An inside look at covert surveillance.
Since the camera and lens are hidden, a covert installation makes it more likely that an unsuspecting violator will be viewed, recorded, and even apprehended while committing the act. An unrelated reason for using covert CCTV is to avoid changing the architectural aesthetics of a building or surrounding area.
But covert installations have unique requirements. Security professionals should make sure these differences are understood before a covert CCTV operation is set up.
Techniques. CCTV lens and camera concealment is accomplished by having the lens view through a small hole, a series of small holes, or from behind a semitransparent window. CCTV cameras are installed in one or more locations in the room, depending on the activity expected.
Covert CCTV systems pose some unique optical problems compared with overt systems using standard lenses. Since the diameter of the front lens viewing the scene must, by necessity, be small so it can be hidden, the lens is designed to be optically fast, collecting and transmitting as much light as possible from the reflected scene to the television sensor.
As a consequence, small-diameter lenses, called pinhole lenses, are used. The term pinhole is a misnomer, however, as these lenses have a front diameter anywhere from 1/8 to 1/2 inch.
There are several misconceptions regarding the factors determining a good pinhole camera or lens system for covert applications. Since the lens, camera, VCR, and system installation are a significant capital investment, an understanding of what constitutes a good system is important.
Figure 1 shows the covert security problem. The lens/camera must receive reflected light from an illuminated scene, the lens must collect and transmit the light to the camera sensor, and then the camera must transmit the video signal to a video monitor or VCR and video printer. Most covert pinhole lenses are designed for 1/3-inch, 1/2-inch, and 2/3-inch camera sensor formats.
Figure 2 (page 57) shows two basic configurations for pinhole lenses and cameras located behind a barrier. The hole in the barrier is usually chosen to have the same or smaller diameter (d) than the pinhole lens front lens element. When space permits, the straight-type installation is used. In confined or restricted locations with limited depth behind the barrier, the right-angle pinhole lens/camera is used. In both cases, to obtain the full lens field of view (FOV), it is imperative that the pinhole lens front lens element be located as close to the front of the barrier as possible to avoid tunneling, or vignetting - the loss of light through a lens or optical system at the edges of the field due to inadequate lens design or an internal obstruction.
When the pinhole lens front element is set back from the barrier surface, the lens is, in effect, viewing through a tunnel, and the image has a narrower FOV than the lens can produce. This appears on the monitor as a porthole-like (vignetted) picture.
Lens pointing angle. An important installation problem often initially overlooked is the lens pointing angle required to see the desired FOV. Many applications require that the lens/camera point down at a shallow depression angle (30 degrees) from the ceiling. This objective is accomplished by using the small-barrel, slow-taper lens.
This feature allows pointing the small-barrel lens over a larger part of a room than the wide-barrel lens. Not all lenses can be mounted at a small angle to the ceiling because of the lens barrel shape. Lenses having a large barrel diameter and fast taper at the front cannot be mounted at the shallow angles required. The small-barrel, slow-taper design permits easier installation than the fast-taper, since less material must be removed from the barrier. In addition, the lens has a faster optical speed, since it is larger and collects more light.
A pinhole lens having a small front diameter is simple to install. The smaller tapered barrel can be mounted at a smaller angle to the barrier than the wide-barrel lens. This feature allows pointing the small-barrel lens over a larger part of a room than the wide-barrel lens.
Lenses. Covert security applications typically use a standard pinhole lens. Most pinhole lenses are designed for 1/3-inch, l/2-inch, or 2/3-inch format cameras and have a manual- or automatic-iris control to adjust the light level reaching the camera. Several generic pinhole lens types are available. The right-angle version permits locating the camera and lens inside a narrow wall or above a ceiling.
Optical speed. The optical speed or f-number (f) of the pinhole lens is important for the successful implementation of a covert camera system. The lower the f-number, the more light reaching the CCTV camera and the better the television picture.
The best theoretical f-number is equal to the focal length (FL) divided by the entrance lens diameter (d). In practice, the f-number obtained is worse than this number because of various losses caused by imperfect lens transmission, in turn caused by reflection, absorption, and other lens imaging properties.
For a pinhole lens, the larger the entrance lens diameter, the more light gets through to the camera sensor, resulting in better picture quality, all other conditions remaining the same.
The amount of light collected and transmitted through a lens system varies inversely as the square of the lens f-number: if the lens diameter is increased a small amount, the light passing through the lens increases by a large amount. If the lens diameter is doubled, the light throughput quadruples.
Many types of covert lenses are commercially available for CCTV security applications. Most of these lenses are designed for 1/3-inch, 1/2-inch, and some for 2/3-inch sensor formats, since these cameras are small, are in widespread use, and provide excellent image quality and resolution: 380 TV lines for 1/3-inch and 450 to 570 TV lines for 1/2-inch and 2/3-inch format cameras.
Some pinhole lenses have a very small entrance aperture, that is, 0.10 inch (2.5 mm), and are therefore optically slow (f/3.5-f/4.0) by design. A lens with an FL of 9 mm and a 2.5-mm aperture has, at best, a theoretical f-number of 3.6. Other lens losses within this type of lens give an overall optical speed of approximately f/4.0.
A covert lens with an 11-mm FL and a 6-ram aperture has a theoretical f-number of 1.83. Other lens losses result in an overall optical speed of approximately f/2.0.
The 9-mm lens with the smaller aperture works well if there is sufficient light. An advantage of the 6-mm-aperture lens is that it can be used in applications where a larger hole adequately conceals the lens and where there is insufficient light available for the 9-mm FL lens with the 2.5-mm hole.
The most important characteristics of a pinhole lens are its optical speed - the lower the f-number the greater the speed - and its ease of installation and use.
When covert operation is required in locations having widely varying light-level conditions, and when a silicon or Newvicon tube, charge coupled device (CCD), solid-state, or other intensified low light level (LLL) camera is used, a pinhole lens with an automatic iris controlling the light reaching the camera sensor is necessary.
Some new types of shuttered CCD cameras may tolerate the use of manual-iris lenses. Security managers should check with the manufacturer for the light ranges over which the camera will operate.
A generic characteristic of almost all pinhole-type lenses is that they invert the video picture and therefore the camera must be inverted to get a normal right-side-up picture. Some right-angle pinhole lenses reverse the image right to left and therefore require an electronic scan-reversal unit to regain the correct left-to-right orientation. Some pinhole lenses have a focusing ring or the front element of the lens can be adjusted to focus an image on the camera sensor.
Convertible kit. Pinhole lenses have been manufactured for many years in a variety of focal lengths (3.8, 4, 5.5, 6, 8, 9, 11 mm), in straight, right-angle, and manual- and automatic-iris configurations. The FL of most of these lenses can be doubled to obtain one-half the FOV by using a 2X extender.
Pinhole lenses with 16-mm and 22-mm FLs are achieved by locating a 2X magnifier in-between the 8-mm and 1 1-mm lenses and the camera. This automatically doubles the f-number of each lens (only one-fourth of the light transmitted). In many applications, the required FLs and configuration are not known in advance, and the user (or dealer) must have a large assortment of pinhole lenses, or take the risk that he or she will not have the right lens to do the job. This dilemma has been solved with the availability of a pinhole lens kit.
With this kit of pinhole lens parts, eight different FL lenses can be assembled in either a straight or right-angle configuration in minutes. An additional four combinations can be assembled for a disguised sprinkler-head covert application. All lenses have a manual iris (an automatic iris is optional).
Several points should be considered when using standard, fully assembled pinhole lenses or pinhole lenses made from the pinhole lens kit:
* Straight lenses invert the picture; therefore, the camera should be mounted in an inverted orientation.
* Some right-angle lenses will show a right-to-left picture orientation instead of left-to-right, as with normal lenses. A scan-reversal unit will correct the problem. Check with individual manufacturers.
* The straight pinhole lens with the sprinkler-mirror attachment displays a right-to-left picture. Use an electronic scan-reversal unit to correct the problem. The right-angle sprinkler-mirror version displays a correct left-to-right picture.
The FOV when using any of the medium-to-long FL lenses is independent of the hole size through which the lens views, providing the hole produces no tunneling. Viewing through a wall with a wide-angle 4-to-8-mm FL pinhole lens may require a cone-shaped hole or an array of small holes to prevent tunneling of the scene image.
Mini-lenses. Mini-lenses are small fixed focal length (FFL) objective lenses used for covert surveillance when space is at a premium. The lenses can have focal lengths of 3.8, 5,5, 8, and 11 mm, for example. They have front-barrel diameters between 3/8 inch and 1/2 inch, making them easy to mount either behind a barrier or in close quarters.
Because these small lenses have no iris, they should be used in applications where the scene light level does not vary widely, or with electronically shuttered cameras. Mini-lenses, like other FFL lenses and unlike standard pinhole lenses, do not invert the image on the camera.
Since the small and short (less than 5/8 inch long) mini-lenses have only three to six optical lens elements, fast optical speeds of f/1.4 to f/1.8 are realized. Pinhole lenses, on the other hand, are 3 inches to 5 inches long and have as many as ten to twenty optical elements and optical speeds of f/2.0 to f/4.0. This makes the mini-lens approximately five times faster (able to collect five times more light) than the pinhole lens.
Off-Axis Optics. A useful variation of the mini-lens is one that is mounted with its optical axis laterally offset from the camera-sensor axis. This offset configuration allows the camera to view a scene at an angle away from the camera pointing-axis. The physical amount the optics must be moved to produce a large offset angle is only a few millimeters, which is easily accomplished with this special mini-lens and its modified mount.
The offset angle is chosen so that, with the camera parallel to a mounting surface, the entire lens FOV views the scene of interest without viewing the mounting surface. This angle is 20 degrees for the 8-mm lens and 14 degrees for the 11-mm lens when using a 1/2-inch format camera; it is 25 degrees and 19 degrees, respectively, for the same lenses when used on a 2/3-inch camera. This technique has a direct benefit when a camera/lens is mounted flat against a wall, ceiling, or other mounting surface.
Optical Attenuation Techniques. Since mini-lenses do not have an iris, they should be used only when the lighting conditions are fairly constant and do not exceed the dynamic range of the camera. If the scene is brightly illuminated by an intense artificial light or by the sun, several techniques can be used to attenuate the light to the lens/camera.
The first technique is to mount the mini-lens behind a light-attenuating filter. This may take the form of a gray, neutral-density filter, a partially aluminized film, or a tinted/smoked glass or plastic material. Neutral density filters are available from photographic supply stores. This technique uniformly attenuates the light across the full aperture of the lens.
A second technique is to mount the mini-lens behind a small hole, a pattern of small holes, a slit, or other hole. This objective is accomplished by either mounting a small cap with the hole on the camera lens or mounting the lens behind a hole in the barrier. The light level reaching the camera sensor can be set initially by locating the camera lens behind a hole that is smaller than the diameter of the mini-lens itself.
This technique attenuates the light reaching the lens but does not do it uniformly. For medium-FL lenses (11 mm and above), almost any shape hole results in a satisfactory image on the sensor. When the 11-ram or 22-mm mini-lens or pinhole lens is mounted behind a viewing barrier, a central hole as small as 1/16 inch is suitable for producing a full image of the scene, providing sufficient light is available for the camera.
When the 4-mm, 5.5-mm, or 8-mm mini-lens or pinhole lens views through a small hole, an undesirable porthole effect occurs, which is eliminated by having the lens view through a central hole and a series of concentric holes located around the central hole. The hole pattern must extend to the outer limits. of the lens so that the full FOV of the lens is maintained. These concentric holes enable the lens to have peripheral vision or wide-angle viewing, and they eliminate vignetting.
Sprinkler-heads. A very effective covert system uses a camera and lens camouflaged in a ceiling-mounted sprinkler head. Of the large variety of covert lenses available for the security television industry (pinhole, mini, fiber-optic), this unique, extremely useful product hides the pinhole lens in a ceiling sprinkler fixture, making it difficult for an observer standing at floor level to detect or identify the lens and camera.
The covert surveillance sprinkler installed in the ceiling in no way affects the operation of the active fire-suppression sprinkler system; however, it should not be installed in locations that have no sprinkler system, so as not to give a false impression to fire and safety personnel.
When installed, most of the pinhole lens and the entire camera is concealed above the ceiling, with only a modified sprinkler head, a small mirror, and small lens in view below the ceiling. For many applications, this stationary pinhole lens pointing in one specific direction is adequate. For looking in different directions, the camera, sprinkler head, and moving mirror assembly are made to pan (scan) via a motor drive. A scanning version of the sprinkler concept has a remote-control, 360-degree panning capability.
Mirrors. Large plastic domes are often used to conceal a panning, tilting, and zooming television surveillance system from the observer so the observer cannot see the direction in which the camera lens is pointing. Most domes are from 7 to 24 inches in diameter and drop below the ceiling by 6 to 12 inches.
Because the CCTV lens must view through the dome, a typical light loss of 50 percent occurs, and the image distorts somewhat because of the imperfect optical quality of the dome. The amount of light loss and distortion depends on the particular dome used and the part of the dome through which the camera is viewing. A more aesthetic and covert camera/lens assembly is made up of a camera, pinhole lens, and small mirror.
The pinhole/mirror system provides an alternative to some dome applications. The system can be fixed or have a 360-degree panning range, or a limited pan, tilt, and zoom capability depending on the design.
The moving mirror system has two advantages over the dome: there is no large protruding dome suspended below the ceiling and installation is easier, since only a small hole about 3/4 inch in diameter is required to insert the lens and mirror through the ceiling. The small mirror scanning system has one limitation in that it cannot view the scene directly below its location.
The dome system has two advantages over the scanning mirror: for one, the dome serves as a deterrent, since the observer sees the dome and believes a camera is active in it but does not know at any instant whether the camera is looking at him; second, the dome system has the added capability of full-range zoom optics.
Fiber-optics. For covert CCTV applications, all of the previous lenses find wide application when the barrier between the scene side and the camera/lens side is only a few inches, so the pinhole or mini-lens and camera can be mounted directly behind the barrier.
What if the camera must be located 6 to 12 inches behind a thick concrete wall? What if a lens is on the outside of an ATM and the camera is 3 feet away, inside the building? Difficult television security applications are sometimes solved by using coherent fiber-optic-bundle lenses.
Fiber optics are used in surveillance applications when it is necessary to view a scene on the other side of a thick barrier or inside a confined area. The lens is installed behind a thick barrier (wall) with the objective lens on the scene side, the fiber-optic bundle within the wall, and the camera located on the protected side of the barrier. The lens viewing the scene can be a few inches or a few feet away from the camera.
There are three optical techniques to "lengthen" the camera' s objective lens; two involve using a small-diameter "extender" that can be inserted into a hole or other enclosure in front of the camera sensor. The third uses a flexible fiber-optic bundle to transfer the image. The first of these methods is the rigid coherent fiber-optic conduit; the second is the borescope lens. These two special lenses can extend the objective lens several inches to several feet in front of the camera sensor.
The rigid fiber version is a fused array of fibers and cannot be bent. The flexible fiber version has hairlike fibers loosely contained in a protective sheath and can be flexed and bent easily. These fiber-optic lenses should not be confused with the single or multiple strands of fiber commonly used to transmit the time-modulated television signal from a camera to a remote monitor site over a long distance of hundreds of feet or miles.
The coherent fiber-optic lens typically has 200,000 to 300,000 individual fibers forming an image-transferring array. By combining lenses with coherent fiber-optic bundles, long, small-diameter optical systems are produced, which require drilling only a 1/4-inch to 1-1/4-inch hole to position the front lens near the front side of the barrier.
A small aperture hole (dependent on light level available) is drilled completely through at the barrier surface, and the camera and lens are connected on the opposite, protected side. This lens/camera system solves many banking industry and correctional-facility security problems.
A minor disadvantage of all fiber-optic systems is that the picture obtained with these systems is not as clean as that obtained with an all-lens pinhole lens. There are some cosmetic imperfections that look like dust spots, as well as a slight geometrical pattern caused by fiber stacking.
These imperfections occur because the fiber-optic bundle consists of several hundred thousand individual hairlike fibers, some of which are not perfectly transmitting. For most surveillance applications, the imperfections do not result in any significant loss of intelligence in the picture.
Flexible Fiber. When the utmost flexibility (angular movement) between the front objective lens (pinhole or otherwise) and the camera is required, an alternative to the remote-head CCD camera is a coherent flexible fiber-optic bundle. The one significant advantage this lens has over a remote CCD is that there are no electrical voltages within 39 inches of the front lens, which may be important in some applications.
The 39-inch-long fiber-optic bundle is encased in a protective, flexible, braided stainless-steel or plastic sheathing to provide environmental protection (from adverse weather, corrosive environment, or mechanical abuse). It can be twisted 360 degrees with no image degradation.
Although this discussion of the principles and techniques applicable to covert surveillance is not all-inclusive, it provides an overview of some of the major considerations involved in this type of installation. Before attempting to set up a surveillance site, security professionals should gain an' understanding of these design fundamentals. They can then implement a practical CCTV security system that will best meet their company's objectives.
RELATED ARTICLE: Covert Checklist
A general understanding of camera capabilities as discussed here can help a security director select the most appropriate equipment for a particular need. Among the main points one should keep in mind are the following:
* Optical speed or f-number is probably the most important reason for choosing one pinhole lens over another. The lower the f-number the better. An f/2 lens transmits four times more light than an f/4.
* Most pinhole lenses have an FL between 3.8 mm and 22 mm and are designed for 1/3-inch, 1/2-inch, or 2/3-inch format cameras. When viewing through a wall with a wide-angle pinhole lens or mini-lens (3.8-mm, 5.5-mm, or 8-ram), the lens may require a cone-shaped hole or an array of small holes to prevent tunneling of the scene image.
* A short FL lens (5.5 mm) has a wide FOV and low magnification. A long FL lens (50 mm) has a narrow FOV and high magnification.
* Medium FL lenses produce FOVs wide enough to see much of the action and still have enough resolution to identify the persons or actions in the scene. A short FL lens sees a wide FOV and objects are not well resolved. Long FL lenses see a narrow FOV with objects well resolved (clear).
* Under most conditions, the small-barrel, slow-taper pinhole lens is easier to install and is the preferred type over the wide-barrel, fast-taper shape. The user must weigh the pros and cons of both types.
* The use of a straight or right-angle pinhole lens depends on the space available behind the barrier for mounting the lens and camera, and on the pointing direction of the lens.
* The fastest pinhole CCTV system is a mini-lens coupled to the camera. This is the best choice where the lowest cost and highest light efficiency are desired.
* A manual-iris lens is sufficient in applications where there are no large variations in light level, or where the light level can be controlled. Depending on the camera used, where there is more than a 50:1 change in light level, an automatic-iris pinhole lens or an electronically shuttered camera is needed.
* Most applications are solved using an "all-lens" system. In special cases where a thick barrier exists between a surface and the camera location, a rigid coherent fiber optic bundle lens or borescope is used. If sufficient light is available, an "all-lens" borescope type should be used to obtain the cleanest picture. A second alternative is a remote-head camera.
* AC power is preferred for permanent covert camera installations. Either 117 or 24 volt AC wall-mounted converters can be used; however, 24 volt AC is preferred since it eliminates any fire or shock hazard and can be installed by security personnel without the help of electricians. Since most small cameras operate from 12 volt DC, a 117 volt AC to 12 volt DC converter is most popular. For temporary installations, 12 volt DC battery operation is used, with rechargeable or non-rechargeable batteries, depending on the application.
Herman Kruegle is vice president and co-owner of Visual Methods Inc., in Westwood, New Jersey. He is a member of ASIS. This article is an edited excerpt from his recently published book CCTV Surveillance: Video Practices and Technology (Butterworth-Heinemann, 800/366-2665).
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|Date:||Mar 1, 1995|
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