Main Functionality of CCD Camera

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  Security System ->The main characteristic of the Lens  

( 1 ) F stop

Iris, like the human eyes, can control the amount of light that passes through a lens in order to provide clear image picture. The ability of a lens to gather light, which is symbolized as F, also know as F stop, is equal to f/D where f is the focal length and D is the aperture of the len . The smaller the F stop number, the larger the aperture and thus the greater the amount of light passing through the lens tothe camera’s imaging device, thus increasing the sensitivity of the camera.

F-numbers conform to an international aperture scale in discrete values such as F1, F1.4, F2, F2.8, F4, F5.6, F8, F11, and F16. The light passed is 1/FxF. For example, a light sensitive F/1.4 lens will pass 1/1.4 2= 0.5 = 50% of available light.

Different F-stop lens

( 2 ) Focal Length

Once the viewing object is defined, the focal length of the lens is dependent on the following factors:

1) The object size

2) The distance from the object to the camera and

3) The CCD size

 The focal length of the len is calculated according to the following formula:

h/H= f/L or w/W=f/L

W- Width of object

H- Height of object

w- Width of format (1/2 format: 6.4mm; 1/3 format: 4.8mm; 1/4 format: 3.6mm )

h- Height of format (1/2 format: 4.8mm; 1/3 format:3.6mm 1/4 format: 2.7mm )

f- Focal length

L- Object distance

Ex: Full image of 4.5m high object :

1/3 format, Object distance: 10m

3.6/4500=f/10000

f=8mm

( 3 ) Field of View

Field of view (FOV) relates to the size of the area that a camera will see at a specific distance from the camera. The field of view isdependent on lens focal length and camera format size.

The FOV width and height can be calculated using the following formulas:

FOV Width= Format (horizontal in mm) x Distance in feet from camera )

Focal Length

FOV Height: 0.75 x FOV width

When the focal length is smaller, the field of view is wider, which is suitable for picture taking within short distance. On the other hand, when the focal length is greater, the field of view is narrower, which is suitable for picture taking in the distance or a close-up shot.

Manipulating the FOV formula allows a calculation of the distance in feet from the camera for a required FOV width. The formula becomes:

Before the FOV for a camera is selected, the minimum desired resolution for an intruder or object to be viewed must be determined(i.e., whether it is desired to identify a person or to just determine if a person is within the scene).

This will limit the maximum FOV width and is referred to as the resolution-limited FOV (image to the right). The resolution-limited FOV width can be determined by using camera resolution in horizontal lines per foot and the number of lines of resolution per foot requiredto identify an intruder.

The following formula is used to calculate the resolution-limited

FOV width:

Resolution limited FOV width = Camera resolution

Number of lines of resolution

A resolution of 16 lines per foot is considered acceptable for identifying most people. If a camera with 350 horizontal lines of resolutionis utilized, the resolution-limited FOV width for a resolution of 16 lines per foot can be calculated as follows:

Resolution limited FOV width = 350/16=22'

The following table presents the horizontal camera format sizes of the image for various size imagers:

Table 8

For example, the maximum distance from a 350-line, horizontal resolution, 1/2-inch format camera with a 75mm lens to the resolution-limited FOV width at 16 lines per foot resolution will be as follows:

Maximum Distance= 22 x 75/6.4 =257.8 feet

( 3 ) Depth of Field

Depth of field is the area in focus ahead of and behind the main object. When an object is focused, there is certain area behind theobject and in front of the object that will still be in focus, although not as sharp.

Depth of field increases or decreases based on the length of the lens, the lens’ aperture, and distance from the camera to the subject.

Depth of Field is calculated by the following formula.

H= f 2

C X F

T1= B(H+f)

H+B

T2= B (H-f )

H-B

F: F number

H- Hyperfocal distance ( the point of focus where everything from half that distance to infinity falls within the depth of field )

f- focal length

H- object distance from the image sensor

T1: Far Limit

T2: Near Limit

C- Circle of least confusion ( 1/2 format: 0.015mm; 1/3 format: 0.01mm; 1/4 format: 0.008mm )

Based on the above formula, the conclusion can be as follows:

• Lens length

Shorter lens (ex. wide angle) = longer depth of field

Longer lens (ex. telephoto) = shorter depth of field

• Aperture

Wide aperture (low F-stop) = shorter depth of field

Narrow aperture (high F-stop) = longer depth of field

3 Distance to object

Short distance = shorter depth of field

Long distance = longer depth of field

 

 F3.2                                                              F5.0                                                                    F9.0
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  Short depth of field                                                                                      Long depth of field