensional array of silicon photodiodes. 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 are moved 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 59dB.
A set of color filters (red, green, and blue) arranged in a Bayer pattern over the pixels
generates color images.
6.1.1 A/D Architecture and Frame Rate Controls
The C5180, C4181, and C4180 image sensors multiplex 80 (C5180) and 64 (C4181 and
C4180) 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 1/64th 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 USB3 interface. The camera compensates for this mismatch in
data output rate versus data capture rate by decreasing the pixel clock rate to match the
computer’s capture rate.
The following figure shows a typical CMOS image sensor architecture.
