- What is resolution?
- What are effective pixels?
- What is lux?
- CCTV Glossary
- All About CCTV Camera
- Selecting the right camera
- Lens selection guide
- Field of view sample
What is resolution?
We've all been in an Electronic store where they have an array of plasma and LCD's mounted side by side. We hear the term "higher resolution" in a sales pitch. When we refer to "resolution", it simply means the "overall picture sharpness" and is measured by counting both horizontal lines (vertical resolution) and vertical lines (horizontal resolution) of an image. Refer to the diagram below. The white lines are also counted as a line in the picture. In other words, resolution quantifies how closely the lines are to each other and yet still be visibly counted. The higher the number of TV lines, the higher the resolution and the more details we can see.
However, when we are talking about "resolution" in terms of a CCTV camera, we only refer to the "horizontal resolution" because we cannot change the vertical resolution, which is limited to the number defined by the TV scanning system. (We can change the horizontal resolution, and it depends on the horizontal resolution of a camera, the quality of the transmission media, and the resolution of the monitor.
The horizontal resolution of a camera is treated a little differently unlike the vertical resolution. The horizontal resolution is also defined by the number of vertical lines which can be captured by a camera and reproduced on a TV screen, but we only count the vertical lines based on the TV aspect ratio of 4:3, to preserve the natural proportion of an image. For an example, 550 TV lines of horizontal resolution will correspond to a maximum of approximately 733 lines (550 X 4/3) across a screen. This is why we call the horizontal resolution as "TV Lines (TVL)" instead of just "vertical lines". In other words, if a camera manufacturer says that their camera supports 550TVL resolution, it means that the camera outputs its TV picture to support a monitor with the resolution of 550 TV lines, based on the TV aspect ratio of 4:3. Remember, the resolution of the camera is only as good as the resolution output of the monitor.
CCTV cameras can be categorized into two groups: high resolution camera and standard resolution camera, in terms of CCD resolution. The CCD resolution is different from the camera resolution. If you check a camera specification sheet, you might find specifications named "Number of Total Pixels" or "Number of Effective Pixels". These specifications tell you what resolution CCD is used in that camera. In general, a camera, which uses a CCD with the resolution of 380K (768 x 494) effective pixels, it is a high resolution camera. If a camera uses a CCD with a resolution of 250K (510 x 492) effective pixels, it is considered a standard resolution camera. The high resolution (380K effective pixels) CCD is used to make a camera with 480 TVL resolution and above. On the other hand, a camera which supports a lower resolution than 480TVL, uses a CCD with 250K effective pixels. Thus, you can say that there might be a considerable difference in sharpness between 420TVL and 480TVL cameras. If you see a camera, which uses a CCD with 250K effective pixels but is advertised as 480TVL resolutions or above, it would be wise to confirm the actual resolution. Otherwise, you will end up paying more than you should.
Now that you have an understanding of what resolution means, some of you might wonder how we can count the TV lines to determine the degree of resolution. There are several industrial/international standards to test the degree of resolution, such as IEEE-208, EIA-1956, CCTV Labs Test, etc. CCTV manufacturers perform resolution tests based on these standards.
What are effective pixels?
According to the definition listed on Standard "DCG-001-Translation-2005", effective pixels are the pixels on the image sensor (CCD), which receives input light through the optical lens, and which are effectively reflected in the final output data of the image. Simply, effective pixels are the pixels which are actually used to produce the final image. The remaining unused pixels are not counted in the Effective Pixel count, but rather are found in the Total Pixel specification.
Refer to the figure below which describes the pixel-related definitions. The rectangular box including the optical black area is a CCD. In the figure, you can see the image circle of a lens. That circle can be adjusted by changing the focal length of a lens. The optical black area is not used. Area C is also a part of the effective pixels but this area is not guaranteed by an opto-electric conversion element vendor. Thus, the maximum number of effective pixels guaranteed by a CCD manufacturer would be up to Area B. Area A is the number of effective pixels limited by an image circle of a lens and this area can be expanded up to Area B by adjusting the focal length of a lens. If the image circle of a lens is bigger than Area B, Area A will be limited by Area B.
Fig1. Pixel-Related Definitions
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Area A: Number of effective pixels limited by an image circle of a lens
Area B: Maximum number of effective pixels guaranteed by an opto-electric conversion element vendor
Area C: Maximum number of effective pixel on a CCD
Area D: Number of total pixels on a CCD
A CCD manufacturer discloses both the total pixel and effective pixel specifications, and most camera manufacturers use the specs for their own purpose. However, some camera manufacturers will only indicate the number of total pixels (or can be described as "number of pixels") on their specification sheet. This is so they can use it to their advantage when marketing their product. This issue has been causing some confusion among consumers when comparing different brands of the same product.
As we discussed in the previous issue, CCTV camera manufacturers predominantly use a CCD with 250K (510H x 492V) effective pixels to make a CCTV camera under 480TVL resolution and a 380K (768V x 494H) effective pixel CCD to make a 480TVL or higher resolution camera, unless they explicitly indicate that they are using a special CCD.
Now that you have a better understanding of what an effective pixel is and the difference compared to total pixel, you can review a camera spec sheet for accuracy and judge for yourself the integrity of the product.
What is Lux?
The lux (symbol: lx) is the International System of Unit (SI unit) of illuminance and luminous emittance. SI unit is the modern form of the metric system and is generally a system of units of measurement. Lux is used in photometry as a measure of the intensity, as perceived by the human eye, of light that hits or passes through a surface. In the CCTV industry, Lux is defined as how sensitive the camera is under low light conditions.If you look at a specification sheet, the sensitivity is defined by a number, but more specifically, it will be followed by a F-Number.
What does all this mean?
Before we can interpret the lux rating, it would be helpful to define the scientific meaning of lux.
What is scientific definition of Lux?
Lux is an origin unit based on lumen. One lux is equal to one lumen per square meter; lumen is the unit of luminous flux, a measure of the power of light perceived by the human eye.
1 lx (lux) = 1 lm/m2 (lumen per square meter)
The difference between lux and lumen is the lux takes into account the area over which the luminous flux is spread.
For simplicity, let's say a flashlight is 100 lumens and is lit up in an area that is one square meter. The actual illuminance would be 100 lux. Now, let's take that same flashlight and take it to an area that is spread out over 10 square meters. This will produce a dimmer illuminance of only 10 lux.
The chart below will give you an idea of how bright a lux is in practical terms.
How is lux determined?
Lux is a subjective value and is determined by each manufacturer's method of testing.You may have seen the sensitivity expressed as 0.5lux, 0.5lux@F2.0, 0.5lux/F1.2, or other variations.There is no standard ISO regulation when it comes to measuring lux, so the same camera may rate a different lux by different manufacturers. The reason is because the manufacturers do not use the same parameters when testing lux.
The outcome will depend on the following factors:
F Stop: F stop of a lens is the area of the aperture that determine the amount of light allowed to enter the CCD sensor.The lower the F stop number, the more light it is able to absorb.
As seen in this diagram above, the lower the f number, the bigger the aperture is, which means the absorption of light is much greater affecting the lux of the camera. ie. 1 lux / F10 equal to 0.01 lux / F1.0 since it will absorb 10 times more light than F10 lens.
The characteristic of visible light that is expressed in Kelvin (K). It is important in which Kelvin environment the camera sensitivity is tested under. As seem in the diagram below, each color temperature has its own wavelength (nm). High color temperature such 5,000 K or more, represents cool blue, which has low wavelength (~400nm). Low color temperature such 3,000K below, represents warm yellowish red color, which has higher wavelength (~800nm). However, this difference of wavelength will bring critical difference in lux testing. Spectrum of 500nm light source will produce more electrons than 700nm light source. Reflection ratio
Reflection ratio of the object and it's background will significantly distort the measurement. An object with 100% reflection rate will generate 100 times more light on target plane then an object with 1% reflection rate.
Unit used to measure composite video signals. 100 IRE equals to 700mV, this is full video signal which will operate at the best image, contrast and brightness. 50 IRE is the half of it and 10 IRE is 10% or original amplitude or 70mV. Generally acceptable video signals are 30 IRE as minimum requirement for fair image. Company A specs it's camera at 0.01 lux @ F1.4 (AGC on). Company B specs it's camera at 0.1 lux @ F1.4 /50 IRE (AGG off). At first glance,Company A's camera has a more appealing lux value, but when both cameras are compared side by side under the same conditions, the Company B's camera will be much more sensitive under low light.
Signal to noise radio
The S/N ratio measures how well a camera signal can transmit in low light conditions.The S/N ratio is expressed in decibels (dB).A highly sensitive CCD will pick up a higher noise level, so in practice the sensitivity is reduced some what in order to minimize thenoise. Noise is also proportional to temperature, so it is inevitable to have a noise free.
CCTV & Video Surveillance Glossary
Auto electronic shutter (AES) Feature of a camera to adjust for light changes without the use of an auto-iris lens.
Auto iris A special camera lens with the ability to open or close its iris automatically in response to changing light levels.
Back-light compensation (BLC) The ability of a digital security camera to adjust for bright background lighting that would normally cause the subject to appear too dark.
C-mount A type of camera lens mount that enables different lenses to be swapped out and used on a security camera.
CCD The light-sensitive imaging microchip found in digital security cameras.
Digital video recorder (DVR) A special computer that converts analog computer images to digital images, compresses the images, and then stores them for later viewing. A DVR replaces the time-lapse VCR, multiplexor and switch found in analog CCTV surveillance systems.
Duplex An electronic device used to record and display camera images at the same time. A full-duplex DVR can record camera images while displaying images from a different camera at the same time.
Housing Protective enclosure that a camera can be placed in to protect it from outside weather conditions.
Lux Unit of measure of light sensitivity for a camera. Sensitive cameras can operate with low levels of lux.
Multiplexer An analog device found in older CCTV systems that allows multiple cameras to be displayed simultaneously on a single CCTV Monitor. Multiplexers can also be used to transmit multiple camera images at once over a single cable.
Pinhole camera A spy camera with a lens that can see through a tiny hole. These camera are usually hidden.
Power-over-Ethernet (POE) Device that allows one to transmit power to a security power through an Ethernet network cable.
PTZ Stands for pan-tilt-zoom. PTZ cameras have motors that allow them to be remotely moved up-down, side-to-side, and the camera lens zoomed in or out.
Quad Analog CCTV test monitor equipment used to display 4 camera images simultaneously on a single monitor.
Real-time recording For digital video, 30 frames-per-second per camera allows no jerkiness in the video.
Remote surveillance The ability to view a camera image that is located remotely, where the video image is transmitted over a phone line or the Internet.
RG-59 A type of coaxial cable used in CCTV systems.
Signal-to-Noise Ratio (SNR) The ratio of video signal to noise. This is a measure of how much signal noise the camera can withstand and still present a good image. The higher this number is, the better the picture quality.
Switch An analog device found in older CCTV systems that takes multiple camera inputs and displays them on a monitor one at a time (unlike a quad).
Time-lapse VCR A special VCR found in analog CCTV systems designed to slow down the recording rate in order to store many hours of video footage on a single videotape.
Varifocal lens A camera lens in which the focus is not fixed and that can be adjusted either manually or automatically.
Video gain Also called video amplification, this is the increase in video signal power by an amplifier.
Watch-dog timer The automatic reboot of a DVR system whenever a problem is detected.
All about CCTV
Bullet cameras are shaped like cylinders, and get their name from the fact that they resemble bullet cartridges. Bullet cameras come with weatherproof housings and are often used as part of outdoor CCTV surveillance systems.
Most bullet cameras have a fixed 4mm lens. A 4mm camera lens lets you to see facial features out to approximately 35 feet. A 4mm lens allows provides about a 70-degress viewing angle, which is the widest angle you can have without suffering picture distortion.
Day/Night Security Cameras
Day-night cameras, also called low-level cameras, employ a very sensitive digital chip that can capture scenes in very low-level lighting conditions. During the day, the camera takes images in color while at night it automatically switches to black-and-white mode when the light level drops a certain amount. These cameras do need some light in order to take images, even if it is the light of the moon or stars.
Some people confuse day-night cameras with infrared cameras but they are not the same. The latter use infrared illumination for their operation to see without any light at all.
Dome cameras gets their name from the plastic dome housing that the camera is enclosed in. They are most often seen in retail businesses and office buildings. Often the dome is dark tinted making it hard to see where the camera is pointing. Dome security cameras are very tough and some models come with high-impact polycarbonate housings that will withstand heavy blows, making them ideal for locations with high-potential vandalism.
Hidden cameras, also called spy cameras or covert cameras, are designed to be very small and either hidden from view, concealed in everyday objects, or disguised. Most hidden camera are pinhole cameras, which feature a small lens that can see through a tiny pinhole opening, such as behind a wall.
Infrared cameras, also called night-vision cameras, use an infrared light source near the camera lens to illuminate the area with infrared light, which people cannot see. This allows the infrared camera to see in conditions with no light at all, such as inside dark offices at night. With just a slight amount of normal light, an infrared camera can take a picture that looks as good as that in daytime. Most infrared cameras take pictures in black-and-white.
One problem that infrared security cameras can experience when placed in outdoor housings is the light reflection from the front glass cover of the housing. By placing the camera lens flush with the housing, this problem can be minimized. Some people confuse infrared cameras with day-night cameras but they are not the same. The latter do not use infrared illumination for their operation.
Outdoor Security Camera
Outdoor security cameras are housed in special weatherproof enclosures or housings that protect them from tough weather and temperature conditions such as rain, snow, wind and the sun. Some enclosures utilize electric strip heaters and fans. The strip heaters help keep the humidity down for the electronics, prevent condensation from forming on the housing cover and camera lens, and provide heat in cold environments. Other security cameras meant to be placed in high-crime or high-vandalism areas feature tough "armored" enclosures that can withstand heavy blows.
Pan-Tilt-Zoom (PTZ) Camera
PTZ security cameras feature a motorized mount that allows the camera to be moved remotely up-down and side-by-side. In addition, the camera has a motorized zoom lens that can be moved in or out.
Digital Video Recorder (DVR)
Digital video recorders (DVRs) have replaced older analog CCTV system equipment such as multiplexers, quads, time-lapse VCRs and videotapes over the last few years.
A digital video recorder allows live camera images or previously-recorded video images to be viewed on your computer over a computer network or over the Internet. All digital video recorder equipment can be setup to record only images where motion is detected on the camera field of view. This saves you from having to play back hours of video footage that doesn't change. Digital video recorders for video surveillance are available with 4, 6, 8, 10 and 16 video camera inputs.
A DVR can be connected to a CD or DVD burner, computer hard drive, or other mass storage device. PC-based DVRs are also available - these consist of a PCI card that is installed in the computer along with software. This kind of DVR with CD burner can be set to email you or even call you on your cell phone if a motion is detected one of the cameras.
Selecting the Right Camera
Ask yourself for what purpose you intend to use a security camera or camera system for: real-time viewing, store surveillance (which means recording and storing video footage), one room of a house, access doors for an office building, etc.
Color vs. B&W
How important is it to have camera images in color? Black-and-white security cameras generally have greater sensitivity at low light levels and thus are cheaper than an equivalent color cameras.
Indoor vs. Outdoor
If the camera is mounted outdoors, be sure and get an appropriate enclosure for the environmental conditions likely to be encountered. This can include rain, snow, heat, cold, sun glare, humidity and corrosive atmosphere.
Fixed lens vs. zoom lens
Fixed-lens cameras are much cheaper than zoom lenses. Ask yourself how important it is to be able to remotely zoom in or out a scene.
Fixed mount vs. pan-tilt camera mount
Similar to the lens issue, ask yourself how important it is to be able to move the camera up, down, or side to side.
Note: One pan-tilt-zoom (PTZ) camera can replace the job of several fixed mount, fixed lens security cameras.
Open vs. hidden camera
Do you care if the security camera is visible or is covert operation of the camera important?
Real-time vs. later viewing
Do you need to be able to view camera images in real-time or is storing images on a video recorder for later viewing acceptable?
Local viewing vs. remote viewing
Do you want to be able to view images from anywhere? This would mean getting a network IP camera that can be accessed over the Internet.
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Lens Selection Guide
The focal length of the lens is measured in mm and directly relates to the angle of view that will be achieved. Short focal lengths provide wide angles of view and long focal lengths become telephoto, with narrow angles of view. A "normal" angle of view is similar to what we see with our own eye, and has a relative focal length equal to the pick up device. Our online lens calculator is a simple to use device for estimating focal length, object dimension, and angles of view.
The lens usually has two measurements of F stop or aperture, the maximum aperture (minimum F stop) when the lens is fully open, and the minimum aperture (maximum F stop) just before the lens completely closes. The F stop has a number of effects upon the final image. A low minimum F stop will mean the lens can pass more light in dark conditions, allowing the camera to produce a better image at night. A maximum F stop may be necessary where there is a very high level of light or reflection, as this will prevent the camera from "whiting out", and help maintain a constant video level. All auto iris lenses are supplied with Neutral Density spot filters to increase the maximum F stop. The F stop also directly affects the depth of field.
C or CS Mount
Modern cameras and lenses are generally CS mount. With CS mount cameras, both types of lenses can be used, but the C mount lens requires a 5mm ring to be fitted between the camera and lens to achieve a focused image. With C mount cameras it is not possible to use CS mount lenses.
Depth of Field
The depth of field refers to the area within the field of view which is in focus. A large depth of field means that a large percentage of the field of view is in focus, from objects close to the lens often to infinity. A shallow depth of field has only a small section of the field of view in focus. The depth of field is influenced by several factors. A wide angle lens generally has a larger depth of field than a telephoto lens, and a higher F stop setting typically has a larger depth of field than a lower setting. With auto iris lenses, the automatic adjustment of the aperture also means constant variation of depth of field. The small depth of field is most apparent at night when the lens is fully open and the depth of field is at its minimum. Objects that were in focus during the day may become out of focus at night.
Auto or Manual Iris
Generally we tend to use auto iris lenses externally where there are variations in the lighting levels. Manual iris lenses are used normally for internal applications where the light level remains constant. However, with the introduction of electronic iris cameras it is now possible to use manual iris lenses in varying light conditions and the camera should electronically compensate. There are several considerations to this option though: the setting of the F stop becomes critical; if the iris is opened fully to allow the camera to work at night, the depth of field will be very small and it may be more difficult to achieve sharp focus even during the day. The camera can maintain normal video levels, but it cannot affect the depth of field. If the iris is closed to increase the depth of field, the low light performance of the camera will be reduced.
Video drive or Direct drive
With auto iris lenses it is necessary to control the operation of the iris to maintain perfect picture levels. Video driven lenses contain amplifier circuitry to convert the video signal from the camera into iris motor control. With direct drive lenses, the camera must contain the amplifier circuitry, and the lens now only contains the galvanometric iris motor making it less expensive. The deciding factor depends on the auto iris output of the camera. Most now have both types.
Field of View Sample
Picture area 49'w x 35'h
Picture area 29'w x 21'h
Picture area 22'w x 16'h
Picture area 1'w x 9'h
Picture area 3.25'w x 2.50'h