Considering “Repeatability” of P/T/Z Cameras

Yes… another detail to consider when reviewing P/T/Z cameras; “Repeatability”. This may be of little concern when the intended P/T/Z camera application uses very low or no zoom within the presets of a tour. High end P/T/Z cameras with large optical zoom ranges tend to (and should) have higher repeatability ratings than other cameras. WHY?

First…
Repeatability, with regard to a P/T/Z camera specification, can be defined as the accuracy within which the camera position of the pan AND tilt axes are able to recall the previously set (aka: preset) position. Repeatability is typically given in degrees: 0.05°, 0.08°, 0.25°, etc.

Second…
The numbers given for repeatability are degree based. In effect, the repeatability specification references the ability of the camera to return to an exact position specified within the circular rotation of an axis. In the case of P/T/Z cameras… both the pan and tilt axes.

With these definitions in mind…
In a wide angle shot (with zoom at or near zero), any factor of repeatability found in today’s P/T/Z cameras is likely negligible. The image shift on either pan and/or tilt of even 0.25° when viewed after recalling a preset position will be very hard to notice.

The difference in repeatability becomes noticeable when a zoom setting has been associated with the preset position. A high zoom setting amplifies the image shift that occurs when the preset is recalled. The 0.25° image shift now becomes visible.

Repeatability can have a distinct impact on the resulting video image provided by the camera for real-time viewing as well as for what may be getting recorded into a DVR/NVR. The positioning of the viewable image provided by the recall of a preset should allow for variations due to the camera repeatability. This means that the user should test the presets several times to ensure the viewable image will still provide the desired video data even if the image has shifted.

Unrelated to the repeatability of the camera itself, yet related to the repeatability of image acquisition from a preset… stability of the camera mount location. The characteristics of the camera mount should be considered when establishing presets and expected image shifts. Mounting to the top of a pole is very common along highways, intersections, etc. Consider the suns effect on the pole material throughout the course of a day. One side of the pole is exposed to the sun during the morning hours while the other side is not; one side “hot” and one side “cold”. The material expansion coefficient, coupled with the height of the pole, can create an angular shift much greater than 0.05°. This angular shift may reverse throughout the course of the day as the sun changes position. Oh yes… and don’t forget wind!

In general, every P/T/Z camera has a number that specifies repeatability. A lower number will tend to indicate a higher quality of manufacturing, and is more likely to be associated with units incorporating a higher optical zoom or specialty video (thermal) capability.

~ David Scales, Chief Technology Officer


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WTI (Wireless Technology, Inc.)

Video Surveillance Products Dedicated to the Broadcast, Global Security and Transportation Markets!

http://www.gotowti.com/ ● http://www.wirelesstech.com/

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Building a Sidewinder Series ~ Part 1: Hydrophilic Glass

Building a Sidewinder Series ~ The Technology of WTI’s Sidewinder Camera:



Part 1: Hydrophilic Glass

The latest in glass technology are “self-cleaning” and “low-maintenance” window glass, two categories which encompass window glass products that are coated or fused with titanium dioxide (TiO2) alone or with under-layers or silicon dioxide (SiO2).

CVD (Chemical Vapor Disposition) is the process of choice due to the inherent longevity of fusing TiO2 to the glass. TiO2 was chosen for use on WTI’s Sidewinder camera for its combined photocatalytic and hydrophilic properties.


Hydrophilicity – This term describes the property of a surface that makes it have an affinity for water. When water hits hydrophilic glass, it flattens and spreads out to form a thin sheet on the glass surface. This sheeting action not only helps rinse away loosened dirt and other organic material it also minimizes spots and streaks by helping the glass dry more quickly and evenly.

A glass surface is considered to be hydrophilic if the contact angle of a water bead on its surface is less than 20 degrees. In contrast, a hydrophobic surface (which promotes beading like that of a freshly waxed automobile) has a contact angle of more than 60 degrees. The contact angle on ordinary glass measures 20-40 degrees.


Photocatalysis – This term describes the property of a surface which allows it to decompose organic material. On a self-cleaning window, the surface of the glass will help break down loose dirt and other organic material. This allows the sheeting action of the water to more easily rinse loosened dirt away. This property also helps the glass maintain its hydrophilicity (ability to sheet water).





For more information on the Sidewinder camera, please visit us on the web at http://www.gotowti.com/.