What is LUX and Light Sensitivity?

Light sensitivity, or minimum illumination, refers to the smallest amount of light needed for the camera to produce an image of useable quality. Minimum illumination is presented in lux (lx), which is a measure of illuminance. In general, provided it is not overexposed, the image will be better the more light that is available in the scene. If the amount of light is insufficient, the image will be noisy or dark. The amount of light that is required to produce a good-quality image depends on the camera and how sensitive to light it is.

To capture good quality images in low light or dark conditions, a day and night camera that takes advantage of near-infrared light is required. For detection in complete darkness and difficult conditions such as smoke, haze and dust, a thermal network camera provides the best solution.

Different light conditions offer different illuminance. Many natural scenes have fairly complex illumination, with both shadows and highlights that give different lux readings in different parts of a scene. It is important, therefore, to keep in mind that one lux reading does not indicate the light condition for a scene as a whole, nor does it say anything about the direction of the light.

Examples of different levels of illuminance.
Illuminance Lighting condition
100,000 lux Strong sunlight
10,000 lux Full daylight
500 lux Office light
100 lux Poorly lit room

Many manufacturers specify the minimum level of illumination needed for a network camera to produce an acceptable image. Whereas such specifications are helpful in making light sensitivity comparisons for cameras produced by the same manufacturer, a similar comparison between products from different manufacturers is flawed by default. This is because different manufacturers use different methods and have different criteria for what is an acceptable image. In other words, the process of measuring light sensitivity is complicated and inconclusive. There are several reasons for this.

Incident light vs. reflected light

Lux meters and cameras collect different light information. Whereas a lux meter records the amount of visible light that hits, or illuminates, a given area (incident light), a camera records the amount of light reflected from the objects in the covered area (reflected light). Thus, the lux readings of two people occupying the same space, where one is dressed in white and the other in black, would turn out identical. However, the amount of light captured by a camera covering the same scene may be lower or higher, partly because the person dressed in black reflects less light than the person dressed in white.

The difference between incident light measured by a lux meter and reflected light captured by a camera.

Lux value examples

Natural scenes often have complex illumination. When a lux value is measured in a scene, it only represents the illuminance at the object in focus. In the images below, for example, the lux value represents the illuminance at the point of the tree or in front of the building. The brighter sky and the darker ground are not taken into account.

5 lux.

80 lux.

4000 lux.

Factors that influence the light sensitivity

A camera’s light sensitivity is dependent on a number of manipulable factors. There are a number of factors that influence the light sensitivity of a camera, including:

  • Exposure time
  • F-stop
  • Sensor quality and size
  • Lens quality
  • Color temperature

Methods are subject to variation due to lack of standard. There is currently no global standard for measuring minimum illumination on the market, which means that not even the market leaders share the same method, making it a challenge for any vendor to achieve fair values and for customers to trust them. To properly compare the low light performance of two different cameras, the cameras should be placed side by side and be viewing a moving object in low light.

Source = http://www.axis.com/sk/en/learning/web-articles/technical-guide-to-network-video/light-sensitivity