Tuesday, October 27, 2009

Guide to Cameras 2

Choosing a Camera Specification

The main criteria of a camera's performance are its sensitivity and resolution. Secondary considerations are colour or monochrome and indoor or outdoor suitability.

Sensitivity is the camera's ability to respond to light levels. Resolution defines the amount of picture detail in the image produced by the camera.

Camera resolution

This is expressed as the number of television lines that the camera is capable of producing. For example the KT&C monochrome bullet camera is classed as 420 lines or usually 420 TVL. The comparable colour model is 380 TVL
These are typical figures for CCD cameras, CMOS cameras are usually lower.
Higher resolution cameras of over 500 TVL are available for select applications. For example, trying to read the number plate of a fast moving car would need the best resolution affordable. You should be able to read a stationary number with a resolution of 380 TVL. Below 300TVL and using CMOS cameras even facial features will be difficult to distinguish. These are very broad guidelines and also depend on the light levels and shadow conditions.

Indoor/Outdoor Cameras

If a camera is to be sited outside and is not going to be mounted in an enclosure it must be classed as weather resistant.
The cable entry points are sealed and most bullet cameras come with trailing leads that allows the connections to be made inside the building. Typically 18ins (45cm) should be sufficient. One bullet camera under review had only a few centimetres of cable. This means the connection would be exposed to the rigours of the weather. Needless to say this model did not meet the selection criteria of 2seetv.

The strength and durability of the casing is difficult to quantify but we have rejected cameras with flimsy plastic enclosures, particularly wireless cameras where the antenna dish looks like it could be easily damaged. The range of KT&C bullet cameras have strong aluminium bodies and sealed cable entry points and were selected for these features.

Light Levels

Choosing the correct camera to operate in the ambient light conditions is possibly the most important although most tricky specification to understand.

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:
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

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. Colour needs a little more. However, colour cameras achieve good night-time vision by switching to a monochrome mode. You will never get good night-time colour pictures without huge amounts of additional lighting. So think carefully about the added cost of colour over monochrome cameras. As most people are used to colour television it sets a standard so it is very common to still select a colour 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. Colour 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.

1 comment:

  1. What is more, using DVR multiplexers with your wireless cameras allow sending data to your PC real time, letting you maintain control over the security of your home or working environment whenever you have an access to the Internet.