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 Light that reflects off objects is collected through a lens onto an image sensor. The image sensor converts the incoming light to digital values forming an image. The amount of light hitting the sensor determines the quality of the image. If the incoming light drops too low, the sensor will fail to register an image at all. The light sensitivity of a sensor is often expressed as a Lux value. Lux is a unit of the amount of light falling on a given area. Manufacturers of video cameras often state a lux value as the bare operating minimum for their cameras. However, the advertised lux performance should not be the only point considered when evaluating a camera system. Many other factors come into play during low light conditions. When light drops low, the camera needs to amplify the weak signal from the sensor. This amplification is called gain. In strengthening the signal, an unwanted side effect is that image noise will also be amplified. Apart from impairing overall image quality, noise also increases demands on bandwidth and storage.

The Right Amount of Light

Shutter speed is an important factor of catching good images. The shutter opens up for a short time allowing enough light to reach the sensor. It then closes again to keep the image from being overexposed. In bright daylight the shutter only needs to be open for a few milliseconds. As light drops off the shutter needs to stay open longer. At some point this will cause motion blur on anything moving in the scene. A fixed scenario may look good in low light with slow shutter speeds, even though anything moving in the scene will become blurred and possibly unidentifiable. The lens also affects the low light performance of a camera. A lens with high quality glass elements and a wide aperture lets more light through and enables faster shutter speeds.
When comparing different camera models for low light situations, make sure they are fitted with the intended lens. A lower F-stop value means the lens has a wider maximum aperture and will perform better in low light. Some cameras automatically adjust the aperture, or iris, depending on the available light in the scene, making them suitable for scenarios where the amount of light may vary. The difference in intensity between the brightest and darkest parts of a scene is called dynamic range. When this range extends beyond the capabilities of the image sensor, parts of the image will be rendered without any detail, just all white or all black. A camera with Wide Dynamic Range capabilities is designed to function better in environments with variations in light intensity. Make sure you test the camera in its target environment.

Possible solutions

A number of solutions and technologies can be used to counter the problems of low light conditions. We will give you a brief overview of the three different technologies. All objects radiate thermal energy, or heat. Special types of sensors have the capability to register this and can form an image based solely on temperature variations in a scene. Cameras with these sensors are called thermal cameras. These images lack natural colors, but are very useful for detecting objects in absolute darkness. Thermal cameras can even see through smoke and light mist making them suitable for perimeter and area protection.
Our second solution is auxiliary lights, and is quite commonly used. Additional lamps are set up to illuminate the parts of the scene that are too dark. You can also choose to illuminate a scene with an IR light. This light will not be visible to people, making it perfect for hidden installations. IR video however lacks natural colors. One of the benefits of adding extra light is that you are widening your range of camera options. A camera with an extremely high resolution sensor for identification purposes but with poor low light performance can be combined with an auxiliary light source.
Lightfinder is a new technology from Axis Communications. It is based on Axis’ know how, in-house technology and a sensor with exceptional light sensitivity. Combined with high performance optics this enables the camera to deliver images in situations where a normal camera would fail. Lightfinder technology is suitable for installations where it is not practically possible to add any lights. The good color reproduction of the Lightfinder platform also makes it a good choice when recognition and identification objectives need to be met.

Wrapping it up

No single camera can handle all real world challenges, and it is important to assess the target scene, and operational requirements before selecting a camera technology. Often, a combination of technologies gives the best result.