SEEING THE LIGHT
The world’s indoor spaces are increasingly being lit by fluorescent light sources. While this trend seems good for energy efficiency, it turns out to be a serious problem for CCD video cameras, writes John Monti and Jeff Jones
The technical problem with video cameras and fluorescent lighting takes shape in slightly different ways worldwide, but it boils down to a video sampling issue that occurs when there’s a difference between the video field capture frequency (which is set by global standards bodies) and local AC power frequency (which varies by geographic location).
This mismatch results in everything from video images whose colour shifts every few seconds to video that ‘beats’ so strongly that operators are physically unable to watch it continuously. A closely related technical issue also affects scenes with LED lighting sources, causing the video images to go dark every few seconds.
Some of the results are simply annoyances, but at other times they can seriously hinder effective security.
These video sampling issues are not readily apparent to people looking directly at a scene lit by fluorescent lights or LEDs, thanks to a natural effect known as ‘eye persistence.’ Unlike video cameras, when humans view a scene our brains ‘fill in the gaps’ between fast-flickering lights. The human eye perceives the lighting as constant – in the same way we perceive the fast-moving frames of projected movies as continuous motion.
Through the years, a number of approaches have been tried to solve the video sampling issues caused by fluorescent and LED frequencies and their mismatch with AC power frequencies. These other approaches, however, significantly reduce the dynamic range of the video camera for fluorescently lit scenes. This loss of information is not an acceptable trade-off for many video security applications.
Pixim’s Digital Pixel System technology has recently solved the persistent technical issues of fluorescent light flicker, fluorescent colour roll and frequency-modulated LED ‘blackout’ – while also retaining ultra-wide dynamic range capabilities, for true natural colour and accurate image capture even under variable lighting conditions.
Pixim’s Enhanced Flicker Reduction (EFR) mode we believe represents the first real solution to this persistent technical issue.
'Security personnel are physically unable to monitor the flickering video for any length of time which can lead to unmonitored security cameras. Historically major compromises are required to reduce the flickering such as drastically reducing the camera's dynamic range & colour reproduction '
Fluorescent light flicker
Like its name implies, fluorescent light flicker occurs because fluorescent light output – like that of any lighting technology that uses a ballast – is not constant, and in fact flickers. When video fields are captured at a frequency that is significantly different from the AC power supply frequency, the flicker becomes very obvious.
Fluorescent light flicker is a significant problem in eastern Japan (it also affects Chile and some other Latin American, Caribbean and Pacific nations). For historical reasons, the western half of Japan was powered using 60Hz AC power and the eastern half uses 50Hz AC power. The video standard used throughout Japan is 60Hz NTSC.
Low-frequency fluorescent lighting flickers at a rate that is twice the frequency of the AC power source that it is connected to. In eastern Japan and other areas using 50Hz AC, the fluorescent flickering rate is out of synch with the video camera’s 60Hz frequency. Unless the camera is line locked to the AC power source, flickering will be evident in the video capture.
This flicker problem cannot be solved for standard CCD sensors. It is also a persistent problem for internet protocol (IP) security cameras, which are often powered by DC voltage or power over ethernet (PoE), neither of which can be line-locked to an AC power source.
Video captured under these conditions displays a constant beating, strobe-light effect that makes watching the video unpleasant and difficult. As a result, security personnel are physically unable to monitor the flickering video for any length of time, which can lead to unmonitored security cameras. Historically, major compromises are required to reduce the flickering, such as drastically reducing the camera’s dynamic range and colour reproduction.
Fluorescent colour roll
Even regions that use 60Hz AC power are not out of the woods with respect to video capture challenges in fluorescent lighting. That’s because video cameras’ NTSC standard of 60Hz is actually 59.94Hz. That slight difference is enough to cause trouble. The spectral characteristics of fluorescent lights change within each AC power cycle as the illumination level changes. If a camera is not AC line locked, the time bases between the 60Hz AC power and 59.94Hz NTSC video frequency drift over time, and the camera will capture the spectral changes.
Specifically, the camera’s image captures will sweep through the entire illumination cycle of the fluorescent light every 8.325 seconds, causing the colour of the fluorescent light to change in the captured video. This effect is known as ‘colour roll’ or ‘colour breathing.’ Video cameras displaying changing colour every eight seconds or so might not sound like a serious issue – unless the accurate colour of a scene is important for security or evidentiary reasons. Often the colour artifact is a burst of bright orange, which is quite distracting.
Frequency-modulated LED blackout
Today, many traffic and signal lights, headlights, flashlights and other devices use light-emitting diodes (LEDs) as their lighting source. LEDs are not continuously powered; rather, their power is modulated, causing the lights to ‘blink.’ This power modulation saves energy and extends the life of the LED, but it creates problems for video cameras in ways that are analogous to the fluorescent lighting issues described earlier.
LEDs blink at 120Hz frequency in most areas that use the NTSC video field rate of 59.94Hz along with 60Hz AC power. The LED time base mismatch means that the LEDs will appear to go dark for a period of time every 8.325 seconds in the captured video.
As a result, LED-based traffic and train signals will not be accurately captured by cameras mounted on moving vehicles such as trains and buses. This situation presents potential safety problems (eg if trains controlled automatically via input from video cameras can’t ‘see’ a signal at a crucial decision point), and it is also very important to accurately capture the illumination state of traffic and transit signals for forensic purposes (eg to tell whether a traffic light or transit signal was showing green, yellow or red at the moment of a traffic collision).
'Video cameras displaying changing colour every eight seconds or so might not sound like a serious issue - unless the accurate colour of a scene is important for security or evidentiary reasons. Often the colour artifact is a burst of bright orange, which is quite distracting '
Pixim’s solution
Previous attempts to address the video sampling issues with fluorescent and LED lights have been limited in both usability and effectiveness.
Traditionally, cameras that are able to accept AC power sources directly can be line locked to the AC signal, which eliminates the roll effects. Unfortunately, the majority of cameras sold and installed around the world have DC power sources. Also, the trend toward IP cameras is recently favouring PoE, which cannot support AC power. In cases where AC line lock is possible, there is the additional problem of the camera capture locking out-of-phase with the AC source, causing a false colour cast on the image. This can be remedied only manually at the time of field installation by a trained technician.
In cases were AC line lock is not possible due to a lack of a low-voltage AC supply, analogue CCD cameras have traditionally used ‘internal sync.’ This mode must be invoked manually at install time by a trained technician, leaving the camera stuck in this mode. The result is reduced colour saturation and dynamic range. Also, internal sync modes require an expensive mechanical DC iris lens, which adds cost to the camera.
In contrast to these approaches, Pixim offers an automatic solution that solves the fluorescent- and LED-induced artifacts while retaining full wide dynamic range capabilities – without intervention from an installer.
Pixim’s Enhanced Flicker Reduction (EFR) mode, available in Pixim’s V3.2 firmware (and above), overcomes video sampling issues including fluorescent light flicker, fluorescent colour roll and frequency modulated LED blackout. In addition, it enables video cameras to maintain a full 15 bits of dynamic range for scenes lit by artificial, modulated light with no manual intervention required.
The unique capability of Pixim’s Enhanced Flicker Reduction video capture mode is another example of the architectural power of Pixim’s all-digital solution, which is able to solve practical video capture problems in ways not possible using older analogue CCD and CMOS image sensors.
Pixim began shipping the firmware with EFR mode in May 2008 for video cameras powered by Pixim’s Digital Pixel System technology, and the EFR capability will be a standard feature with all future version of Pixim’s firmware. The first cameras employing EFR are expected to reach the market soon.
 John Monti is vice president of marketing at Pixim and Jeff Jones is Pixim’s director of product marketing. For more info visit: www.pixim.com (opens a new window)
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