What is a CCD Sensor?
Most CCTV cameras are now almost exclusively 'CCD' sensor type. CCD stands for Charge-Coupled Devices. These are light-sensitive chips, the sensitive area being split into a large number of individual pixels (Small dots on the chip).
An image from a scene is focused through a lens onto the chip surface and charge is built up in each pixel proportional to the intensity of the light falling on it, hence an electrical representation of the optical image is formed.
Typical active pixel counts for CCD sensors are between 250,000 and 500,000 pixels. Cameras are available in both monochrome and color types.
Note: WECU Surveillance supplies cameras without lenses filled. This is because there are many different types of cameras and even more types of lenses and so it would be impractical, and very expensive, to stock all possible permutations and combinations, to meet each application.
Please - REMEMBER THE LENS!
The main criteria of a camera's performance are its sensitivity and resolution. Sensitivity is a measure of the minimum amount of light required by the camera to give a 'useable' Image. Resolution defines the amount of picture detail in the image produced by the camera.
The CCTV industry has many ways of stating an individual cameras sensitivity performance. Most common is minimum scene illumination required to give useable picture at a particular lens aperture.
This is inherently subjective.
Another, more objective definition is the minimum scene illumination required by the camera to give full video output
I.e. 1v peak to peak. Few manufacturers use this definition! Most manufactures provide a figure in LUX which is actually a light level the figures quoted are often like this 0.1 LUX.
The problem with this is that to get a true representation of the actual sensitivity of the camera the lens apertures must also be compared. Lens apertures, i.e. the size of hole that light can pass through in the lens, must be compared in order to get a true comparison of performance.
This appears to say that camera 2 is twice as sensitive as camera 1 because it needs half the light to give a useable picture. In fact the cameras have equal sensitivities because the aperture through which light is allowed to fall on the sensor of camera 2 is twice the area of that for camera 1.
- Camera 1 requires a minimum scene illumination of 0.1 lux at a lens aperture of fI .4 to give a useable image.
- Camera 2 requires a minimum scene illumination of 0.05 lux at f 1.0 to give a useable image.
Typical sensitivities (defined as minimum scene illumination required for useable picture with lens aperture at fI .4) for current CCD cameras are as follows:
Note: Monochrome CCD cameras are still approx. 5 times more sensitive than equivalent color cameras.
- Medium Performance (General purpose) Monochrome 0.5 - 0.25 lux
- High Performance (Low-Light/Outdoor) Monochrome <0.20 lux
- Medium Performance (General purpose) Color 3.0 lux
- High Performance (Low-Light) Color < 2.5 lux
Typical light levels
Choosing the correct camera to operate in the ambient light conditions is possibly the most important although most tricky specification to understand.
- Full Summer Sunlight: 50,000 Lux
- Dull Daylight: 10,000 Lux
- Shop/Office environment: 500 Lux
- Dawn/Dusk: 1 - 10 Lux
- Main Street Lighting: 30 Lux
- Side Street Lighting: 0.5 - 3 Lux
Light levels are usually measured in Lux. This is a measure of the light energy arriving on an area 1m2 of surface per second.
Typical light levels are:
The golden rule when deciding which camera to use for a given lighting condition is not to choose one that will only just give a picture; Try to give the camera approximately 10 times its quoted minimum scene illumination. Most cameras will be able to cope with excess light. The major problem is when they do not have enough light to produce a picture.
The sensitivity of covert cameras with pin-hole lenses are often quoted as 0.1Lux @f1.4. This seems to indicate that the camera will work in ¼ moonlight. Actually the pin-hole lens will have an aperture ratio of something like f4 and so the camera will need approximately 1 Lux to produce a picture.
Unless your house is directly under main-street lighting the light level is probably less than 1 Lux at the front and even lower at the back. Bright security flood lights in theory help but often produce dark shadows a short distance from the house.
A monochrome camera rated at 0.05 Lux will produce reasonable results. Color needs a little more. However, color cameras achieve good night-time vision by switching to a monochrome mode. You will never get good night-time color pictures without huge amounts of additional lighting. So think carefully about the added cost of color over monochrome cameras. As most people are used to color television it sets a standard so it is very common to still select a color camera knowing it will switch to monochrome mode at night.
Monochrome cameras respond well to additional IR (infra red) lighting. With this in mind true night-vision cameras include a ring of IR LEDs. Color cameras also offer IR illumination but as stated previously will switch to monochrome at night even with the IR LEDs turned on. These are true night-vision cameras and are rate at 0 Lux.
During the day the amount of illumination reaching a scene depends on the time of day and atmospheric conditions. Direct sunlight produces the highest-contrast scene, allowing maximum identification of objects. On a cloudy or overcast day, less light is received by the objects in the scene, resulting in less contrast.
The field of view (FOV) How much will the security camera see?
The field of view (FOV) is based on the camera & lens. WECU Surveillance.com has many different types of Lenses to suite any and all of your CCTV security camera needs. Wide angle lenses are suitable for most applications due to their ability to view a large area. A 4mm wide angle lens will be able to view an area 20' high x 26' wide at 21' away.
As an example, a 15' x 15' room is shown in the diagram below. Observe that the 4mm lens (green arrows) allows better wide angle viewing coverage than the 12mm lens (red arrows). In applications where a closer view is needed (such as above a cash register or over a greater distance), an 8 or 12mm may be desired. The same camera (above) at 21’ away with a 8mm lens will have a 10' vertical x 13' horizontal FOV.
At 21' with a 12mm lens, the FOV will be approx 6' vertical x 9' horizontal. Increasing the focal length of the lens decreases the perceived distance to the viewing area. See the FOV diagram below for approximate views with different focal length lenses.
- Resolution is measured in lines and normally only horizontal resolution is quoted.
- Vertical resolution of a camera is of little interest because pictures are generally speaking limited by the scanning system used to produce the image.
In the UK and Europe this is the CCIR system, which defines that a picture is built-up from 625 lines across the screen. Of more interest in measuring camera performance is horizontal resolution. This is the maximum number of black or white areas that can be identified across the picture. This is always specified per monitor height. A typical general purpose monochrome camera has a horizontal resolution of 380 lines. This means that it should be possible to resolve 380 x 1.33 i.e.. 505 individual actual lines of black, white or grey along any one line of a CCTV picture image. (The factor of 1.33 comes from the fact that a TV picture has an aspect ratio of 4 : 3).
Typical resolution performances for CCD cameras are as follows:
- Medium Performance (General purpose) Monochrome 380 lines
- Medium Performance (General purpose) Color 380 lines
- High Resolution Monochrome 600 lines
- High Resolution Color 480 lines.
Cameras will be quoted as 1/2", 1/3" or occasionally as 2/3". Newer cameras will be referred to as 1/4". This refers to the format of the camera and defines the size light-sensitive area of the imaging device used in the camera. In itself it gives no measure of a cameras performance although generally the smaller the format size, the smaller the light gathering area of the sensor. It is provided for information to enable the installer to match the camera to the correct lens. Camera formats are getting smaller in order to reduce the costs of the imaging devices and hence the cameras themselves as well as the lenses put onto them.
The format size of a lens must be equal to or greater than the format size of the camera it is being used on.
If the lens is of smaller format size than the camera then the corners of the scene being viewed will be cut-off (vignetting).