Day and Night Functionality
The electromagnetic spectrum extends from x-rays to microwaves. Light is a particular range of frequencies that can be detected by the human eye. Within the visible spectrum differences in wavelength manifest themselves as differences in colour. The visible range extends from approximately 400 nanometres (violet) to 700 nanometres (red). Below blue is “ultra-violet” and above red is “infrared”. The term “Black Light” is commonly applied to the radiation that borders the visible-light region of the electromagnetic spectrum.
All objects emit radiant energy merely by virtue of having a temperature above absolute zero. The higher the temperature, the greater the amount of energy emitted. All objects emit radiant energy merely by virtue of having a temperature above absolute zero. The higher the temperature, the greater the amount of energy emitted. Warmer objects such as people and animals stand out from typically cooler backgrounds. Under infrared illumination grass and the foliage of deciduous trees will appear white due to the high near infrared characteristic of green chlorophyll and the high infrared reflectance of the underlying cellulose structure. [The typical reflectance of open countryside (grass, trees) is approximately 20% for visible light but can be 60 to 70% for infrared illumination]. In addition to emitting, all substances are capable of absorbing radiation. [see Reflectance]
The CCD arrays used in colour cameras are sensitive to infrared light, which is present in normal daylight. The infrared light produces false colour signals from the CCD that affect the purity of the colours reproduced by the camera and can produce a “focus shift” that reduces image sharpness. To prevent these undesirable effects, all colour cameras are equipped with an “IR-cut” correction filter that is placed between the lens and the image sensor to remove infrared light. This also means that infrared illumination cannot be used with normal colour cameras.
One solution to this problem is to have a small motor that moves a colour correction filter in front of the CCD in colour mode but retracts it in monochrome mode. This is the solution adopted in the Axis 221, Axis Q1755 Network Cameras.
The actual CCD image sensor in a colour camera comprises an array of pixels like a monochrome camera. Each pixel is, however, subdivided into three smaller light sensitive areas that are constructed to be sensitive to red, green and blue light respectively. Consequently the pixels are larger in size than for monochrome CCD and the number of pixels that can be fitted on to a colour CCD of a given size is less than a monochrome CCD of equal dimensions. [In general, monochrome cameras have a resolution that is higher than colour cameras]. The colour correction filter and the colour sensitivity of the pixels also tend to make colour cameras less sensitive than monochrome cameras. Typically, colour cameras have sensitivities between 1 lux and 2.5 lux whereas monochrome cameras have sensitivities between .01 lux and 0.1 lux.