CCD vs CMOS

The image sensor is the camera component which captures light and begins the process of turning it into a digital image. There are two types of security camera sensors: CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor). Which of the two you choose depends on your application.

With a CCD sensor, every individual pixel's charge is transferred through an output node, which is converted into an electrical signal. The signal is then buffered and sent as an analog signal. Because the pixels are devoted to light capture, the image quality is usually pretty high.

With a CMOS sensor, every individual pixel performs its own charge-to-voltage conversion, and the sensor performs amplification and noise-correction. The sensor also includes digitization circuits, which allow the component to output information in a digital format. Because of the complexity of this design, the area devoted to light capture is reduced. And because each pixel must perform its own conversion, uniformity (image quality) is lower.

Both technologies have their place in the marketplace. If your goal is to have good low-light-level surveillance, then a CCD camera is the best. If there is little to no light, CCD sensors used with IR illuminators produce an excellent picture. If indoor lighting is adequate, and megapixel quality is your goal, then CMOS technology may be your best bet.

Make sure you test your camera with the lighting that will actually be present in the field of view. Do not rely on manufacturer’s data sheets for the minimum useable picture. This figure is often subjective, and your customer may not find the results acceptable. CMOS cameras are more prevalent in the IP network world. These cameras are typically equipped with built-in Web Servers, which enable remote viewing of video from other locations, by using IP network protocol.

In conclusion, CCD sensors are the best for outdoor applications, low light scenarios or when a higher quality image is required. CMOS sensor cameras should mostly be considered for indoor applications, though with adequate lighting conditions, they may also work well outdoors.

Lighting for CCTV surveillance

When capturing video in CCTV surveillance applications, there are three essential elements involved.

Camera:

Make sure the camera has the correct sensitivity for the application.
Sensitivity measures how the camera will respond to a certain level of light to obtain an acceptable picture. Sensitivity is typically measured in lux (1 lux = 1 lumen per square meter), with all camera manufacturers stating the minimum lux level needed to produce acceptable images. Camera manufacturers rarely explain where the minimum LUX level should be measured. Is it the minimum light on the scene, at the front of the lens, or on the camera image chip?

Lens:

The lens must match the camera for the proper amount of light to pass through it.
The lens aperture, or f-stop, determines how much light passes through it to the camera chip. The lower the f-stop, the more light the lens will pass. Many manufacturers will specify a certain f-stop lens with a specific camera. Using the lens specified will guarantee that the proper amount of light will pass through the lens, providing an adequate picture at a certain light level.

Illumination:

Unless we have light, we will have no video images from the camera. There are many sources of light. Some are good and some are not so good for CCTV surveillance.

1. Incandescent lamps - The bulb life is limited and these lights are very inefficient. The maintenance costs on this source of lighting are very high.

2. Fluorescent lamps - These lamps are more efficient than incandescent but still waste up to 60 % of the input energy to heat. These are commonly used indoors and are not used in outdoor applications.

3. High Intensity Discharge (HID) lamps - These are very good for CCTV applications due to their 60 – 80% efficiency and their ability to provide excellent color rendition. Low-pressure sodium is a form of HID lamp but is unsuitable for CCTV applications due to its yellow tinge. The best HID source in use today is metal halide. Metal halide bulbs provide cool white light and have excellent color rendition.

4. LEDs - LEDs are now considered to be the green solution to outdoor lighting. They are extremely efficient, and their reliability cannot be matched.

The proper field of view

When designing CCTV camera systems, many people fail to consider what they want to see in the camera’s field of view. John Johnson, in his foundational work for the United States Army, developed what he called DCRI surveillance levels.

Detection: A target is determined to be in the camera’s field of view. Detection applications typically require wide-angle lenses. The lower the lens number, the wider the field of view. For example, a 4mm lens would have a wider field of view than a 12mm lens.

Classification: A target can be distinguished between an animal and an upright human.

Recognition: A human target is determined to be a threat based on the type of clothing and/or the equipment being carried.

Identification: A human target can be identified as a specific individual.

In order to obtain the proper field of view, the correct lens must be installed on the camera.

The best type of lens to use to ensure the proper field of view is obtained is the varifocal lens. There are many different types of varifocal lenses. Some are manual iris, and some are auto-iris lenses. A typical varifocal lens would be 5 to 50mm. For more information on lenses, visit different manufacturer’s Web sites. For example, try Bosch’s Web site at http://products.boschsecurity.us/en/TAMS/products/bxp/CATM40b8fcf5b76c79e176c117fb36cba772.