CCTV SYSTEMS LIGHTING AND ILLUMINATION
CCTV SYSTEMS • A video camera requires light to work properly, without sufficient light the video camera can’t receive enough information to form an image. • During a 24 hour period, light conditions outside can change, the lighting can go from extremely bright to extremely dark.
CCTV SYSTEMS • Indoor lighting conditions can change as lights are turned off and on, depending on the time of day and other factors.
CCTV SYSTEMS • Indoor light can be affected by light entering through windows, and light levels can change significantly by opening or closing a door. • Outdoor lighting conditions can also change during the switch from natural to artificial light sources.
CCTV SYSTEMS • A camera receives light through its lens, the lens iris is used to adjust the aperture of the lens to allow more or less light to reach the imaging element. • Factors such as environmental and atmospheric conditions can affect the amount of light received by the lens. • High levels of dust or liquid spray in the area can not only affect the amount of light, but can severely limit the field of view.
CCTV SYSTEMS • Atmospheric conditions such as heat or humidity can cause lighting levels to fluctuate, distorting incoming images. • In many cases the lens iris can compensate for varying degrees of light. LENS IRIS APERTURE
CCTV SYSTEMS • The point here is that the camera used in certain spaces either has to compensate for the changes in lighting conditions or be aimed away from direct light sources. • Cameras installed outdoors are outfitted with a sun shield that acts like the lid on a baseball cap to shade the lens from direct sunlight.
CCTV SYSTEMS • Natural light is light received from the sun. • The human eye is sensitive to solar radiation with wavelengths between 400 to 750 nanometers (nm), the human eye is most sensitive to light at 555 nm. • What color do you think is in the 555 nm wavelength? GREEN
CCTV SYSTEMS • The wavelength of light determines it’s color. • The longer wavelengths are at the red end of the spectrum, while the shorter wavelengths are towards the violet end of the spectrum.
CCTV SYSTEMS • A camera, like the human eye, must receive light in this range in order to create a video image. • Artificial light is produced by a number of processes, including heat, chemical reaction and photoelectrical activity. • The most popular types of indoor lighting is incandescent and fluorescent lights.
CCTV SYSTEMS • Outdoor lighting consists of halogen, high pressure sodium, metal halide, mercury and aluminum oxide lamps.
CCTV SYSTEMS • Artificial lights, depending on the materials with which they are constructed, generate light at various wavelengths and therefore different colors.
CCTV SYSTEMS • When measuring light we use photometric units. • The basic measure of the amount of light produced by a light source or light output is the lumen, LED flashlights use this unit of measure. • Another measure of light is called candlepower (CP) or simply put, a candle.
CCTV SYSTEMS • Candlepower was determined as the intensity of light emitted by a standardized candle at a distance of one foot and was expressed as a foot candle. • The concept of candle (candlepower) was later replaced by the candela.
CCTV SYSTEMS • The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540×1012 hertz and that has a radiant intensity in that direction of 1⁄683 watt per steradian (a solid angle). • The candela is sometimes still called by the old name candle, such as in foot-candle and the modern definition of candlepower.
CCTV SYSTEMS • A candela is a metric value and provides a much more precise and consistent measurement. • The candela is based on the temperature of platinum where it converts from a liquid to a solid state.
CCTV SYSTEMS • When working with CCTV and video cameras, we have to take into account the reflectivity or illuminance of objects surfaces.
CCTV SYSTEMS • When a light source hits an object, the amount of light reflected by the object determines what is seen by either the camera or the eye.
CCTV SYSTEMS • Different materials reflect light to varying degrees. • Snow reflects light up to 90% of the light that falls on it. Black velvet fabric reflects less than 20% of the light that falls on it • The challenge with measuring reflectivity is that materials absorb some wavelengths while reflecting others. • This is important when selecting or using a camera.
CCTV SYSTEMS • A black and white camera is immune to spectral shifts. • Color cameras are extremely sensitive to spectral shifts. • The further away from a light source, the more the intensity of the light diminishes.
CCTV SYSTEMS • When engineers, designers and manufacturers talk about reflectivity, they use a metric scale called lux.
CCTV SYSTEMS • Lux is used to describe illumination, or the density of light falling on an object. • Lux as defined in the dictionary……. • Lux: is equal to the direct illumination on a surface that is everywhere one meter from a uniform point source of one candle intensity or equal to one lumen per square meter.
CCTV SYSTEMS • When we are talking about illumination or lux, we are referring to the actual light available at a given distance.
CCTV SYSTEMS • Lumens are used to measure how much light is produced by a light source. • Lux and foot candles are equivalent measures, they both measure the same thing, but use different scales. • Use these two rules to convert between lux and foot candles, a foot candle is 10.57 lux and a lux is 0.093 foot candles.
CCTV SYSTEMS • Lux is equal to one lumen per square meter. • Lux is the unit of measurement used by cameras, the lower the lux rating the better the camera can see in low light.
CCTV SYSTEMS • Most CCD cameras are sensitive to light ranging from bright sunlight (10,000 foot candles, or 107,500 lux) to twilight (1 foot candle, or 10.7 lux) • This means a camera can operate in relatively high or low light levels. • Special cameras that are low light sensitive are called LLL cameras, and are extremely expensive.
CCTV SYSTEMS • Another solution for a dark CCTV environment is the use of infrared light sources and infrared sensitive cameras. • Infrared light (IR) is light below the threshold of human vision.
CCTV SYSTEMS • the IR spectrum is separated into two categories, near infrared and far infrared ranging from 800nm to 6,000nm.
CCTV SYSTEMS • Near infrared ranges from 800nm to 3,000nm. • Infrared sensitive cameras use the infrared wavelengths between 800nm to 900nm. • Light in this range behaves as visible light and can illuminate a scene for surveillance. • Light in this range is undetectable to the human eye.
CCTV SYSTEMS • Infrared can be detected by instrumentation designed for visible light. • There are two types of infrared sensitive cameras. • The first type of infrared camera requires an external IR light source. • The second type of IR sensitive camera amplifies existing light.
CCTV SYSTEMS • Cameras that require an external IR light source use an array of led lights to produce infrared light in the range of 900nm to 1,100nm. • Zero lux cameras contain a CCD for imaging, and some cameras are designed with an infrared led array built into the camera housing.
CCTV SYSTEMS • The challenge is to produce enough light to illuminate a scene. • Cameras with built-in infrared LED’s typically have a short illumination distance of only 10-20 feet. • To increase the distance, an external infrared illuminator is required.
CCTV SYSTEMS • External infrared illuminators come in varieties: • Infrared led array…already mentioned • Special high intensity lamps with a visible light filter, lamps with filters can still emit visible light of 0.01 lux. • LED arrays are state of the art in lighting, and are most likely to be used today.
CCTV SYSTEMS • For typical CCTV applications, we are looking for cameras that can serve dual lighting purposes. • We want a camera that works with both visible light to zero to no light. • These cameras are called night vision or zero lux cameras.
CCTV SYSTEMS • Night vision cameras work by first converting the incoming light into an electrical signal. • The signal is then amplified and converted back into light. • When available light is converted for use by a CCTV camera, it is converted into green light.
CCTV SYSTEMS • Remember that green light is right in the middle of the visible spectrum and the human eye is most sensitive to green light. • By converting to green, the range of amplification is greater, for this reason, images from low light cameras are usually green. • Red and blue both have narrow wavelength bands in comparison.
CCTV SYSTEMS • Some cameras switch from color to black and white to capture images during dawn and dusk hours.
CCTV SYSTEMS • One problem with available light cameras generate a certain amount of visual noise or visual artifacts during the conversion process. • Because they amplify available light, light from sources such as the moon or street lights may actually be too bright to capture. • Most cameras provide automatic gain control (AGC), which allows the camera to compensate by adjusting its sensitivity.
CCTV SYSTEMS • The images provided are more useful, but details in shadow areas can be blacked out as a result.
CCTV SYSTEMS • Backlighting can be an unwanted effect in CCTV, it means that you have a video image where the details are blacked out because of a bright light source behind the object you’re viewing. • To solve this cameras provide either a backlight compensation (BLC) or automatic light control (ALC) adjustment.