The video camera is the heart of any security system. Stated simply, one could say that CCTV is nothing more than "Electronic Photography." The purpose of the camera is to convert a visual image into a series of electronic images. The information is then converted into an electronic signal that can be amplified and transmitted via a coaxial cable to the display link of the CCTV system.

An understanding of how live video is created helps to answer some very common questions.

In order to achieve greater recording times (12/24/96/120 hour), compromises must be made. The first is to eliminate of one of the two fields producing the video image. This reduces the number of information lines by 1/2 and thus the image quality will suffer.

Each frame of video consists of two fields of information. In a full frame the image is made up of 525 individual scan lines( North American Standard) and, therefore, each field will be 1/2 half of that amount, or 262 1/2 lines.

If a recorder only records a field of information (262 1/2 lines), the playback image will have half of the information of the live video and lower quality image.

As mentioned previously real-time video is 30 frames per second, whereas real-motion is approximately 15 to 20 frames per second. In order for a recorder to record longer periods of time, the number of frames per second must be reduced. This reduction causes a jerky motion playback. It is the installers or users responsibility to program the settings on time-lapse recorders to obtain the desired result for each application.

Roll over the Composite Video to view an actual waveform produced by a camera.

This signal controls switching, character generation (time/date, camera titles, ...), locking of the monitor picture as well as locking of the tape recorder. Any distortion can result in:

Horizontal Information - Each field consists of many single scan lines of information. The diagram below shows just one of those scan lines.

Full Video or Recommended Video Levels

Behind every successful system you will find a well-matched camera, lens assembly, and an installer that is properly trained. This combination of a camera, lens and installer will always be the determining factors in the overall picture quality of a system.

To understand the progress of the digital camera, a quick look at the history of surveillance cameras is necessary. Digital conversion of a video signal has been around for many years. Many CCTV industry advancements started in the consumer electronic arena. However, unlike the consumer video market in which the operator or the subject being viewed can be positioned to create an acceptable picture, the surveillance market must rely on special features or enhancements of the camera itself in order to obtain the same results.

Camera sensitivity determines how well the camera can reproduce a picture in low light levels. All cameras produce images in well-lit applications; however, how low light levels can be reduced and still produce a quality images is based on the sensitivity of the camera selected. Camera sensitivity is measured in foot-candles or lux and appears on the data sheet as follows:

Resolution is a measurement of the camera's ability to reproduce detail. That is the total number image elements that can be reproduced with good definition. It is usually measured in TV lines (TVL) and today's cameras range from 380 to 580 in lines of resolution overall.

Sample    

All cameras will provide usable images during daytime conditions. It is the overall sensitivity of the cameras CCD that indicates how well the camera will perform in low-light applications.

The installing technician has some control over the image quality using settings found on the camera. These settings are described in detail in the Camera Control section.

The first thing to remember is that a surveillance system by itself does not constitute a security system. Cameras cannot respond to alarms, put out fires, or call for help. Cameras provide a visual extension to the area that can be viewed by one person. They are also a means for gathering a permanent video record of the area or incident being viewed..

The degree of protection determines whether a fixed or PTZ camera is used. A camera location that uses a pan/tilt or scanning device will offer a somewhat lower degree of security.

The amount of elapsed time it would take to view the same portion of the scene would be unacceptable. High-speed dome assemblies that have rotation speeds more than 240-degrees a second can help this situation. However, even with this type of system the degree of security may not be sufficient for critical installations. To achieve a highest degree of security, consider using fixed or non-moveable camera locations.

The explanation of coverage can be described as different zones of protection. The outer most zone, known as the Perimeter, is the area where the actual protection begins and can be as simple as a "No Trespassing" sign.

As you approach the protected areas or items, the degree of surveillance increases. There are three basic forms of surveillance.

This method is used in warehousing applications or for viewing large areas. Fixed or pan/tilt cameras can be used for this type of protection. However, the choice is usually determined by the amount of money available for the system installation and not by the actual application requirements.

This third type of surveillance extends to various levels of interests or concerns. These systems usually require the addition of an operator to control the actions of the security system.

So many to choose from and so many different manufacturers. Why so many different types of cameras? Camera selection is based on the system application. Since cameras are the heart of any security system they are usually the first piece of equipment selected. Camera selection can be broken down into five basic categories:

The main difference between indoor and outdoor applications is the need for auto-iris lenses and camera housings. The setup procedure for outdoor cameras includes the use of AGC as well as back focusing of the camera and lens combination.

Resolution is the detail or how well the camera can reproduce the scene it is viewing. The higher the resolution the camera can resolve, the better the picture quality. Applications for this camera include medical, banking, machine vision and robotics, but can be used in many other situations if extra picture detail is desired and the user is willing to pay the extra cost. High-resolution cameras have about 570 to 600 TV lines of resolution.

Low-light cameras can produce a picture at extremely low-light levels--some as low as 0.00001 foot candles of light (starlight condition). The application for this type of camera is limited due to the cost of such a camera. The basic camera starts at around $8,000 to $10,000 each. However, if necessary, this camera is an excellent choice for perimeter applications.

This type of cameras covers a very diverse area and us used in limited quantities. Such applications would include covert, underwater, reverse video, explosion-proof and pressurized cameras.

This question could become a hard one to answer, especially if you do not know where to get the answer. you should never assume is that if your eyes can see it, then the camera should be able to see it also. The human eye is a very sensitive device and can usually see in light levels well below that of cameras. Compare CCTV to photography. In fact, CCTV is nothing more than electronic photography and follows the same rules. There are light meters available for measuring foot-candles or lux (both are measurements of light used in the security industry). Lux is a 10 to 1 conversion from foot-candles.

Understanding that all cameras are not created equal will help solve many problems for installers. First rule of thumb...If a camera produces a quality image during daylight hours, and at night the image becomes unacceptable, the probable cause is normally improper camera selection.

Unfortunately the camera does not look at the light source, it looks at a scene. This refers to the reflection factor or how much light is going to be transmitted from the viewed surface or object. This factor also determines the image quality, especially during low-light level applications. Since this factor varies from 5 to 95%, installers need to know the application for the camera design in order to properly set the limit stops (for PTZ operation) or the direction and angle for a fixed location.

Selecting the proper camera, involves interpreting the specification sheet provided by the manufacturer of the camera. These data sheets list the sensitivity of the camera, but you must know how they determine the figures for their sensitivity. Most major manufacturers use the same guidelines for measuring sensitivity.

If you can match all three requirements in a real-world situation, the camera sensitivity listed on the data sheet would be correct. However, most scenes viewed in security applications usually fall short of the 89.9% reflection factor stated in the data sheet.

Note: as the sensitivity of a camera increases (to see at lower light levels) the more the camera is going to cost.

For installers, knowledge about the areas highlighted in RED are important for the overall success of any system installation and operation.

Any installer who has been in the industry for a few years is very familiar with manufacturer specification documents associated with CCTV equipment. This "data sheet", as it is called, is designed to give all of the information needed to properly select and evaluate a particular camera.

The DATA SHEET..........Fact, Fiction or Confusion.

The selection process of any camera usually starts with camera sensitivity and resolution.

Camera sensitivity tops the list when selecting a camera. It is also the most confusing. Sensitivity was described in the previous section as the measurement used to describe how well a camera reproduces a video picture with a certain amount of light. This light measurement is described in either foot-candles (fc is the light intensity of a one square foot surface, one foot from a source of one candela), or lux (lx is the light intensity of a one square meter surface, one meter from a source of one lumen). However, this number is not always determined by a fixed set of parameters. Each manufacturer has his or her own set of rules on how to determine this figure and this is the area that can become confusing.

To understand camera sensitivity, let's review the camera requirements needed, with respect to available lighting, to produce a video signal. To begin with, the light falling on the scene viewed by the camera is known as the incident or direct light and is measured in foot-candles or lux. This light can be either natural or man-made. The camera does not look at this light directly; it does, however, receive light reflected off the scene.

This reflected light is the light the camera utilizes to produce a video picture. This reflected light is measured in foot-lamberts or nits and can vary from between 5 to 95% of the incident light, depending on the contents of the scene. For this reason, installers will see differences in image quality at low light levels. This is also one of the main reasons for the limit stop setting found in pan and tilt applications. The higher the reflection factor the greater the amount of light that will reach the camera sensor.

The picture quality of a camera is determined by the amount of light reaching the camera sensor. The more light reaching the sensor, the greater the output video signal will be. The value full or recommended video output signal value is one (1) volt peak-to-peak video signal. This output signal is also measured as 140 IRE units. This was the measurement used by the Institute of Radio Engineers in the 1950’s and is still used today by different manufacturers.

We now describe the difference between a full or recommended video signal and that of a usable or minimum video signal. The terms "usable" or "minimum" are used to describe a video signal level that is but a small percentage of a full video signal. This percentage may be as low as 20% or as high as 50%. The percentage factor is not standardized and varies between manufacturers. The main question to ask then when determining whether a picture is usable or not ...Usable to Whom? Each person has their own idea of what a usable picture is and that is the reason for the large percentage range. Again, the lower the percentage used for usable picture, the better the number for camera sensitivity will appear on the data sheet. So between the different reflection factors, the percentages used for a usable or minimum picture quality, the camera sensitivity listed on the data sheet can be very confusing especially if you are trying to compare cameras or camera manufacturers when making any selection.

To further confuse matters, the f-stop of the lens used when testing the camera’s sensitivity is not standard. Each manufacturer can choose their test lens. This can lead to misunderstandings about the numbers associated with camera sensitivity. To understand the effects of lenses on camera sensitivity, let's review basic lens characteristics.

The resolution of a camera defines how much detail the camera can resolve. The greater the number of pixels the finer the picture detail will be. Today's chip cameras list resolution by the number of pixels (picture elements) the camera's sensor incorporates or TVL (TV line pairs). The resolution of cameras can vary from 380 to over 600 TVL (or 5066 to 8000 pixels?) for black and white and 330 and 500 for color. The main term to be aware of when comparing the resolution of one camera with another is the number of active pixels. If the camera data sheet does not list the resolution in TVL (TV line pairs) but only in pixels, a quick conversion can be obtained by multiplying the pixel count by 75%. This gives you the TVL resolution of the camera. Remember, only use the active pixel number, not the total pixel count.

Apparent Resolution is a number sometimes associated with the resolution of a camera, but is not a real number. This number usually is a combination of many different camera features and is used mostly as a marketing tool to help sell the camera. The number is generated from the actual resolution, amplitude response, and aperture correction characteristics of the camera. When combining all of these features, the overall picture will appear to have more contrast and better detail. However, when comparing camera resolutions, compare only the actual number and not the apparent values.

The following is a brief listing of some of the major camera features and their purpose.

The vertical phase control adjusts the vertical sync pulse of multiple cameras that are powered by different AC line phases to ensure proper video switching. Many large applications have 3-phase power systems and therefore it becomes the installers responsibility to make final adjustments.

If a camera has an adjustable, vertical phase control (V-Phase), it should only be adjusted:

An electronic circuit used to automatically adjust the gain of a signal as a function of its input signal. I have a better name for this: "Artificial Gain Control", because this feature takes the existing video level and amplifies it to produce a picture when light levels are very low. This feature not only amplifies the video signal but any noise that is associated with that signal. Because of this , AGC produces a grainier picture at lower light levels than some people want.

There was a time when the following statement was completely true. "No camera or lens combination can produce a good picture when viewing an object with a very bright background." This is no longer the case. A camera feature known as backlight compensation has changed that statement. This feature allows the camera to reduce the video level in the bright areas and then reproduce the overall video signal as an average video. Most cameras now have this BLC feature and more advanced cameras have the means to set parameters for multiple screen applications.

DC or Video controlled lenses automatically control the opening and closing of the iris to allow the proper amount of light to reach the camera's sensor. The difference between the two types is how they accomplish the iris control. In a video-input type of auto-iris lens, the lens requires two inputs. The first input is a positive DC voltage that operates the electronics and the iris motor. The second is a video signal. This video input along with the internal electronics of the lens is used to control the opening and closing of the iris. Too much video and the lens iris closes, not enough video and the iris opens. One can state that this type of lens is self-correcting and does not require any help from the camera to operate. On the other hand, a DC controlled auto-iris lens does not have any electronic circuitry to control the iris opening, and therefore has to rely on the camera to supply this information. Not all cameras have this type of interface and therefore one must be careful when selecting a camera/lens combination.

How to approach camera setup in the CCTV marketplace seems like a mysterious process, since many different cameras, lenses, monitors, and accessories need to be integrated into a system.

The first step for installers to understand is the purpose of camera settings and which are important for each system application. Many adjustments are made prematurely, causing problems in the field.

The line-locking feature of a camera means that the camera's vertical sync pulse locks to the incoming AC power line frequency. This is to insure that all the vertical pulses from many different cameras all occur at the same time. This factor is extremely important when installing a video switcher.

Note: DC operated cameras can not be line-locked

If a camera has an adjustable vertical phase control (V-Phase), it should only be adjusted:

The purpose of vertical phase control is to adjust the vertical sync pulse of multiple cameras that are powered by different AC line phases to insure proper video switching. Many larger applications have 3-phase power systems and, therefore, it becomes the installer's responsibility to make the final adjustments.

The use of a master power supply allows for all cameras to obtain power from the same phase of the AC line. This feature insures proper vertical sync generation and prevents rolling of the video images when switchers are incorporated in the system. However, the installer must remember that all camera AC inputs have a polarity. For this reason, connecting of 24 volt AC cameras to the power source can affect the performance of video switching.

Caution

Reversed polarity to 24 volt AC camera inputs will result in a vertical roll during switching.

There was a time when the following statement was completely true: "No camera or lens combination can make a good picture of an object against a very bright background." This is no longer the case. A camera feature known as backlight compensation has changed that. This feature allows the camera to compensate for video level in the bright areas and then reproduce the overall video signal as an average. Most cameras now have this BLC feature and more advanced cameras have the means to set the parameters for multiple screen applications.

Not all cameras are created equally. The red areas on the first two images indicate the area in which BLC will operate properly. The last image indicates that the entire screen incorporates backlight compensation. Installers MUST know the BLC sensing area to properly position the cameras.

Click here for a BLC Example

With the introduction of smaller solid-state camera formats (1/2 inch, 1/3 inch, 1/4 inch), lenses have also changed. The normal lens used was known as a "C" mount type. This meant the following:

This was a standard used by everyone. Solid-state cameras are now using what is known as a " CS " mount lens. The thread sizes on each of these lenses are the same but the focusing distance has changed. CS lenses require a shorter focusing distance between the lens and the CCD.

Any "C" mount lens will work with a "CS" mount camera by the addition of a 5mm adaptor ring.

NOTE: You can never place a "CS" mount lens on a "C" camera. It will mount properly but the camera/lens assembly can never be focused.

In older camera designs, an auto-iris lens was required to produce the proper video level output from the camera during changing light levels. CCD imager was always at full power. Shuttering or CCD Iris of cameras now changes the sensitivity of the chip, which allows the use of manual iris lenses in applications with small light level variations.

DO NOT use CCD-Iris controls with auto-iris lenses.

An electronic circuit used whereby the gain of a signal is automatically adjusted as a function of its input signal. A better name for this is "Artificial Gain Control" because this feature takes the existing video level and amplifies it to produce a picture when light levels are very low. This feature not only amplifies the video signal but any noise that is associated with that signal. Therefore, AGC produces a more grainy picture at lower light levels.

A note to installers: the rated sensitivity of most cameras includes the AGC circuit amplification. The AGC circuit is selectable and is usually shipped from the factory in the OFF position. Since AGC only operates when light levels are too low for normal camera operation,we strongly suggest that the AGC setting always be placed in the ON position.

As applications increase for cameras, so does the need to increase the quality of information being displayed. Not only does an operator have to be able to evaluate a situation, but he or she must be able to give an accurate description of that evaluation. One way to accomplish this is through the use of color. Color generatesm25% more detail than black and white cameras and this increased detail helps when trying to properly identify an object or person during a security event.

Changing an existing security system to color appears to be a straightforward job -- just replace the existing cameras and monitors with color components. However, a few of the basic design rules do change. Although the basic rules for selecting proper video cables, camera housings, pan/tilts and mounting hardware remain the same, in the areas of the camera sensitivity, type of external lighting, multiple camera synchronization, and lenses, differences do occur. These areas require additional explanation and are applicable to installing a new CCTV system as well as updating one.

Color cameras require about twice as much light to produce a picture with the same quality as that of a black and white camera. Color cameras also require higher minimum light sensitivity to produce useable video. For black and white camera systems, usable picture levels measure between 20 and 30 percent of a full video signal. (Full video picture is equal to 1 Volt p-p or 140 IRE units) for a color camera system to produce a usable picture requires 50 per cent of full video, or 50 IRE units. Below the 50 percent, a color picture will begin to lose its registration, as seen by the human eye. Knowing this, one can see that when updating an existing black and white security system to color, especially for exterior applications, there must be a sufficient amount of lighting available.

Because more lighting is required to produce a useable picture in a color system, what kind of lighting is best? In black and white camera systems almost all forms of lighting can be incorporated - including infrared. Light sources such as: mercury vapor, low and high-pressure sodium, and tungsten, have been widely used for these applications. However, with color, a few more considerations must be taken into account. First, all color cameras have a restricted light bandwidth. The bandwidth for color cameras is between 400 and 790 nanometers, more commonly known as the visible light spectrum. Because of this factor, a color camera cannot take advantage of extended red or infrared light sources (whose IR range is between 800 to 1200 nanometers). This characteristic alone, in some cases, limits the overall sensitivity of a color camera and may cause many headaches for the person designing a system that has available to it a limited amount of lighting.

When checking light sources for using color, whether for updating or for a new system, one must not only consider the amount of light but other areas parameters as "color rendering index" (CRI) and color temperature (measured in degrees Kelvin). Color rendering is how a measure of well a light source is able to produce the actual color of the viewed object without causing a shift in the color. Color temperature determines the basic background color of a light source.

To fully understand what is meant by CRI, an example may help. If you view a green piece of paper with the human eye and the light source is sunlight, the paper will appear green to the eye. If you change the light source from sunlight to a high-pressure (HP) sodium lamp, the same piece of paper will now appear to be bluish in color. Certain types of light sources are better than others for reproducing the "true" color of a sample object. Chart #1 shows the CRI of certain light sources and the range for which each reproduces color. Incandescent or tungsten best matches that of true sunlight in producing proper color rendition; however, the operating costs for these types of systems are very expensive. Most new lighting systems for color today are of the metal halide type. The cost versus performance value of this light source is superior to most other available l

                                                                                                

The overall color accuracy of the system largely depends on the color of the light source. The color from different light sources can range from a dull red to bluish-white. This color range depends on the temperature (degrees Kelvin) to which the lamp is heated. Because of these differences, a color camera must also have the ability to automatically compensate for these changes. This compensation is known as the "white balance" of the camera and is required to produce the proper color reference on the monitor. Most color cameras have an automatic white balance that can adjust between 2800 to 7000 degrees Kelvin. We can also dismiss the use of light sources that do not operate within the normal range and therefore would not be considered a good selection for a color CCTV system.

Just like its black and white counterpart, a color video signal contains sync information, blanking information and video or luminance information. A color signal also must contain a chrominance signal. This part of the video signal contains all the necessary color information needed by the monitor to properly reproduce a color image. By changing the phasing characteristics of this color burst signal the hue displayed on the monitor can also change.

This phasing error is caused by the delay generated by the video cable. The length of the video cable determines the amount of delay. A good example of this would be color cameras used in quad application. If the cameras are viewing the same object and each camera is connected with different lengths of cable, the hue from each camera would not appear the same on the monitor. There would be a color shift between cameras. To correct this situation, one must incorporate external synchronization (called gen-lock or black-burst) for the color cameras in the system. This procedure can add a great deal of extra cable and installation time to a system. Synchronization of color cameras still remains a requirement in today's industry. However, more and more manufacturers are incorporating "time-based correction" circuitry in color switching equipment to help reduce installation time and costs for color systems.

For installers, the need to gen-lock the system is determined by the application and about 99.9% of the time this alignment is rejected.

The last area of discussion is on lenses used with color cameras. As previously mentioned, before most people updating a color system will only replace the cameras and monitors. Proper lens selection for a color system can be just as important as selecting the proper light source. Lens selection for color cameras do not refer to viewing angles or formats as these considerations are the same for B/W cameras. Color cameras, require one to evaluate additional lens parameters. The term used to identify these parameters is known as "color corrected optics". Without color corrected optics the lens will still pass all colors of the light spectrum; however, each of the colors will not focus at the same point on the image plane. This is known as chromatic aberrations of the lens and occurs because the angle of refraction differs for each wavelength or color. To correct this situation, the lens elements must be coated with a special material that allow all colors in the light spectrum to focus at the same point on the image plane. Most lenses in today's marketplace are color corrected. The only precaution suggested here is that when using existing lenses for system updates, insure that the optics will work properly for color applications.

When updating or designing a new system using color cameras, remember a few simple rules.

A composite video signal is the CCTV industry standard. This signal consists of all sync information, video information and color information required by the system. These are combined to produce what we call a composite signal. This signal type is compatible with extended cable runs, important in the CCTV industry. However, there are some drawbacks. The most important one is that not all the information produced by the camera is in color. This is because of the limitation of the color burst used by the composite signal.

The color burst signal operates at a frequency of 3.58Mhz. Since every 80 lines of horizontal resolution requires 1Mhz of bandwidth, this converts to a actual color resolution of 280 to 300 lines. For most applications, this is acceptable, but for those requiring greater resolution , a different approach is sometimes used. For these, S-VHS or Y/C component video is incorporated. In component video the signal is divided into luminance and chrominance. In other words there are separate connections for color and for the video, freeing the system from the 3.58 Mhz color burst limit.

This is why S-VHS images appear brighter and more defined than an image generated by a normal signal.

The answer in most cases is yes. However, there are a few monitors that have problems in this area. If a color monitor does not incorporate special circuitry that eliminates the effects of the absence of the color burst signal, the display might show randomly generated color bursts across the monitor screen. This situation can only be corrected by monitor replacement.

As previously discussed, color camera systems normally require more light to produce satisfactory images during low light conditions. Increasing ambient light levels to produce high quality images is much too expensive and, in some cases, is objectionable to many end-users. However, by selecting lenses which are known as fast or high-speed, you can sometimes overcome this situation. As shown below, special high-speed lenses can increase the overall light reaching the camera sensor by a factor of 4x.

These lenses are costly, but they will produce better images at lower light levels without the need for additional lighting.

For standard color cameras, the answer to this question is NO. The reason is that all color cameras incorporate an IR blocking filter. This is done to insure proper color reproduction. Because the demand also exists for image production at lower light levels many camera manufacturers now include color cameras with automatic switch-over circuitry to B/W. That allow these camera to be used with IR illumination.

The no-iris lens means just that; no adjustment of the iris is possible. The ratio between the lens focal length and the diameter of the iris opening determines the f-stop rating and cannot be adjusted. The number of "no-iris-lens" applications is small, however, with the introduction of automatic shuttered cameras (CCD Iris) , the no iris lens has gained some popularity.

Manual iris lenses have adjustable iris openings between F1.4 to F-22. The lower the F-stop number, such as F1.2 or F1.4, the greater the amount of light that can pass through the lens assembly.

This adjustment of the iris can be accomplished manually or automatically. In the manual mode, the iris is adjusted to produce the proper video picture with a given amount of light. If the light level changes, a manual iris lens must again be adjusted to compensate for these changes. Like the no iris lenses, manual lenses require fixed lighting applications or the use of shutter cameras to produce a proper video image. The no iris and manual iris lenses also have a limited amount of light attenuation (maximum f-22) and cannot be used in an application where light levels are extreme.

DC or Video controlled lenses, automatically control the opening and closing of the iris to allow the proper amount of light to reach the camera's sensor. The difference between the two types is how they accomplish the iris control. In a video input type of the lens requires two inputs. The first input is a positive DC voltage that operates the electronics and the iris motor. The second is a video signal. This video input along with the internal electronics of the lens is used to control the opening and closing of the iris. Too much video and the lens iris closes, not enough video and the iris opens. One can state that this type of lens is correcting and does not require any help from the camera to operate. Whereas the DC controlled auto-iris lens ,however, does not have any electronic circuitry to control the iris opening, and therefore has to rely on the camera to supply this information. Not all cameras have this type of interface and therefore one must be careful when selecting a camera/lens combination.

                                                                                                                                       

For outdoor applications or applications with large light level changes, DC or Video controlled auto iris lenses are used. These two types of lenses automatically adjust the iris element as the light level changes. Besides automatically adjusting the iris, most of these lenses also incorporate a spot filter. The purpose of the spot filter is to enable the lens to attenuate the light by a higher degree. This allows the proper amount of light to reach the camera sensor without overload when light levels reaching 10,000 to 13,000 foot-candles.

Note: A spot filter is a neutral density material placed on the lens element to attenuate the light passing through the lens. The spot filter size can increase the range of the lens to as high as F-1500. Normal range is between F-360 to F-500.

The next area is the lens iris and how it affects light passing through the lens. Like the human eye, as the light becomes brighter, the iris of your eye closes to allow the proper amount of light to react with your retina to produce a good quality image. It is the same with lenses used in CCTV. The type of iris used by lenses can be divided into four groups; non-iris, manual iris, video and DC-controlled iris.

NOTE: Each increase in the f-stop number will close the iris by 1/2 thus decreasing the amount of light reaching the camera's sensor by a factor of 50 %.

                                                                                               

The word "fixed" when referring to lenses means that the angle or the area that the lens views remains constant or is not variable. The focal length (mm) of a lens is the distance between the image plane (camera sensor) and the center of the optics of the lens. As the focal length increases, the area being viewed decreases or becomes more telephoto. The focal length of a lens is measured in millimeters. Like the the focal length of the lens, the format of the camera's sensor also affects the angle of view. The formats used in today's cameras are 1 inch, 2/3 inch, 1/2 inch and 1/3 inch. Adding to the confusion 1/4 inch chip cameras have just been introduced into the marketplace.

The viewing angle of the lens or how wide an area the lens will see is measured in millimeters. This number is found on each lens. The more common fixed focal length lenses are:

2.1 mm - 4.8 mm - 8 mm - 16 mm - 25 mm - 50 mm - 75 mm

The two lens mount assemblies are known as C and CS. The mounting parameters are the same for both. The difference is the distance required to focus each of the lenses.

Like the camera sensor, lenses also have different formats.

                                                              

As an added note, smaller formatted lenses placed on a larger formatted camera produce an image with shaded corners. ( Refer to CCTV 101 Module 2 Camera Theory)

What do you want to be able to view?

This question can be broken into a number of sub-topics that can help in the selection of a lens for any application.

Depth of Field is the area within which an object located in the field of view remains in focus. As the lens aperture closes (higher F-stops) the greater the focused depth becomes.

This is very important especially in applications such as hallways where everything should remain in focus.

As the iris of a lens starts to open (smaller f-stop rating) the depth of field distance decreases.

A common way to increase the range of any given lens is the use of an extender. This piece of glass is situated between the camera and the existing lens and usually results in a multiplication factor of two.

The word "fixed" when referring to lenses means that the angle or the area that the lens will view remains constant or is not variable. The focal length (mm) of a lens is the distance between the image plane (camera sensor) and the center of the optics of the lens. As the focal length increases, the area being viewed decreases or becomes more telephoto. The focal length of a lens is measured in millimeters. Like the focal length of the lens, the format of the camera's sensor affects the angle of view.

The next group of lenses is the varifocal lens. This lens combines a normal and wide viewing angle into a single lens assembly. The same characteristics discussed for fixed focal lenses are true for the varifocal lens. This type of lens allows the installer more flexibility when trying to accurately set the view angle for each customer. The normal viewing angle for varifocal length lenses is usually about 30 degrees, with the wide angle being between 60 and 80 degrees. Since 65% of all applications select lenses in this normal to wide range, the varifocal lens has gained much success in the CCTV industry.

The last of the three basic lens groups is the zoom lens. Like the fixed focal length and varifocal length lenses, zoom lenses have the same overall characteristics for iris control and mounting, with one exception: the focal length of the lens can change. This lens is designed so that its focal length can be adjusted by manual movement of an external ring on the lens or by a remote controlled motor assemblies. Unlike the varifocal length lenses, zoom lenses offer much greater focal length range. Because of this, zoom lenses are classified by the zoom range: a 6 to 60 mm range , for example, is called a 10X zoom lens. However, not all 10X zoom lenses are 6 to 60 mm. When selecting a zoom lens make sure you list the range of the focal lengths that you require, not just the power of the lens. Since the focal length of the lens can be changed, provisions to remotely focus the lens must also be available. Zoom lenses are normally associated with systems that incorporate a pan/tilt unit offering wider flexibly to the CCTV operator. This increased flexibility requires a corresponding increase in system setup.

One of the main problem associated with lenses, whether fixed, varifocal, or zoom, is the need for back focus and focus/zoom tracking. The purpose for these two procedures is to keep the lens in proper focus during day/night operation and throughout the zoom range of the lens. Most of the time, this procedure is forgotten and the net result is a system that is unsatisfactory to the customer.

For many applications, standard lenses or configurations may not be what your customer wants. The need for special lenses for either covert, color or IR applications may be required. A brief description of some of these lenses is described below.

A pinhole lens is a lens that has a very small front opening diameter. This opening usually is 0.06 to 0.25 inches in diameter and is used for covert security. The lens variations include straight and right angle, manual or auto-iris, narrow taper or stubby front design. Due to the special lens optics required, the minimum f-stop rating of these types of lenses are relatively high and extra light may be required in order for the camera and lens assembly to produce a quality picture.

Aspherical lenses offer a higher light transmission factor with less-severe image aberrations and distortion then that of a normal spherical lens. The shorter the focal length (the wider the viewing area), the more likely one will see a pincushion effect when using a normal spherical lens. Aspherical lenses should be considered when a wide angle, low distortion, high speed lens is required. This lens, in combination with low light sensitive cameras, may sometimes eliminate the need for additional lighting or intensified type of cameras.

When upgrading to a color system, many system installers will only replace cameras and monitors. Proper lens selection for a color system can be just as important as selecting the proper light source. Proper lens selection for color cameras does not include viewing angles or formats as these considerations are the same for B/W cameras. Color cameras, however, do require additional lens parameters. The term used to identify these parameters is known as "color corrected optics". Without color corrected optics, the lens will still pass all colors of the light spectrum. However, each of the colors will not focus at the same point on the image plane, causing an out-of-focus condition. This distortion is known as chromatic aberration of the lens and occurs because the angle of refraction differs for varying wavelengths of colors. To correct this situation, the lens elements must be coated with a special material. This coating causes all colors to focus at the same point on the image plane.

.

                                                         

In the same way, color applications using infrared lighting has the same distortion if properly corrected lenses are not incorporated. The focal points for visual and IR light are different and therefore cause focus problems between day and IR (infrared) operation. This is very noticeable when using Day/Night/IR cameras with auto-focus.

The formula used to determine the lens size for any given application requires the following information:

From the formula, one can see that the format of the lens is a not a requirement in determining the viewing angle. As a result, changing the format lens used does not cause any change in the viewing angle. The only requirement is that the lens format be equal to or greater than that of the camera used. This prevents shading of the video picture, more commonly known as the porthole effect. Apparent change in the focal length of the lens occurs only when the camera format changes.

Most lenses from manufacturers are set for standard applications. However, each and every installed lens should be adjusted for optimum performance.

The level control is found on all auto-iris lenses. This control is used to set the operating level of the video output of the camera. The control settings on a cameras/lens assembly is as follows:

The level control is factory adjusted to ensure proper 1-volt video output signals from the camera. If adjustment is required, the following is a simple procedure to pre-set the level control for a reasonable image.

NOTE: This adjustment MUST be done during daylight hours.

If the image is still not acceptable, bench testing and configuration of the lens may be required. This procedure requires the use of a video measuring device (oscilloscope or video meter).

Some lenses may also have an ALC (automatic light control) adjustment. This adjustment determines the response to light by the level control of the lens.

For objects requiring identification that appear in bright areas of a scene, set the lens to peak (P) or fully CCW.

From the same initial scene conditions, if the area of importance is not in the bright area the lens should be set to the average (A) mode of operation. The installer has the opportunity to adjust this control to obtain the best result. Normal setting is usually mid-range.

The term "Back Focus" refers to the position of the CCD sensor behind the lens. This adjustment must be accurate or the camera performance will be out of focus during low light operations.

.