Seeing in the dark isn’t just a science, it’s an art. For the 31 years I have been in the security industry, the goal has been to see in the dark. We have combated the empty voids with white light, infrared light, excited electrons in third- and fourth-level intensifiers. We have tweaked and tuned our technology until it feels as though there is nothing else that can be done. And then, right when you think it’s over, someone comes along and gives it another shot in the arm. So we continue to attack and defeat our dark side.
But there are plenty of obstacles to contend with. We have lens light-loss factors. We have distance issues. We have image clarity problems. We have color issues. We have inconsistent light reflectance issues. We have crap in the air to deal with. We have the technical process of deciding between intensification or just building a better mousetrap. And last, we have the dark itself ... the void ... the absence of light or reflective light. The absence of the very thing our eyes and cameras require to create an electrical pulse that is deciphered into an image.
Any time light passes through a density, it refracts or bends. This refraction is caused by the slowing of the various light waves as they pass through the lens or glass, and it causes the light to lose intensity. The intensity of the light is the very thing that generates the energy to create an image.
We measure this light loss in f-stops. One f-stop gain is equal to a 50% reduction in the light passing through the lens. One f-stop loss is equal to a 100% increase in the light. All lenses have light loss. If they didn’t, their rating would be f-0.
A camera’s sensitivity is based upon the lens that it was tested with—usually an f-1.4. So the first step to low-light vision is to improve the quality of the lens. Not necessarily an easy or inexpensive thing to do. However, with our improved technology, most low-light images could be greatly improved if the designer would only install a better-quality lens. The lenses are out there and very available.
There is a little-known rule called the inverse light law that states that light diminishes in inverse ratio to the square of the distance from the source—a fancy way of saying that as light travels, it spreads out and therefore diminishes. The formula for this is E = I/D?. E is energy or light, I is intensity and D is distance. In simpler terms, if you have a light source that produces 6 fc of light at 60 feet away, you would have .75 fc at 120 feet and .33 fc at 180 feet.
The bottom line is a catch-22. If you want to increase your light intake to the camera, you use a wider-angle lens. However, if you want to identify your subject, you need to use a telephoto or zoom lens. By going telephoto, you narrow your light-gathering area dramatically and you increase your lens light-loss factor.
How clear is the image? How much detail is lost in white noise or electronic garbage? Inside our cameras, we have various circuits that are designed to enhance or promote our image under adverse situations. One such circuit is the auto gain circuit (AGC), which is designed to amplify the video signal whenever it drops below an acceptable range—usually about .8 volts peak to peak. These circuits are even in our IP cameras, they just work a bit differently.
Auto gain circuits were originally designed to keep the image consistent as the image travels through shadows. The unfortunate side effect of this circuit is that in the process of amplifying the video image, it also amplifies the noise level, creating a very grainy image and throwing clarity out the door. So in most low-light camera technology, the AGC must be greatly filtered or dropped completely.
Intensified cameras use an electron exciter mounted in front of the CCD. As light enters the exciter, the electron flow of the light is intensified, thus giving the illusion of amplified light. The natural side effect of this process is an equal distribution of white noise in your image as the light levels drop. Thanks to digital technology, a great portion of this noise is now able to be filtered out of our images.