By Bob Mesnick.
IP camera manufacturers provide product specification sheets that help in the selection of the right camera for individual IP security and surveillance systems. But which specifications are important? They include such things as resolution, minimum light sensitivity, lens, wide dynamic range, signal to noise and so on. The importance of each of the camera specifications depends on the objective and application. For example, the low-light specification is important if the camera is to be used outdoors where it can get dark. If the IP camera will only be used indoors, the viewing angle width may be more relevant. This article reviews the important camera specifications and how to avoid being fooled by ‘specsmanship’ (from the marketing department).
This is an important specification. Resolution is usually defined by the total number of pixels in the sensor or the horizontal and vertical pixels. Marketing people also use the terms ‘HD’, ‘720p’ or ‘1080p’ and, most recently, ‘4K’.
- Megapixel camera: This is a general term used for any camera that has over one million pixels in the sensor. There are many cameras that have over one megapixel resolution. The pixels are organised in a matrix of horizontal and vertical pixels. The relationship between the horizontal and vertical pixels is called the aspect ratio. The aspect ratio (vertical to horizontal ratio) is usually 4:3 or 9:16 (wide). The latest sensors (especially the ones that claim 4K resolution) have different aspect ratios that are similar to the very wide formats used in the cinema market.
- HD camera: This is more of a marketing term and is defined as either a 720p or 1080p type HD camera. This specification comes from the video broadcast market rather than the security market. The 720p refers to 720 horizontal lines. The ‘p’ indicates that the lines are progressive rather than interlaced. There is only one scan per frame that includes 720 lines. The 1080p HD camera has at least 1080 horizontal lines.
720p cameras usually have a sensor with at least 1.0 megapixels. Pixel resolution is 1280 x 1024, or it can be 1280 x 800. 1080p cameras have at least a 2 megapixel sensor and are considered to be the higher resolution HD cameras. To add to the confusion, some manufacturers call their 3 megapixel or 5 megapixel cameras 1080p as well.
- 4K camera: This usually refers to a camera with over 8 megapixels of resolution. It has approximately 4,000 horizontal pixels. The number of vertical and horizontal pixels and the aspect ratio are defined differently. For example, Sony announced that their new (coming soon) camera has a chip with 4096H × 2160V pixels, which runs at up to 60fps. Axis announced a camera with 3840 x 2160, which runs at up to 30fps (which is called Ultra HD). The marketing people call all these cameras 4K.
The resolution of a camera is defined not only by the sensor, but also the lens and the electronic circuits. Megapixel IP cameras are sometimes advertised for under $200. Be careful; the sensor may have the megapixels, but the lens may be plastic and the result is a very low quality image.
Minimum Illumination (or Low-Light Sensitivity)
The minimum illumination is the lowest light level (measured in lux) that provides a reasonable image from the IP camera. This can be very subjective and depends on what the viewer thinks is an acceptable image. Low-light images are not only dark, but can also be very noisy.
At the low-light level, the amplifiers are working very hard and there can be circuit noise that affects the video image. This is called the signal to noise (S/N) ratio. The better manufacturers also include the relative level of the signal (IRE), which is a measure of how hard the amplifier has to work. For example, a camera that is operating at 30 IRE is receiving 30 percent of the signal from the sensor circuits, while one operating at 50 IRE is receiving 50 percent of the signal. The lower the number, the harder the amplifier has to work to boost the signal so it can be seen. The noise level can be as high as 20 percent of the signal, so the resulting video can look very noisy when the signal level is very low.
The minimum light level is also affected by the shutter speed, which relates to the frame rate. The longer the shutter is opened, the more light can reach the sensor. The longer the shutter stays open, the lower the frame rate. There are some camera specifications that indicate very low minimum illumination (0.0001 lux), but this is measured at a shutter speed of 0.5 sec. This translates to a maximum frame rate of 2fps.
The minimum illumination level is also determined by the lens. The lower the f-number of the lens, the more light it will let through.
Many IP cameras come with a lens. A wide angle lens could be used to view a small room, while a narrow angle lens (with more magnification) can be used to see an area that is far away. The lens can also affect some of the other specifications, such as minimum illumination, frame rate and resolution. If the IP camera does not include a lens, it usually includes a standard type CS (or C) mounting capability, so various third party lenses can be used.
- Lens angle: The lens is measured in millimetres. The lower the number, the wider the viewing angle. A 2mm lens may have an angle of about 110 degrees, while a 50mm lens has an angle of about 5.5 degrees. The angle of the lens depends on the size of the sensor and the distance from the sensor to the lens.
- Lens light capability: The f-number of the lens indicates how well the light is transferred through the lens. A camera with an f-number of f1.2 can pass more light than one that has an f2.0 lens. The lens angle can affect the f-number– the wider the angle the more light can get in, so the illumination specification is usually measured at the widest lens angle (when a variable lens is included).
- Types of lenses: There are fixed, variable and zoom lenses. A fixed lens, as the name implies, has only one mm or angle setting. A variable lens can be manually adjusted through a range of angles. A zoom lens is similar to the variable lens, but it can be controlled remotely.
- Iris: Iris control adjusts how much light falls on the sensor and affects the depth of field. The smaller the iris opening, the longer the depth of field. When the scene is very dark, the lens iris opens and the field of view is reduced. This means some areas that are close or far away are not in focus. There are manual iris controls, DC auto iris and p-iris lenses.
- Manual iris is manually adjusted and depends on how much light is expected in the scene. A manual iris lens is usually used with indoor cameras where there is a small light variation.
- DC auto iris lenses are usually used with outdoor cameras. The camera electronics adjust the iris opening depending on how much light it detects. At night it opens the iris and when there is bright sunlight it closes the iris.
- P-iris lenses are similar to the DC auto iris lens, except they add additional intelligence to the lens opening. When the iris is closed all the way, it can reduce the clarity of the image (when used with megapixel cameras). A p-iris camera system works with the camera electronics to prevent the iris from closing all the way. Conversely, when the view is very dark, the camera tries to open the iris to let in as much light as possible. As the iris increases, the depth of field is reduced. A p-iris lens prevents the lens from opening all the way and compensates by increasing the camera amplification of the video signal.
- Focus: The focus of the IP camera can be adjusted either by adjusting the lens or by remotely adjusting the distance between the lens and the sensor (back focus). Lenses can have a fixed focus (it cannot be changed) or manual focus. Back focus is not part of the lens, but is usually listed with the lens specification. It makes installation much easier, enabling the focus to be adjusted at the computer instead of at the camera.
- Infrared (IR) corrected lenses: Lenses bend the light to achieve the right focus and magnification. IR light can bend at a different angle than visible light when the wrong type of glass is used in the lens. IR corrected lenses compensate for the focal difference and provide a much clearer image. This is most apparent at higher resolution.
Special Applications that Require Specific Capability
- Wide dynamic range: To view an area with challenging lighting conditions, it is best to select a camera that provides good, wide dynamic range (WDR). For example, a camera that either provides back light control or, better yet, WDR, will be needed to view a lobby with a large window. Wide dynamic range has been dramatically improved in the latest cameras. Older cameras provide about 50dB of WDR; newer models provide over 120dB.
- Frame rate: Frame rates are not particularly critical in most applications, but most megapixel cameras support reasonably good frame rates. What is reasonable? It depends on the application.
Video is very smooth even at 10fps. There are a few applications where higher frame rates may be needed, such as in gambling casinos, or in special testing situations, where there is very fast motion; but in most cases, users can save bandwidth and storage by reducing the frame rate.
In certain cases, the frame rate has to be reduced. For example, frame rates tend to be lower in very high-resolution cameras (over 3 megapixel). The frame rate can also be affected by very low-light situations. By increasing the time that the shutter stays opened, users can improve the low-light performance of most cameras. But be careful; increasing the shutter speed to 0.5 seconds changes the maximum frame rate to only 2fps.
- Audio: Audio capability can be very important in some applications, such as when connecting an intercom to the camera. In this case, a camera with audio input capability is required. Some cameras have a built-in microphone, but be aware that it is always best to have the microphone close to the person talking.
- Input and output (I/O): Depending on the application, input and output connections may be required. For example, to release a door lock, an output signal to control a relay that can open an electric lock is needed. To detect that a door has been opened and start recording video, then an I/O input connection to the camera is required.
Understanding the specifications allows users to select the right camera for their IP camera system; however, before reviewing the specifications, users should fully understand their required application and objectives.
Bob Mesnick is President and owner of Kintronics, an organization specializing in providing network attached IP camera surveillance and security systems. With over 30 years experience in the technology industry, Bob also provides specialist advice on optical storage systems and network attached storage systems. He can be contacted at www.kintronics.com