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.
The color issue with low-light imaging could fan into three or four issues, but I will keep it in simplest terms. The first consideration is that colors reflect differently as the intensity of the light decreases. The human eye is based upon cold light, or blues, while most camera technology is based upon warm light, or reds. Because of this, it is actually easier for the human eye to see in the dark than a camera. The eye is designed in such a way as to shut down color recognition in extreme light conditions, so in low light, colors slowly blend into each other and appear as shades of gray.
Thanks to digital technology, video now has the ability to restore colors to their original intensity. This has opened the door for a whole slug of good, low–light, color cameras. Whereas in the past we were restricted to light levels of 1 fc or more for color images, today we are able to reach into the depths of .01 fc and less.
Inconsistent Scene Illumination
A bright spot, or a combination of deep shadows and bright areas, can play havoc with auto-iris lenses. A bright spot or high reflection causes the iris to close. This action decreases the light coming from the whole scene and consequently creates a dark, grainy image. Thanks again to digital technology, we are able to electronically balance a scene and therefore bypass the auto-iris lens completely.
Weather and Dust
Crap in the air is still the number one issue with low-light technology. Dust, snow, rain, and fog all add up to interference in the image. The first factor is the density of the stuff. How thick is too thick to see through? The second concern is the reflective value of the interference. If you’ve ever turned on your car’s brights during a foggy night, you know the feeling of being blinded by the thing that is supposed to help you see. The third factor is just the opposite: How much light is lost to reverse reflection and/or absorption of the debris?
We have attacked fog, snow, rain and high dust with yellow, red and infrared lighting techniques. This works because the longer wavelength of light can move through and between the various molecules of interference, avoiding a great percentage of reflectance. However, it does not solve low-light viewing problems.
For this, technology again steps in. Using a combination of extreme resolution and digital filtering, we have cameras that not only see through these various debris, but do it at distances of up to 10 km. Just the kind of stuff that is needed and being used on borders and ports and in jungles.
So, all said and done, where are we with low-light cameras? We are in great shape and getting better all the time. Color cameras are available in a wide range of sensitivities going all the way down to .01 fc and further. It all depends upon your application and budget. Black-and-white technology drives down the sensitivity to .000005 fc. Then, of course, we have specialty technology for viewing extreme distances at extreme low levels of light that I just can’t add enough zeros to. The bottom line: Your applications in the dark are easier to see than ever before. What used to cost us tens of thousands of dollars, we are now producing for less than half or even one quarter of the cost. Open your eyes and you will see.
Charlie R. Pierce is director of integrated security technology for IPC International Corporation. Mr. Pierce welcomes your questions and comments. He can be reached at email@example.com.