The Low-Light Resolution
One of the biggest problems with camera systems has always been producing viable, working images in the dark. I'm not just talking about low light, I mean in the dark.
One of the biggest problems with camera systems has always been producing viable, working images in the dark. I'm not just talking about low light, I mean in the dark. Yes, we have had the technology to look around in the dark for the past 30 years, but could we afford it, and could we do it in color? Let's start with a quick overview of the problems that face image production in the dark.
In the Dark
The first mistake most designers make is measuring the ambient light in an area and considering that measurement to represent the light the camera uses. However, cameras see reflective light as well, so with only an ambient light measurement, we come up anywhere from five percent to 95 percent short in our lighting. For example; if the ambient light at the darkest point in a parking lot is .01 foot candle (fc), we must remove 95 percent of that to determine the actual working light of the camera, because asphalt has a five percent reflectivity. The consequence is that we really have .0005 fc of usable light in that parking lot at night.
Light loss caused by the lens is another impact that may be missed. Light loss from the lens can be as much as two to three f-stops. An f-stop is a unit of measurement assigned to light gain or loss. One f-stop gain is equivalent to a 50 percent decrease in light, and one f-stop down is equal to a 100 percent gain in light. Therefore, if our lens has a two f-stop light loss factor, we must decrease our .0005 fc by 50 percent two times (once for each f-stop): .0005 fc / 2 = .00025 fc / 2 = .000125 fc. Now we don't have to know what a fc of light is to be able to understand that there is a huge difference between the original ambient light measurement of .01 fc and the final .000125 fc.
Our dilemma lies in the lack of useable reflective light for the camera and the lack of color reflectivity below 2 fc of ambient light.
Options and Pricing
Five years ago, the sensitivity range for a BW camera directly corresponded with the price. The higher the sensitivity, the higher the cost of the unit. But BW cameras were still the most affordable cameras, ranging from $800 to $1,200 for up to .0001 fc. The intensified camera of the past averaged between $8,000 and $12,000 without housing or lens and averaged a sensitivity of .00002 fc (half moonlight).
The IR-enhancement light scheme would include one or two large, expensive IR lamps with a potentially expensive camera. As a set, the average IR lamp/camera would cost about $3,000 to $5,000 complete.
The thermal camera of five years ago started at around $50,000 and could work up to $300,000 very quickly (dependant upon lens and image enhancements). The sensitivity of the IR-enhanced and thermal cameras is a moot point, because the IR camera uses IR light enhancement as an aid and the thermal camera does not recognize light.
Color at night. We are quickly realizing color at night. The new color cameras are toting sensitivity ranges as low as .003 fc. This is a huge improvement over the sensitivity of previous versions, and it is literally changing many, if not most of our night views. The door to color at night is wide open and applications are streaming in.
Day/night cameras. The next most exciting step forward are the day/night cameras-color by day, BW by night. They work on three principals.
1)Double imagers: A color imager that creates an image until the video signal output drops below an accepted point and then automatically switches over to a more sensitive BW imager.
2)Double scan imaging: A color imager that drops out the color portion of the image when the output falls below an accepted level.
3) Mechanical IR cut filters: The IR cut filter ensures that the color balance of the image will remain true without being affected by the upper ranges of red or heat in full-light situations. By using a mechanical device to remove the filter from in front of the imager in lower light levels, the color CCD can gain significant improvement in sensitivity, partially because of the removal of the filter (up to 1 f-stop) and partially because of the enhanced IR sensitivity without the filter.
These bad boys and girls of the day/night group tote an average sensitivity range of
.0001 fc. There are two problems, however, that have presented themselves.
The first is related directly to lighting. Some units have a problem switching into the night mode if there are street lamps in the image. This is due to the spike in the video signal that is caused by the intensive point of light from the lamps. The apparent problem is that the electronics of the camera are not designed to quell such spikes and so the unit thinks that it's still daytime.
The second problem comes in the form of white balance. White light is made of equal levels of all colors-red, orange, yellow, green, blue, indigo and violet. The white balance circuit is designed to make sure that colors stay true by measuring the incoming light against a true white source that is built into the camera and then adjusting the final image against impurities. Since these cameras are so sensitive there are a few models that appear to be having problems with their white balance circuits in the lower light levels. That is, cold lights (florescent) tend to turn the images blue, while warm lights (tungsten) tend to turn the images yellow. It is a problem that can only be solved by replacing the camera, model and all. The best suggestion is to test and demo on site for a day or two. Overall, the day/night cameras range from $800 to $1,500 and are proving to be an extremely valuable asset to the design process.
Night vision cameras. New to the market are night vision cameras. These new intensified cameras are bragging and proving an incredible sensitivity of .00000046 fc sensitivity. This is huge and borders on thermal imagery. The best part is that they are affordable within the range of $6,000 to $7,000, including housing and lens. Never before have we been able to monitor such a range of areas along perimeters and unlit areas so effectively. For those areas where the light level is just too low, these BW units are able to work with IR lighting enhancements. I say to these new cameras, welcome to the camera package.
Thermal cameras. Of course, to leave out the thermal camera would be to ignore a viable work horse that is fast becoming a valuable asset to the security design where lighting is just not an option. These cameras work by measuring the temperature of the area of view and then displaying the readings in the form of colors on the screen. Warm objects such as humans will appear in various shades of red, while cold objects are varying shades of blues and blacks. The net result is that these cameras see in sub-zero lighting with no enhancements. The best part is that they can be used for a huge variety of applications, from perimeters to waterfronts.
Yes, they work just fine in full daylight, because they tote visible light filters that block out 100 percent of the visible and IR spectrum. The downfall is that they must use specialized lenses and so are just a little bit restrictive. The best part is that these cameras are more than affordable today, especially when you consider the options and costs involved in increased lighting. Ranging from $6,500 to $300,000, these cameras are now to be considered full members of the CCTV industry.
To sum it all up, the night has never been brighter or more affordable. However, I have always promoted and will continue to promote that testing in the field should always be done, whenever and wherever possible, to prove results ahead of investment.
By Charlie R. Pierce
LeapFrog Training & Consulting