Getting the Most from Low Light

Bringing you up to date on changes in camera technology

• By Dave Smith
• Dec 01, 2009

System designers, installers and end users have experienced the frustration that comes when cameras fail to work well in lowlight conditions, producing grainy, noisy video. The video compression used in most IP cameras and DVRs depends on the fact that there is little change in most scenes over time. MPEG and H.264 are able to squeeze data out of the video for areas of the picture that don't move, saving bandwidth and valuable disk storage space. When the picture gets noisy, the compression engine sees the noise as motion, and network bandwidth and hard drive use increase dramatically, causing increased transmission and storage costs.

Recent camera technology has improved the ability to provide good quality video in low-lighting situations.

Newer cameras include an amplifier known as automatic gain control. However, when the AGC boosts the signal, it also boosts noise and eventually the noise makes it impossible to see any picture.

Most cameras allow you to set the maximum level of AGC. Experiment with this setting. Setting the AGC to low means less noise, but the picture will get washed out sooner as lighting fades. Setting it higher, the camera will see better, but it will get noisier.

Here are a few tips to help you sort through the options:

Select a camera that offers high performance in low light. Look at the minimum illumination specifications, but do your own comparisons to find the camera that works best. There are no standards for measuring minimum illumination so manufacturer's specifications are only a general guide. Look for features such as frame integration or Sens-up. Avoid wide dynamic range models unless you have a specific need as most WDR cameras are not as sensitive in low light.

Select a camera model with day/night capability. Day/night cameras switch from color during the day to black and white at night to provide the best picture quality for the lighting condition. The simplest form of a day/night camera is known as electronic day/night. When it switches to night mode, this type of camera shuts down the color signal, which contains most of the noise. Without it, the camera can apply more AGC boost to overcome the poor video contrast.

True day/night or infrared cut filter removal cameras turn off the color signal and physically remove a filter, located between the lens and the image sensor. In color mode, this filter must be in front of the imager in order to produce proper color. In night mode, removing the filter allows IR light and more visible light to reach the sensor. A true day/night or ICR camera costs more but has significantly more sensitivity than an electronic day/night camera. If you use an IR light source, a true day/night camera is required.

Select a lens with the lowest f/stop rating. A one f/stop change means double or half the light coming through the lens, so an f/1.0 lens is twice as good as an f/1.2 lens. Generally, adjust the f/stop to just below the point where the whitest parts of the picture start to bloom and wash together.

Use frame integration or Sens-up. Frame integration is another good way to improve light sensitivity. You can make up for lower light levels by increasing exposure time. Video cameras were traditionally stuck with a fixed exposure time of 1/60th of a second due to television standards. Now, cameras with frame integration are capable of lengthening the exposure time to multiples of one TV field by storing the exposed picture and refreshing it slower.

If Sens-up is set to x2—two fields—the exposure time goes from 1/60th of a second to 1/30th of a second. The refresh rate of the video changes from 60 to 30 fps, thereby doubling the cameras' sensitivity. Going to x4 mode makes this 15 fps, and x6 makes it 10 fps.

The disadvantage is that longer exposures have less ability to freeze moving objects. On tight views with rapidly moving objects, the images will blur. Cameras with frame integration will go to much longer exposures than are practical in security applications, but some will allow you to program a maximum value for Sens-up. As light levels decrease, the camera will increase Sens-up until this maximum is reached. Setting the maximum Sens-up value to x6 is safe for most applications and provides a dramatic improvement in light sensitivity.

Use digital noise reduction. Some newer cameras offer digital noise reduction for low-light applications. If more noise is removed, the AGC boost can be increased to allow operation in lower light levels. Higher levels of noise reduction often result in motion artifacts.

Whether you are using IP or analog cameras, paying attention to the performance features should dramatically improve the quality of video while reducing network load and storage requirements by as much as 70 percent. A few hours spent testing these features and comparing available cameras will result in big benefits for the end user.