A User's Guide to Digital Video Surveillance

LIGHTING CONDITIONS


The human eye adjusts to changing light conditions by the iris dilating and constricting. Without it, think of what it would be like – it would be either too bright or too dark depending upon how your iris was fixed.

Similarly, the camera needs to adapt to changing light conditions. One of these adaptations is performed by a function known as “auto-iris”. Auto-iris is a lens component for controlling light intake electronically. The auto-iris function is built into the CCD imaging circuit of the camera and works in conjunction with the auto-iris lens. It works similar to the human eye allowing more light in when it gets dark, and reducing the intake of light when it's bright out. A manual iris serves no purpose in changing light conditions, as you are not going to climb up on a ladder and make changes to the lens throughout the day. A manual iris becomes applicable where you have fixed lighting conditions, such as in an interior office or hallway, which is not subject to ambient light from outside windows or skylights.

One mistake many people make is pointing a camera right towards the sunlight. Avoid this at all costs; if not completely possible, angle it so it is not direct or try to get a camera that has a sunshade on it. Pointing the camera right at the sun will virtually blind it regardless of the camera iris.
There are other lighting conditions to consider. The auto-iris reacts to what it sees and in some conditions may be misleading; requiring some other corrections. Back lighting can be an issue, and as a result washed out images may appear. To correct this, the CCD has a backlight compensation (BLC) function that may be automatic or may be turned on and off via a dipswitch located on the camera. Back Light Control dims intense back light automatically for a brighter picture of an object.

Another issue is when the light gets too low, the iris can no longer compensate. As the iris is an open and close function it needs other parameters to be addressed. CCD's have a function known as Automatic-Gain-Control (AGC), which compensates by brightening images under low light conditions.

Different CCD's have different abilities to adapt to low light conditions. This is known as a lux rating; the lower the number the better the adaptation to low lighting. Black-and-white cameras actually adapt better to low light than color cameras. In fact, in lighting conditions such as nightclub environment most likely a color camera will give unsatisfactory results where a black-and-white camera would excel.

Below is some sample lux ratings:

Direct Sunlight107,000 – 140,000
Full Sunlight11,000 – 22,000
Overcast Day 1,000
Retail Shop  500
Office Setting  300
Twilight        10
Deep Twilight   1
Full Moon      0.1
Moonless Night    0.0001
Overcast Night 0.00001

For extreme darkness, the human eye as well as the camera iris has their limitations. There are two solutions to this, each having their advantages and disadvantages. The first is the Day/Night technology, which features a structure that is highly sensitive in the “near” infrared light regions, a range that is invisible to the human eye. This makes it possible to see and record in the dark by irradiating near infrared light on the recorded media.

Light in the near infrared light region which has long wavelengths is converted into photoelectricity deep in the seminconductor's (CCD) silicon. Conventional CCD structure, sensors were incapable of efficiently gathering the charge which has undergone photoelectric conversion. Day/Night technology allows a charge due to near infrared light, which was ineffective in conventional CCD devices, to be used as video information.

Day / Night Comparison

While Day/Night technology can adapt to low light or “near” infrared light regions it cannot adapt to total darkness or the infrared region, in this case the use of infrared cameras are in order. Total darkness means total blackness, so if there is moonlight or street lights then Day/Night is more than capable. If you have an interior room with no windows and no lights this would fall in the infrared range.

Infrared ( IR ) radiation – is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. The name means "below red" (from the Latin infra , "below"), red being the color of visible light of longest wavelength. Infrared radiation spans three orders of magnitude and has wavelengths between 700 nm and 1 mm.

The Earth's surface absorbs visible radiation from the sun and re-emits much of the energy as infrared back to the atmosphere. Certain gases in the atmosphere, chiefly water vapor, absorb this infrared, and re-radiate it in all directions including back to Earth. Infrared is used in night-vision equipment, when there is insufficient visible light to see an object. The radiation is detected and turned into an image.

You will notice that most infrared cameras have a series of LED's. These LED's are what cast out into the darkness. Think of it as a flashlight you can only see out to the distance of the illumination capacity. You will see infrared cameras rated at distances. Remember these distances may be exaggerated and under optimal conditions. One need's to understand also the field of view is limited to the illumination area cast onto by the LED's which may be small. You are not going to read license plates 100 yards away in the darkness with standard reasonably priced commercial infrared cameras. This is not Rambo.

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