Shedding Light on Lux Ratings

We examine whether megapixel cameras are ready for their low-lux debut and what else to look at aside from minimum lux levels. The outlook for low-light megapixel cameras also depends on global economic conditions, which could limit uptake.

How low can you go? From the data sheets, every camera is capable of capturing sharp images with barely perceptible illumination and no IR illumination. There seems to be a competition to put the most zeros in a lux rating, with 0.1 to 0.001 lux models all claiming to be capable of this feat.

Actual camera footage proves to be disappointing, resulting in camera shootouts to see whether camera performance is really up to snuff. “In many cases, we test the cameras to make sure that what the manufacturer says is real,” said Bob Mesnik, President of Kintronics.

The manufacturer lux ratings should be considered as reference for buyers, but not an absolute. “A camera might work at 0.001 lux but to what distance?” said Michael Brown, MD of VideoControlRoom. “And what detail do you need to record at that distance? If you want real light at night, integrate the security system into the building lighting.”

Lux meters measure illuminance, but lux readings can be misleading. The lux meter's reading does not equate a camera's light sensitivity, as a camera does not collect light in the same way. Rather than recording how much light hits an area, a camera records the amount of light reflected from objects in the scene. “In other words, each camera must be measured under the same conditions — this is the key to minimum illumination,” said Alex Iida, PM of Security Solutions, APAC Professional Solutions, Sony Electronics.

There are several ways to boost a low-lux rating, but some of them do not actually improve image quality. “For example, a long shutter speed will improve the low-lux measurement, but will result in blurry images,” said William Ku, Director of Brand Business for Vivotek.

Unscrupulous manufacturers may optimize performance during testing by using expensive, high-end lenses that are not included or ones that the average customer is unlikely to consider. “Conditions need to be the same for an apples-to-apples comparison between cameras,” Iida said. “It's possible to put a ‘wonderful' yet deceiving spec on a brochure by taking out figures,such as a low IRE with slow shutter speed.”

Other tricks make low-lux ratings look better. “The gain level can be set high, which improves the lux level but also increases the noise,” Ku said. “Only when all camera settings are listed — IRE, shutter speed, aperture and gain control — will the lux rating then be credible.”

Challenges
Megapixel cameras can handle nighttime imaging, but getting quality images is more expensive compared to analog cameras. Image sensors are essential, but not the sole factor for low-light imaging. True differentiation lies in robust algorithms that can refine raw images into clear footage.

The diminutive physical size of image sensors limits developments. “To achieve higher sensitivity in high-resolution cameras, we can enlarge the size of each pixel, or improve the efficiency of collecting light for each pixel,” Iida said. “Since the general image sensor size has been standardized at one-third of an inch, enlarging the pixel size dramatically would be theoretically difficult. The latter way is a more realistic one to improve the sensitivity in a high-resolution camera.”

Price is a factor for low-light performance, as expensive models will support more features and use high-quality components.

IR cameras are best at producing quality images in low light or dark conditions. “For detection in complete darkness and difficult conditions such as smoke, haze and dust, a thermal network camera provides the best solution,” said Philip Siow, Senior Consultant at Axis Communications.

The wavelength of light affects nighttime imaging. While the human eye is sensitive to wavelengths from 400 to 700 nanometers, CCD and CMOS image sensors can capture up to 1,000 nanometers.

“The benefit of the visible band is that it contains color information that is easily interpreted by our brain, but as night kicks in, security cameras turn into the near IR (NIR) spectral band,” Yanai said. “NIR is covert and ideal for security tasks. Its wavelength is close enough to the visible band to get almost natural contrast images, which allow for face recognition and letter reading similar to daylight.”

As NIR cameras require IR illumination, they are not technically low-light cameras. However, they are used for low-light applications. Thermal cameras do not use additional illumination to “see” in complete darkness, producing images from invisible heat radiation based on temperature differences between objects.

“An IR camera is only as good as the IR illumination, which needs to be either built into the camera or mounted separately,” Brown said. “The trade-off with thermal is that in return for outstanding nighttime warm object differentiations, you get a daytime picture that lacks visiblelight detail — like faces and license plates that you capture with a typical IR camera.”

Unlike a thermal imaging camera, a low-lux camera will not produce an image in total darkness. “Thermal imaging systems sometimes combine one or more thermal imaging cameras with a low-lux camera,” said Flir Systems in a prepared statement. This gives operators the ability to see during daylight and low-light conditions.

Megapixel cameras have come a long way for light sensitivity and can peacefully coexist with other solutions. “Megapixel cameras provide the extraordinary ability to cover a wide field of view with a great deal of details during the day, while other, more specialized technologies utilize the IR spectrum for night security tasks,” Yanai said.

Future Outlook
Megapixel cameras are getting better for low-light performance. However, other cost-effective solutions deliver stronger nighttime performance, resulting in a range of cameras standing guard at night.

Some video surveillance systems comprise megapixel cameras, PTZ cameras, low-light analog cameras, thermal imaging cameras and cameras with very long-range IR laser illuminators. “It's very simple to integrate low-light analog cameras,” Mesnik said. “Thermal cameras and long-range cameras with laser illumination can be more difficult, because they sometimes require special commands that control the camera functions.”

As the global economy contracts, buyer budgets are likely to shrink as well. While network video providers said the double-dip recession has had no effect on their 2012 forecasts, it could have a chilling effect on demand for more high-end megapixel cameras.

Megapixel cameras do perform at night, but no camera fits all conditions. Before purchases, it would be wise to consider site-specific needs. “A system contains more than just the camera,” Mesnik said. “The latest direction is to determine the best components for the system, rather than carrying a line of cameras that can be used in every application.”

Sensor improvements and more robust processors have yielded megapixel cameras with better low-light performance. While auxiliary illumination improves imaging results, megapixel cameras have overcome previous sensitivity issues, making them viable for nighttime surveillance.