One of the mystery areas of CCTV cameras has always been camera sensitivity and minimum scene illumination. Often, camera systems are specified for a particular application only to find out later the scene illumination was not bright enough for good video evidence. How can that be? The lights and the camera are the same as when they were originally sold.
The problem is that we were focusing (no pun intended) on the scene lighting or illumination and not on the reflected light to the camera, which we will refer to as reflectivity. As we will see, scene reflectivity can vary considerably and cameras and lenses must be selected in order to get a good image.
Remember, minimum scene illumination indicates the minimum light required at a scene in order to get an acceptable video picture. Though this sounds simple, there are a number of variables in a camera system that can influence how the camera really performs vs. the manufacturer’s specifications.
Shedding Light on Illumination
There are some key terms and factors that influence getting a good and well-lit video image.
Illumination intensity — The power of what is often referred to as the primary light source. The source will either be natural (sun, moon or stars) or artificial (tungsten, gas or solid-state). It is measured in “candelas,” which are the amount of light energy generated by an ordinary candle.
Lux — The most common CCTV term dealing with camera sensitivity. It is measured according to the principle that 1 lux equals 1 lumen falling on 1 square meter. The physics of this term can get quite involved, so I will stop here and leave the rest to our science readers to investigate further on their own. The rest of us just need to know that this is a common term for scene illumination. It is a term based on the metric system, as the British counterpart is the foot-candle; 1 foot-candle = 10 lux.
The lux value can vary considerably for different light conditions. It is important for the security professional to have a good feel for what the scene illumination lux ratings are for different conditions (see Chart 1 on page 28 of December issue).
One question that comes up when dealing with light sources is “How far does the light shine?” When working with light sources and cameras, there is a physical law of light we must address known as the “Inverse Square Law.” This basically states that illuminance (E) at a distance (D) is proportional to 1/distance squared (E = 1/D2). For instance, a light source providing a level of 30 lux at 20 meters will provide 7.5 lux at 40 meters and only 3.3 lux at 60 meters.
F-stop — The capability of a lens to collect light. High quality lenses can collect more light. Each F-stop downward will collect twice as much light as the previous setting. A lens of F 1.4 will collect two times more light than an F 2.0 lens. Make sure to note the F-stop in camera specifications.
As a bit of an aside, the F-stop of the human eye is about F-2.8. How did we find that out? The focal length (FL) of the human eye is about 17mm (distance from retina to lens). The maximum iris (I) opening is about 6mm. The F-stop formula is F = FL/I or 17mm/6mm or 2.8.]
IRE — Not often mentioned, this is important in camera sensitivity and performance. It is an international measurement of video amplitude. Remember, your camera sensor needs to be able to adequately convert light energy to electrical energy. The maximum amplitude for CCD cameras should normally be set at 100 IRE or 700 millivolts.
Automatic gain is often used to boost the IRE, however extra noise can result. A camera specification should include an IRE rating. Be cautious of those below 30 IRE.
Color temperature — A measurement of the color appearance of light, not the actual temperature. It indicates the hue of the color. Different color temperatures can give different camera sensitivity results, and some specifications will list the color temperature at which it was tested. A color temperature of 3500°K is considered a neutral color setting, such as an office.
Reflectance — This is how much an object reflects light and is often referred to as the secondary source. It is the true source of camera illumination as it is the light the camera actually sees. It is expressed in a ratio or percentage of the source illumination.
As you can see in Chart 2 on page 30 of the December issue, the reflectivity can vary considerably and is often a challenge for northern parking lots that can go from bare asphalt to snow-covered in the same afternoon. This should be a lesson to those who think they can cut corners and not use auto-iris camera lenses on outside cameras.
CCTV guru Charlie Pierce advises to use caution when specifying cameras for parking lots with older, lighter gray-colored asphalt. At some later date, the parking lot will get recoated with a new and considerably blacker asphalt coating. Since the older blacktop reflects more light, the reflectance will be considerably lower when the lot is recoated.
Exploring a Hypothetical Example
Let’s look at an example and see what we have learned. We have two cameras specified; one at 0.8 lux with an F 1.0 lens and another at 0.8 lux with an F 1.2 lens. Which camera is more sensitive?
Did you select the first camera right away because it has the faster lens and will let more light in? Well, camera two is the correct answer because we have to consider the lux level. If I can get a 0.8 lux at the CCD sensor with an F 1.2 lens, I will probably be able to get around 0.6 lux if I use a F 1.0 lens on the same camera.
Now let’s get out our slide rule — oops, wrong generation, I meant calculator — and look at calculating how much light (LCCD) reaches a camera’s CCD chip sensor when the source illumination is known. We will look at an interior office location with a camera that has a lens F-stop setting of F-16 and is in a recessed dome.
Light CCD = 2.52 L / (4 F2)
In this formula, L is the light to the lens and F is the F-stop. We will estimate from our charts that the office illumination is about 500 lux, the overall reflectivity of the office décor is 50 percent and the light attenuation of the dome is 30 percent. Our calculations now show the following: Light CCD = 2.51 x (500 x 0.5 x 0.7) / (4 x 162) = 0.17 lux. The camer’s CCD sensor is receiving 0.17 lux.
Notice the influence of the reflectivity (0.5) and the complement of the dome light attenuation (0.7). If the F-stop was adjusted to F-1.4, we would get 22.3 lux and the camera’s built-in electronic iris compensation would be needed or the video would be washed out.
Click here to see Bob Dolph answer your questions in this month’s “Tech Talk Q & A!”