Stay sharp and focused in low light

Stay sharp and focused in low light

For installations that require low-light monitoring, several things distinguish great low-light HD cameras. Besides the high-resolution sensor, a good lens and suitable iris type, processor performance, good electronics that amplify the video signal and reduce noise at low-light levels, and reliability make or break low-light HD cameras.

There is no question that high definition (HD) cameras look terrific in the day, but it is completely a different story at night as all cameras require some light to operate, except for thermal imaging ones. This part highlights what to pay attention to when it comes to sensor selection.

As the whole physical security rapidly migrates to HD, the industry is working even harder and faster to develop related cameras. Megapixel or HD cameras indicate that more pixels are crammed onto the same piece of silicon. This offers crystal clear image; however, on the other hand, this also affects light sensitivity.

The eye of a camera lies behind the lens, residing on wafers of silicon comprising an image sensor. 2 types of image sensors are deployed: CCD or CMOS. The key advantage of CCD sensors used to be their sensitivity to light while CMOS design enabled smoother integration with other technologies, scoring a point in its favor. In other words, CCD sensors have been thought to produce better-looking images with less visual noise and distortion in low-light conditions. However, megapixel CMOS advances are being made in such a prompt speed that megapixel CMOS sensors are not overshadowed by megapixel CCD sensors.

The Rise of CMOS
Image sensors boast more pixels, sensitivity, and dynamic range. They deliver real results for effective video surveillance. “We are seeing more demands from larger imager sizes as HD cameras prefer imagers with bigger surfaces for more pixels. There are great numbers of camera manufacturers adapt 1/1.8”, 1/2.5”, 1/2.7”, and 1/2.8” CMOS sensors, and the numbers are increasing,” said Chang.

More than Sensors and Lux Ratings
Camera performance depends a great deal on components. However, in the IP video world, sensor performance does not determine everything, as powerful processors can handle more complex algorithms. Iris type and shutter speed also significantly impacts the sensor's exposure to light and performances of low-light cameras.

The iris is the opening that allows light to pass through the lens and to the camera. It controls the amount of light that strikes the sensor. It is just like human eyes, when it is sunny the iris is small and at night it gets large. The type of iris in the lens should match low-light cameras. There are fixed iris, manual, auto iris lens, and p-iris lens. For example, if the camera is used indoors then a manual iris will work, but outdoors where the light can vary quite a lot, an auto-iris type or P-iris lens might be more suitable. The P-Iris works together with the camera to adjust the picture according to the light present.

How P-iris works
The P-iris is a complex system that includes the motor for controlling the lens iris, microcomputer software and camera gain. In bright sunlight, the standard auto-iris will close as much as possible, but the p-iris limits the smallest size of the iris to avoid blurring. In low light conditions the iris would normally open as much as possible. Since this can reduce the depth of field, the p-iris reduces the size of the iris opening and increases the gain of the camera. The P-iris is only available in some cameras.

Shutter Speed
The shutter also influences the sensor's exposure to light. The shutter determines how long the sensor is exposed to light and is typically expressed in fractions of a second (1/1000s, 1/100s, 1/30s, 1/3s, and more). Many cameras use both iris and shutter speed control to regulate light input. One important issue to bear in mind is slow shutter speeds. In dark scenes, slow shutter speeds can make an image bright but it creates blurring and ghosting of moving objects.

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