The Marvel of Vision
If eyes are said to be the windows to the soul, then IP cameras with their lenses and CMOS sensors serve as a gateway to computer vision. Much like our eyes, camera sensors transform light into electrical signals that computers can interpret.
The Evolution of Video Sensors in IP Cameras
Before the advent of solid-state sensors, vacuum tubes were the primary technology. Early analog video cameras employed a type of electronic phototube known as Vidicon or Plumbicon for image sensing. These tubes utilized a light-sensitive cathode ray tube.
The cathode gun in the tube emitted electrons that were directed by vertical and horizontal plates. This electron beam scanned the photoconductive layer of the tube. The lens projected the image onto the Vidicon’s screen surface, and an electronic amplifier detected the resulting variations in current.
The earliest video systems scanned the surface line by line to create images with 525 TV lines (NTSC standard in the US). Scanning was done either in an interlaced or progressive manner. Interlaced scanning involved two passes over the surface, each covering half the lines, with the two fields combined to form a single frame. Progressive scanning covered all lines in one pass.
Resolution back then was measured by the number of discernible lines on a test chart.
How IP Camera Sensors Operate
An IP camera is a digital device that connects to a network. It comprises an optical sensor, lens, video processing circuits, and a network interface. The sensor is a crucial component of the camera. An IP camera system also includes a video recording system, video management software, network switches, and computer displays.
Modern CMOS solid-state sensors offer significantly higher resolution compared to older analog sensors. Resolution is now measured by the number of pixels on the sensor, with some solid-state sensors exceeding 20 megapixels.
The Role of the Photodiode
A photodiode is a key component in the camera sensor, converting light into an electrical signal. A diode is a device that restricts current flow primarily in one direction. Photodiodes are specialized diodes with a window that allows light to enter.
The amount of light affects the current flowing through the photodiode, which is then amplified and converted into a digital signal by an analog-to-digital (AD) converter. Multiple photodiodes are used in light-sensing image semiconductors.
CMOS Switch Circuit
The Complementary Metal-Oxide-Semiconductor (CMOS) technology employs complementary and symmetrical pairs of p-type and n-type MOSFETs for logic functions. CMOS technology is used in microprocessors, memory chips, and various digital logic circuits. It is also utilized in analog circuits like image sensors (CMOS sensors).
A major advantage of CMOS technology is its lower power consumption compared to technologies that use only p-channel or n-channel (nMOS) circuits. CMOS switching circuits are integral to semiconductor sensors, with CMOS sensors incorporating numerous such circuits.
How Semiconductor Sensors Convert Light to Digital Images
There are two primary types of semiconductor sensors: charge-coupled devices (CCD) and complementary metal-oxide-semiconductor (CMOS) sensors. CCD sensors were among the first solid-state chips used in cameras. However, CMOS sensors gained popularity due to their lower power consumption.
CMOS technology, invented in 1963 by Frank Wanlass, became widely used for imaging applications in the 1990s. Advances in CMOS design, including smaller pixel sizes, reduced noise, and improved image processing algorithms, contributed to this shift.
In a CMOS sensor, the charge from each photosensitive pixel is converted to a voltage. This signal is multiplexed by row and column to multiple on-chip digital-to-analog converters (DACs). Each pixel consists of a photodiode and three transistors that handle resetting or activating the pixel, amplification and charge conversion, and selection or multiplexing.
Creating Color Images
Digital cameras use filters to separate the light spectrum and produce colored video. Typically, green, red, and blue filters are employed to capture all colors. There are two methods for filtering light: using three sensors each with its own filter, or using a single sensor with three small filters for each photodiode.
IP cameras utilize three tiny filters for each pixel location, requiring three photodiodes to define a single effective pixel. Some sensors offer over 20 megapixels, providing 4K resolution.
Image Signal Processing
Image processing begins in the amplifier of the CMOS sensor. Modern CMOS devices have improved their performance in low light and noise reduction. However, they still depend on the image signal processor (ISP) and network interface to deliver optimal digital video to the network.
The ISP, sometimes referred to as the digital signal processor (DSP), enhances, compresses, and processes video for display on a computer monitor. Major camera manufacturers have developed proprietary ISPs, each boasting superior performance and features.
The ISP provides functions such as noise reduction, low-light enhancement, wide dynamic range (WDR), video compression (H.265), digital image stabilization, and defogging. Advanced ISPs offer intelligent analytics, including motion detection, image recognition, object removal, and object detection, which trigger alarms in IP camera systems.