Are camera manufacturers deceptive with camera sensitivity specs? No. The following few CCTV tips help to explain why.
Lighting and its effects
Light affects everything from picture clarity to focus. The bottom line, is that if there is no, or little light, virtually no picture can be obtained. The converse is true too. If there is good lighting available, the potential to achieve a good picture from your CCTV cameras is greatly enhanced.
Common units of measurement
Lumen (lm) - The rate of luminous energy emitted from a light source and called luminous flux, expressed as lumen. Luminous flux as incident on the surface of the scene is called luminance or illumination and is defined as: luminous flux per square metre or 1 lux.
Lighting considerations
Usually daylight presents no problem as far as light is concerned. Daylight will range from 107 to 10,764 lux typically on an overcast day, and from 10,764 to 150,690 lux on a sunny day. These lighting conditions will be handled with ease by any chip-type camera equipped with an auto-iris lens.
Interior lighting, on the other hand can cause problems. Although a scene may have constant light applied at all times, that light may not be sufficient for a low-end CCD camera to work properly. At this point a sensitive camera may be needed. Interior lighting luminance is easily calculated using a light meter at the scene to be monitored.
Camera sensitivities
Camera sensitivity has to do with the various light levels that will allow a camera to produce a good picture. Camera sensitivity can be found in the manufacturers specification (spec) sheet. All manufacturers list the camera's sensitivity based upon the minimum amount of light required in a scene (scene illumination).
This listing can be deceptive to the CCTV designer since cameras do not see light as measured from the source. They do see what is known as reflective light. What your eye and the camera actually see is the light that is bounced off an object. Knowing that cameras view reflective light versus incident light is extremely important to the designer.
Light
White light (visible light) is composed of seven different colours of equal intensity but different wavelengths. These seven colours make up visible light spectrum and can most easily be remembered with the help of a simple acronym, ROYGBIV (Richard of York gave battle in vain), R - Red, O - Orange, Y - Yellow, G - Green, B - Blue, I - Indigo, V - Violet.
A good example is the red rose. A red rose will reflect the red light waves of the visible light that strikes and all the other colours will be absorbed, creating the illusion to the eye (or camera) that the rose is red.
Since a camera is designed in such a way that it reproduces its image from reflected light, the amount and type of reflected light needed to produce a video image is what determines the sensitivity of the camera, not the actual scene illumination.
A good example of this would be to compare blacktop pavement to snow. Which of the two has the higher reflectance levels? Obviously, the snow does.
Light reflectance chart
Asphalt (black) 5% reflectance
Concrete (new) 40% reflectance
(old) 25% reflectance
Red brick 25% reflectance
Grass (green) 40% reflectance
Face (Caucasian) 18% reflectance
Snow 95% reflectance
If I have 5 lux units of light in a parking lot, how much actual reflective light does my camera need to have to work?
The answer is very simple. Let us look at the reflectance chart; we find that asphalt has a reflectance of 5%. We can use the formula 5 lux X 0,05 = 0,25 units of actual light. If I had made my decision based upon the manufacturers spec sheet, I will be very disappointed with my picture because I based it upon scene illumination. So I have to choose a camera with a sensitivity of less than 0,25 lux to get a favourable picture.
Source: Frank Street - tel: (011) 838 4515.
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