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The CCD Camera
Introduction
As with any kind of photography, lighting of the subject is the most important factor to consider when conducting a surveillance. Unfortunately, light is often completely out of the control of an investigator. Waiting for the sun to rise higher or move from behind a cloud is not an option when filming an event that must be captured immediately. Filming at night can be problematical in that the available light may not be adequate to clearly identify the subject and the activity taking place. If additional light is added, it may draw attention to the presence of the camera or otherwise alert the subject that something in the environment is unusual. Even though an investigator may not have control of artificial lighting at a given surveillance site, knowing the characteristics of certain light sources can improve the chance of success.
Fortunately, the camera is one area where an investigator has some control. Video cameras and camcorders have been developed using many solid-state devices. These devices have reduced the physical size and enhanced performance over earlier types of video cameras. Of special interest to an investigator is a category of CCD camera known as the "micro camera". These cameras are small, inexpensive, and very rugged. Understanding some of the design features of these cameras and how they are affected by various light sources will aid an investigator in selecting the best camera for an application; further improving the chance of a successful surveillance.
The CCD
At the heart of every solid state camera is a component known as a Charge Coupled Device (CCD). This component is designed to convert the light image formed by a lens into an electrical charge that drives the image forming video circuits of a camera. Because they are placed directly behind the camera lens, they are often referred to as the camera sensor or target. Sensors are available in various size formats, with 1/2² , 1/3² , and 1/4² being the most common. The common names for these micro cameras are "circuit board cameras" or "chip cameras". "Circuit board camera" refers to the fact that these cameras are often sold without an enclosure, while "chip camera" refers to the fact that semiconductor chip technology is a large part of the make up of these cameras. Besides micro cameras, charge coupled devices are also used in camcorders and security system cameras.
A Visible Difference
It is important to know that these camera sensors view the world in a way that is far different from our sense of sight. Our eyes respond to light waves within a light spectrum of 400 nanometers (violet end of the spectrum) to 700 nanometers (red end of the spectrum). This comprises the visible light spectrum. Our eye’s response to light forms a bell shaped curve that peaks more or less centered in the visible spectrum at the 550 nanometers. CCD camera sensors respond to light beginning at 500 nanometers in the visible spectrum and end far into the infrared (IR) spectrum at 1100 nanometers. A CCD sensor’s peak sensitivity occurs at 980 nanometers, which is well above the range of our vision.
Both color and black and white cameras use the same type of CCD sensor, the only difference being that an IR filter is placed over the sensor in a color camera. This is done because the high IR sensitivity of the CCD sensor interferes with the automatic color balance circuits of a color camera; a circuit that maintains true color rendition under varying light levels. For this reason color cameras do not do as well as black and white cameras in low light situations where light sources with IR components may be a factor. Recent improvements in color cameras have improved low light camera response, but black and white cameras still perform better in low light situations.
Artificial Light
Because of the differences in sensitivity between the human eye and the CCD camera, the actual light available for the camera may be far greater than the apparent light as viewed by the human eye. This effect will be enhanced or diminished, depending on the light source and camera used. This is because some light sources have a high IR content and others have little or none.
An important example is found in the tungsten element of an incandescent light bulb. These lamps give off some light in the visible light spectrum, but produce much more energy in the IR range. For the purpose of illustration, suppose you are investigating employee theft, and you set up a black and white camera in a room. To your eye the room is dimly lit by two 40 Watt table lamps. This is the apparent light visible to you. However, when you check the camera output on the monitor, to your surprise the room appears to be brightly illuminated. This reveals the actual light available in the room. The high IR radiation from the 40 Watt bulbs, combined with the CCD camera’s IR sensitivity, produced a brilliantly lit scene when viewed on the video monitor.
Conversely, a low pressure sodium light (such as those used in parking lots) produces light that is confined almost entirely to the yellow segment of the visible light spectrum. There is no IR component to these lights and when a color camera is used to surveil a scene using this illumination, much detail is lost. This is due to the fact that the camera can only render yellow objects well. All other colors will be rendered as brown or black. For this reason, color cameras are best used in daylight situations or with artificial lighting that produces a wide spectrum emission. A color camera would be a poor choice for a parking lot at night.
Infra red (IR) illuminators are available that will extend nighttime use of back and white cameras to areas where total darkness would prevent their use. Because the IR spectrum is outside the visible range, the scene would appear unlit to the unaided eye. Most of the smaller, inexpensive illuminators have a range of 25 to 30 feet. They are often made up of an array of IR light emitting diodes (LED). Some cameras have a few (4-6) LEDs right on the circuit board next to the lens. These will illuminate 10 to 15 feet.
Natural Light
Outside, sunlight will produce more than adequate light for most situations with either black and white or color CCD cameras. Of course, in the natural world light intensity will vary from very bright to very dark. CCD cameras are equipped with a means to maintain the quality of the video picture between these two extremes. In a 35mm film camera, the exposure of the film to light is controlled by the shutter (length of time the film is exposed) and the size of the iris opening in the lens (quantity of light to strike the film). Because pinhole and mini-mount CCD cameras do not have an iris, picture quality is maintained only by reducing the amount of time the sensor is allowed to build a charge for creating the picture. This is called "electronic shuttering" (from above: time = shutter). In actuality there is no mechanical shutter, but conventional shutter speeds are used as an equivalent index. As the light level rises, the "shutter speed" increases by means of automatic electronic circuitry, decreasing the time for a charge to develop. If the scene should darken, the shutter speed would slow, increasing the time for a charge to build.
The circuitry that regulates this shuttering measures light on an "averaging" basis. "Averaging" means that the camera measures the entire picture frame to determine the amount of shuttering to properly expose the CCD. This is very effective for evenly lit, low contrast scenes. However, it can be ineffective for scenes with deep shadows or high contrast. As a result, micro cameras with auto shuttering will work best if the scene to be filmed is evenly lit. However, special circumstances warrant more sophisticated camera control than can be found on inexpensive micro cameras.
For a higher cost, micro cameras are available with a manually adjustable shutter, back light compensation, and white balance (color cameras only). This allows the investigator to override the automatic settings and manually adjust the camera for difficult situations. A manual shutter allows a slower shutter speed to be selected to compensate for deep shadows in a scene, while a faster shutter selection compensates for a brightly lit scene or rapid motion. Back light compensation corrects the light metering for a scene with a dark foreground and a light background. White balance control allows the color to be calibrated to a white surface, thus truing all other colors as well. If the camera has a C or CS mount, then a conventional lens with an iris can be used, allowing further control of light. These lenses may feature a manual and/or auto iris.
Camera Sensitivity
Light sensitivity of a camera’s CCD sensor is measured in a unit called Lux, the lower the number, the better the low light capability of the camera. A value of 0.05 Lux for a black and white camera and a value of 0.45 Lux for a color camera can be found in the marketplace today. These are presumed to be the lowest light levels at which the camera will produce a usable picture. For comparison with the above figures, a full moon will provide 0.2 Lux of illumination, while full sunlight will provide 100,000 Lux.
