New sensors are improving the performance of color cameras used in low-light situations, such as in medical operating rooms, outdoors at dusk or dawn, or on conveyor belts and assembly lines. Camera manufacturers such as Imperx offer their customers many benefits by integrating sparse color filter sensor technology into their product lines.

Sparse Color Filter Array Technology

The Bayer color filter array (CFA) is the most popular pattern used in almost every digital imaging system throughout the world. Its 4-color pixel kernel has 2 green, a blue, and a red pixel, so an imaging sensor using this type of CFA receives all of its illumination through color filters. Color filters, however, absorb some amount of light, providing less than 100% transmission.

A sensor without color filters on it will experience higher low-light transmission efficiency. Many low-light applications, such as surveillance, sacrifice using color and switch to a monochrome camera without color filters in order to maximize low light sensitivity. This is especially true where the lighting is not controlled and there is a need for dawn-until-dusk imaging. But this approach is unsuitable for many applications that require high sensitivity and want to distinguish colors. Users performing surveillance may need to know the color of an individual's shirt, a task that is not achievable using a monochrome camera. Users also wish to monitor many situations occurring at great distances where illumination cannot be provided.

A new color filter pattern increases light sensitivity and provides color images. Compared to a standard Bayer color sensor, this new technology increases light sensitivity by approximately two f-stops. Truesense Sparse Color Filter Pattern technology from Truesense Imaging adds panchromatic pixels to the red, green, and blue elements that traditionally form the image sensor array, providing 4 channels of output. A panchromatic pixel is one with a clear micro lens that is sensitive to all wavelengths of visible light. They collect a significantly higher proportion of the photons that enter the pixel than those that are color filtered and offer improved sensitivity when used in light-starved applications.

Image processing algorithms combine both luminance information from the clear pixels and color information from the color filtered pixels to create color images with the same sensitivities as monochrome cameras.

Cameras Take Advantage of the New Sensor Technology

The Truesense Sparse CFA sensor has 5.5 micron-sized pixels with better smear performance and improved responsiveness when compared to prior generations containing 7 and 9 micron pixels. With blooming characteristics reduced by a factor of 300, this new generation of sensors allows Imperx to improve light performance into a wide variety of cameras in their portfolio. Owners of the company's cameras that use a standard Bayer CFA with Gigevision (with Power over Ethernet), Camera Link and Coaxpress cameras who wish to enhance their sensitivity can upgrade easily. With the same form, fit, and function as the previous line of cameras, Imperx cameras all share the same command set and physical dimensions so users can upgrade with a drop-in replacement by selecting the Sparse CFA color sensor.
The new Truesense technology is adaptable to cameras with almost any lens, including zoom iris focus capabilities with optimized gain and iris control.
The commercial off-the-shelf Imperx cameras continue to have features that appeal to users in low light markets, such as:

• Desired performance, range, thermal characteristics and noise properties
• Inputs and outputs that synchronize with other cameras and contain optimized schemes for delays and programmable exposures
• Automatic gain, automatic iris and automatic exposure capabilities.

Combining these features with improved sensors with low-light sensitivity offers system designers and camera end-users many benefits, such as reduced lighting requirements, faster shutter time performance and ultimately reduced costs.

Low-Light Applications Already Seeing BenefitsApplications that benefit from the feature set and the new sparse CFA sensor are panoramic imaging or wide area surveillance, aerial mapping, intelligent transportation, manufacturing, medical imaging and more.
In airborne Imaging or persistent surveillance applications, Sparse CFA sensors don't require as much light, extending the period of time during the day that users can fly. Also, the increased sensitivity of color allows viable images to be obtained using faster shutter speeds. This means users can fly faster while mapping since these sensors do not require as much light.

The same benefits help users in machine vision applications, which take place in controlled indoor environments that are usually well lit. Typically, machine vision cameras measure items moving on a conveyor or assembly line using fixed shutter times, gains, and illumination levels, so they operate under a known set of conditions. Cameras with faster exposures allow users to freeze action at higher rates, producing shorter integration times. With 2-4x better light sensitivity, users can reduce the amount of illumination needed for a measurement, saving the cost of electric power. These cameras can now be used in lower-light applications where there previously wasn't enough light for a camera at all.

In a true light-starved application, such as paper scanning, paper flies by at a very high speed. Typically, monochrome cameras, area rate scanners or line scan matrix cameras are used with a tremendous amount of illumination, generating a lot of heat and the need for extra illumination. The analog gain is typically increased, producing a lot of noise and false positives. Cameras with increased sensitivity allow users to run with lower gain settings, improve the accuracy of the inspection system and eliminate many misreads and false positives.

Cameras using sensors with improved sensitivity also benefit the intelligent transportation industry, which provides automated license plate recognition, monitors transportation systems overlooking a harbor or port as well as other surveillance applications that rely on ambient light outdoors. They allow users to extend their useful monitoring times at dawn and twilight when a normal camera would produce dark images.

Other areas that these sensors benefit are in medical operating room theaters. Some cameras that require more illumination are used during procedures. The improved sensitivity sensors require less illumination, reducing the power load and intensity. They also open up the possibility that other wavelengths can now be used to look for particular markers or cells.

For more info see
Truesense Sparse Color Filter Array
Imperx Cameralink Cameras