Capturing Image

Introduction Specification of the right CCTV camera for a project is not always the easiest of processes. There are many factors that have to be taken into account: technical specifications, the application and its requirements, as well as any physical constraints the site may impose. With ever increasing product ranges available in the marketplace, and technology constantly evolving to optimize performance, reliability and functionality, it is quite a challenge to make an informed decision to meet the requirements for the job whilst remaining within projected budget. Understanding the many variables within CCTV camera technology today can only be an advantage in helping you make the right choices.

From here, the rest of this white paper explains the internal features of Video Camera and the working principle of Lens.

CCD Camera Sensor
VisionNet Video Camera consist of CCD image sensor as it produces high quality image, especially in 3D environment. The CCD is comprised of about 500,000 light sensitive cells called picture elements (pixels) which convert the light falling onto its surface into an electrical signal. The performance of the camera affects by 3 factors:
1. Image Sensor
2. Lens
3. Illumination
However, Video Camera Image Quality are more dependent upon the quality of the CCD than any of the other camera components. Currently, the popular formats are:

B/W and Color Camera
Basically, Video Camera divides into two main categories: B/W and Color Camera.
Broadly speaking, all video camera sensors are B/W, color is obtained by inserting red, green and blue filters in front. As in reality, those color filters differ some part of incoming light that results less light going to the image sensor successfully. This is why color cameras have less resolution than B/W cameras. Also due to the filters, color cameras are not sensitive to infrared light. Therefore all the discussion on camera sensitivity and suitability for infrared illumination is confined to B/W cameras. This is except for the Dual Mode cameras now becoming available which potentially offer the best of both worlds - VPC-400N.

Light and Illumination
In the world of Video Camera, light is vital to produce any image. The type of lighting used, and the correct positioning, are vital to maximum security. Everyday White or Daily light is actually a mixture of colors of different frequencies. Video Camera is more receptive to certain color combinations than others. The best results are usually obtained by matching the spectral response of the camera to the light illuminating the scene. The amount of light reflected from an object determines how ‘bright’ it appears.

During the designation of a security deployment, lighting takes the first account of choosing the right camera. Amount of Light is measured by Lux. One unit of Lux can be represented by the amount of light generated from a burning candle.

Only natural light provides absolutely even illumination, although it is of course affected by clouds and shadows. All forms of artificial light suffer from the fact that as the distance increases from the light source so the illuminance reduces. This is due to the inverse square law of illumination where the illuminance falls to a quarter of its value if the distance is doubled..

Inverse Square Law Of Illumination
As the luminous flux travels away from the light source the area over which it spreads increases, therefore the illuminance (lux) must decrease. The relationship is expressed by the inverse square law and illustrated below:

Inverse Square Law Of Illumination

The relationship between illuminance and it's effect at a distance is given by:

This factor is particularly important in considering the light available for a camera. For instance a light source providing a level of 30 lux at 20 Metres will provide 7.5 lux at 40 Metres and only 3.3 lux at 60 Metres. The other effect of this is that the wide range of light levels can cause problems with automatic iris lenses. Unless set up correctly, the foreground light will cause the iris to close and lose definition in the distance. The reverse is if the iris is set to the distant light level in which case there will be a lot of flare in the foreground.

Sensitivity
Color cameras generally perform less well in low light than B/W cameras, all other things being equal. Sensitivity is usually measured by reference to minimum illumination lux level at which a camera can produce an image. The lux valuation, although usually referred to as a metric unit of measurement of light, is very often applied subjectively to the ability of a camera to produce images.

For instance: VPC-400N Video Camera produce image with minimum illumination of 0.03 at F1.2 Lux.
*Actual amount of Lux falls into the image sensor differs considerably from the factors of Lens and Illumination.

F-Number
The f-number of a lens is the ratio of the focal length to the effective object lens diameter. It is a mechanical ratio and does not infer the efficiency of a lens. It does affect the amount of light energy passed to the sensor and will play a significant part in the resulting picture. In simple terms the smaller the f-number the more light is passed to the sensor, therefore f1.2 is better than f1.8. The percentage of light passed by different apertures is listed below. This shows the percentage of light falling on the lens that is passed to the sensor.

Here is a simple illustration of how much Lux in different environment
 

For more information:
Video Camera Selection Guide - White Paper