Slow Shutter Speed
CCTV Technical Series - Slow Shutter Speed
Picking up good video images during the day can be hard enough with all the random factors you can't control, but picking up good images at night is always more difficult. And night time is when people become much more concerned about image pickup. Slow shutter speed is a technique that, essentially, allows the video sensor of a CCTV camera to accumulate enough light to operate in conditions the human eye would perceive as complete darkness. It is sometimes called Sens-up, Sense Up, or similar variations, and depends on controlling the electronic shutter integral to electronic video sensor operation. To understand Sens-up (or Sense Up), you must have at least a basic understanding of electronic shutter operation.
Understanding Automatic Electronic Shutter (AES) Operation
The shutter mechanism of a standard (film based) camera works by very briefly opening (and then closing) when you press the shutter control button. Light passing through the lens passes through the shutter and strikes the film. The film is coated with photosensitive chemicals that change when exposed to light. The amount of time the shutter is open is adjustable (on better cameras) so you can control the amount of light striking the film. The exposed section of film is then removed and replaced with fresh film. When exposed film is developed (exposed to more chemicals) you get a picture.
In digital cameras, video cameras and CCTV cameras there is no film. A piece of layered semiconductor material - a CCD or CMOS sensor - takes the place of the film you would use in a standard camera. Depending on the sensor, the camera may have a mechanical shutter or it may be purely electronic. With regard to CCTV video cameras, the shutter type is usually an automatic electronic shutter (AES).
Without complicating things greatly, let's just say the camera chip set and the video sensor (CCD or CMOS) determine exactly how the AES operates when you turn the camera on, based upon available light. The object is to maximize exposure to gain a good image. Like mechanical shutters Automatic Electronic Shutters have a speed rating that determines exposure time, usually from 1/10,000th to 1/60th of a second. Since it is automatic you generally have no control, unlike a standard film based camera where you have mechanical adjustments (on good cameras, anyway).
NOTE: The following description is very simplified for the purpose of illustration. To fully describe the operation of CCD and CMOS video sensors and how they interact with shutters (mechanical and electronic) would take a lot more space, and a full explanation is not necessary in order to gain a working understanding of Slow Shutter/Sens Up operation. Don't base your research on my (admittedly poor and perhaps incorrect in places, and certainly over-simplified) description.
What does the shutter speed rating mean? If the AES is operating at 1/10,000th of a second, that means the video sensor accumulates light for 1/10,000th of a second and then sends the signal to the image processing circuits, which then send it to memory or down the video cable to your monitor. Meanwhile the video sensor (if it is a CCD sensor - CMOS sensors work slightly differently) gets "wiped clean" and reset by its own control circuits, and is ready to acquire another image (or frame). Like clockwork, every 10,000th of a second (for example) a burst of digital data gets sent to the camera chip set for processing and the video sensor gets reset. Over and over. If for any reason there is not enough available light in that time frame, then the electronic shutter adjusts automatically (hence AUTOMATIC electronic shutter). For instance it may go to 1/5000th of a second and if enough light is available, out goes the image. Too dark? It will adjust again, 1/1000th of second, then 1/500th of a second, etc., until it can get a usable batch of digital data to assemble and send down the pipe. Over and over and over, constantly adjusting based on available light. Not enough light at 1/60th of a second? No picture. It's automatic - you can't adjust it. End of story, end of imaging (until more light is available anyway).
Slow shutter speed lets you adjust the amount of light striking the sensor, and essentially determines when the video sensor sends out its batch of data for processing, REGARDLESS of image quality. If this was a standard camera, the operation would be analogous to holding the shutter open for ridiculous periods of time (1/2 second, 1 second 2 seconds, etc.) and allowing light to flood the film for that period of time. Digital slow shutter (DSS) methods really have nothing to do with a mechanical shutter (since it doesn't exist in a CCTV camera), they merely tell the video sensor to accumulate MORE light before sending the data for processing.
Most CCTV cameras equipped with DSS options usually have a way for the user to program this option through an on-screen display (OSD) menu (although other methods may be used). The options in the menu are usually referred to, in one way or another, as a sens-up (or Sense up, sense-up, etc.) limit and can range from 2x to 128x or more. The setting refers to the amount of light being allowed to hit the sensor - 2-times the "normal" light up to 128-times the "normal" light. This method can allow for brilliant, clear color images to be taken in nearly total darkness. Common usable Sens-up settings run from 2x to 10x and rarely more than that. You'll have to experiment in your surveillance environment to find the best setting. Sounds great, right?
Drawbacks to DSS
I'm sure you've seen photographs that are blurred because the subject moved, or the person holding the camera didn't hold it steady. The same thing can happen when using DSS. If the subject moves while the video sensor is accumulating light the resulting image will be blurry or you may see some strange effects. But you can work around this problem. If you can control the subject somehow, hold them still, then slow shutter speed can produce very desirable results, like capturing a license plate on a car that has to wait at a controlled exit of a parking garage, for instance. Or vehicles at stop lights. For people, if you can somehow delay their movement with a barrier or other device you can get good results. The key is to slow your subject down or ideally make them hold still for a short period of time. Sometimes that's possible, sometimes it's not.
While DSS cameras can help you get color images in total darkness, beware of claims from anyone telling you they are better than infrared cameras. In areas where there is a lot of movement and no way to control your subject, a good infrared camera will probably yield better results. If you can determine how your subject is likely to behave, you can figure out which kind of camera will work better for you.
I hope this helps to explain what DSS/sens up/sense up is, and when you want to use it and when NOT to bother with it. Since CCTV cameras with slow shutter speed options are often more expensive then cameras without, understanding this feature can be helpful when determining where you want to spend your money when designing and implementing your surveillance network.
DSS is Here: COR-552W & COR-598HIM
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