PTZ cameras have improved a lot in recent years. Now they come with HDTV-quality resolution, better light sensitivity, higher zoom factors and smoother, more accurate movement. But the one area that has continued to be a challenge for manufacturers is to build a PTZ camera with a robust and instant autofocus.
In indoor installations with good lighting, finding the right focus usually is not a problem; however, in more challenging lighting environments – such as backlighting, dusk or dawn low light, or fog conditions – PTZ cameras sometimes struggle to find the focus target. This creates a “chasing” phenomenon whereby the camera keeps seeking focus while it continues to stream video that lacks sharpness. As a result, important evidence such as license plates or other crucial details can be lost.
So far, attempts to develop better contrast-based autofocus algorithms for PTZ cameras have not been able to totally resolve the problem.
Thinking Outside the Box
From frustration, however, comes inspiration. Why not incorporate advanced laser technology into PTZ cameras to help them focus on the fly? Then, perfect autofocus could be achieved under virtually any condition.
How it works: A laser emits coherent and monochrome light rays all aimed in the same direction. The beam can remain narrow over great distances, making it suitable for industrial applications such as autofocusing a PTZ camera. In camera applications, the laser uses invisible infrared light with longer wavelength at intensity well below the harmful thresholds for the human eye. The transmitter in the PTZ camera sends out a laser beam that bounces off an object and returns to the camera receiver, instantly providing the camera with a focus reference point. This reference point then assists the camera’s focus algorithm to automatically fine-tune the focus to the correct pixels.
How this differs from autofocus: Unlike autofocus, the laser range finder measures the distance to the object by measuring the time it takes for the laser beam to reach the object and return back to the camera. By knowing the exact distance to the object, the camera can focus much more reliably and nearly instantaneously.
While the theory of laser range finders is simple to follow, applying the technology to mass produce high-zoom PTZ camera is actually quite complex. Especially in environments where the target is at a significant distance from the camera, the laser needs to be precisely aligned with the field of view of the camera to ensure a sharp image.
Coping with a Dynamic Environment
The superiority of the laser range finder is most evident in mission-critical camera installations like city and perimeter surveillance, critical infrastructure, airports and harbors where instant focus on moving objects and fast-changing events is essential. The sole purpose of the laser range finder feature is to automatically and continuously verify focus as the scene changes, performing the entire process within a fraction of a second.
Laser focusing helps overcome four key issues that autofocus PTZ cameras typically wrestle with: tracking, noise, contrast, and apparent light sensitivity.
Tracking: Normal autofocus cameras often find it challenging to focus on distant objects, especially under challenging lighting conditions; and if that object moves, maintaining a sharp focus is nearly impossible. Laser-enhanced autofocus PTZ cameras, however, continuously recalibrate the distance to the target as it moves so as to maintain a sharp autofocus.
Noise: If there is a lot of noise in a scene due to low light, a normal autofocus PTZ camera struggles to maintain focus because it cannot distinguish between noise and the real contrast in the scene. Laser range finders, on the other hand, have no difficulty measuring the distance to a target and fine-tuning the autofocus whether the scene is dimly lit, in complete darkness or reflecting a lot of light.
Contrast: Autofocus cameras typically use contrast to calculate the right focus. In scenes where there is no contrast, such as an inside or outside wall, a normal autofocus camera would be hard-pressed to find the right focus. Laser range finders, on the other hand, rely on bouncing a laser beam off an object rather than contrast in the scene to instantly calculate the perfect focal point.
Apparent light sensitivity: In low-lit scenes, conventional autofocus cameras find sharp focus difficult to maintain. As a result, the final focus position seldom reflects the optimal position. This leaves the presented image as slightly blurred. Laser range finders, on the other hand, have no difficulty finding the perfect focus, even in extremely poor lighting. In fact, the laser actually increases the light sensitivity of the PTZ camera by focusing the photons onto the correct pixels, making it possible to see details in darkness that in the past were virtually impossible to notice.
Sharper is Better
With laser focus enabling PTZ cameras to achieve perfectly sharp image quality almost instantaneously, operators can quickly and effortlessly identify people, license plates and other critical details. But laser focus provides other benefits as well, such as less bandwidth consumption and less storage requirements, since sharper images generate smaller video files.
At this juncture, laser technology is being applied to autofocus. But clearly the additional information it enables a camera to capture can also be used to enhance analytics applications. For example, instead of relying only on scene changes to trigger an action, the laser module itself could be used to trigger an action. This opens the door for more advanced and more accurate event detection.
As laser range finding technology matures, future designs may incorporate multiple beams or scanning beams, providing the possibility to extract even more information from a given scene. As manufacturers continue to think outside the box, they inevitably will explore ways to leverage laser range finder technology in completely new types of applications that today we can only imagine.
Fredrik Nilsson is the VP, Americas for Axis Communications (www.securityinfowatch.com/10212966). He has more than 15 years of experience with IP video systems and is the author of “Intelligent Network Video: Understanding Modern Video Surveillance Systems,” published by CRC Press.
