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TV LINE RESOLUTION
Both CCD and Monitor Resolutions are often given in terms of "TV line resolution," which is a test performed using an EIA Test Pattern target. Unfortunately, this specification has no physical units of measurement (such as mm or inches) and can therefore be rather ambiguous. To convert TV line resolution into lines per mm (for example), the following conversions can be used:
We recommend you use electronic components of increasing or equivalent resolution as components are added to the camera. Resolution, as well as contrast, is higher in monochrome systems due to lower signal degradation. Contrast is determined in discrete increments by using an EIA Gray Scale Pattern target.
PIXEL COUNT RESOLUTION (NUMBER OF PIXELS)
Many of our customers are concerned with pixel count resolution. Like TV line resolution, pixel count resolution can be misleading if not interpreted correctly. Most CCD's do not offer perfect rows and columns of pixels and often contain dead spaces in between. Even cameras that have no dead space can yield imperfect center to center pixel resolution. Factors such as signal-to-noise and pixel jitter all contribute to the camera's actual spatial resolution. Where pixel resolution proves to be the most beneficial is in the use of video capture devices. When using video capture devices, the only way to match the capture board's capabilities with that of the camera's is to compare the pixel count.
PRIMARY MAGNIGFICATION
Many of our lenses list a primary magnification for calculating the field of view, which is dependent on CCD size. The following equation shows the relationship between primary magnification and field of view (taking into account overscan of an analog monitor.)
Most lenses in our catalog specify horizontal FOV for a 1/2 inch CCD format (although the lens may be designed to fill a larger CCD format). Clearly, the field of view is dependent on the size of the image sensor. For some applications, it might be necessary to select a camera based on chip size, in order to attain the desired field of view.
CCD SENSOR FORMAT
Aspect ratio for most Charge-Coupled Devices (CCD) is 4:3. Note that the sensor format size is not equivalent to the sensor's active area. Video lenses can be used with any CCD camera, provided the lens design format is larger or equal to that of the camera. If the sensor is too large, vignetting (tunnel vision) will occur.
C-MOUNT
This is a threaded mount (1-32 TPI) and is used on most industrial CCD cameras and lenses. The flange (back focal) distance to the sensor is 17.5mm. CS-Mount thread is the same, but the flange distance is 12.5mm. C-Mount lenses and CS-Mount cameras are compatible, using a 5mm spacer. (#03-618).
CCD CAMERA SENSITIVITY
Above is a typical response curve for silicon CCD's. All of our black and white cameras have a similar response (except for NIR versions). The color cameras typically include an IR blocking filter that cuts off at 700nm. Given the wide range of illumination products that are currently available, it is important to consider how well a camera responds to different wavelengths or colors of light. Certain applications demand high sensitivity to wavelengths that are not visible to the human eye, such as ultraviolet (UV) or near infrared (NIR).
ANALOG VS. DIGITAL
Analog video cameras transmit a continuously variable electronic signal in real-time. The frequency and amplitude of this signal is then interpreted by an analog output device as video information. Both the quality of the analog video signal and the way in which it is interpreted will affect the resulting video images. This method of data transmission has both pros and cons. Typically, analog cameras are less expensive and less complicated, making them cost-effective and simple solutions for common video applications. However, analog cameras have upper limits on both resolution (number of TV lines) and frame rate. One of the most common video signal formats, called NTSC, is limited to about 800 TV lines and 30 frames per second. Analog cameras are also very susceptible to electronic noise, which depends on many factors that are commonly overlooked, such as cable length or connector type.
On the other hand, digital video cameras transmit binary data (a stream of 1's and 0's) in the form of an electronic signal. An output device then converts the binary data into video information. There are several key differences worth noting here. First of all, the video signal is exactly the same when it leaves the camera as when it reaches an output device. Secondly, the video signal can only be interpreted in one way. These differences eliminate errors in both transmission of the signal and interpretation by an output device. Compared to analog counterparts, digital cameras typically offer higher resolution, higher frame rates, less noise, and more features. Unfortunately these advantages come with costs: digital cameras are generally more expensive than analog ones. Furthermore, feature-packed cameras may involve more complicated setup, even for video systems that require only basic capabilities. Digital cameras are also limited to shorter cable lengths in most cases.
SIGNAL FORMATS
There are several basic signal types used in electronic imaging. Analog standards include: Composite (NTSC/PAL, EIA/CCIR), Y-C (S-video), and RGB. Of these, NTSC (PAL/RS-170A/Color) and EIA (CCIR/RS-170/Monochrome) signals are the most common and will accommodate most applications. Y-C and RGB signals provide superior image quality by separating color information into discrete channels. Analog cameras can be connected to analog acquisition hardware for image capture, or directly to a TV monitor for viewing.
Two common digital signal types are CameraLink™ and IEEE-1394 (Firewire). Other formats include RS-422 and RS-644. Digital cameras require a computer interface to display images on an output device. An important point to consider is that Firewire cameras can be connected directly to computers that have a Firewire interface using only a cable, whereas a separate acquisition card is needed for CameraLink™.
CONNECTOR TYPES
Composite analog cables are generally terminated with BNC or RCA connectors. Y-C signals require a 4-pin DIN type connector to transmit a separate color channel within the cable. RGB signals go one step further and divide color into three separate cables (branched at ends of main cable), which are each terminated with BNC or RCA Connectors (note: typically RGB cables use a fourth cable branch for Sync). A single composite connection can limit the performance of color analog cameras. Since Y-C signals offer greater fidelity, it is preferable to use Y-C connectors over BNC whenever possible. Increased cable length may also degrade the quality of analog video signals.
IEEE-1394 (Firewire) cables have small, multi-pin connectors that transmit digital information, and may also provide the camera with power (note: laptop computers typically require a special connector that diverts power for the camera to an external source to save batteries). CameraLink™ uses MDR 26-pin connectors on an LVDS cable, which is limited to about 10 feet in length.
AUTOMATIC GAIN CONTROL (AGC)
This is an electronic option that automatically compensates for low light levels. Manual Gain Control is a similar option that is used to adjust the camera to changes in the system environment.
ELECTRONIC SHUTTER
This is an electronic function that controls the amount of light in a system in order to prevent overexposure and blooming. The shutter works in stepped increments, decreasing collected illumination by 50% for each increment. The shutter decreases exposure time in eight steps, from 1/60th second to 1/10,000 second.
Color vs. Monochrome Cameras
Using a Single Chip
Color Camera
Using a Black and
White Camera
Although this distinction may seem unimportant, it is actually one of the most fundamental points to consider when choosing a video camera. When choosing a camera for an industrial application, many system specifiers instinctively select color because they feel a monochrome image is inferior. That, however, is incorrect. Monochrome (black-and-white) cameras have a single sensor that outputs grayscale images. Effectively, each pixel on the sensor is assigned a numeric value based on the amount of incident light. The smallest number represents black (zero), the largest number represents white, and everything in between is a shade of gray. Therefore, each pixel generates information only about intensity, and not color. A mosaic filter is required in order to detect color, which limits the resolution of a single sensor. As a result, monochrome cameras typically have 10% higher resolution than comparable single-chip color cameras. Monochrome cameras have higher resolution, better signal-to-noise ratio, increased light sensitivity, and greater contrast than similarly priced color cameras. Although color imaging may be preferable, the eye perceives spatial differences more clearly in gradients of black and white. In addition, industrial applications requiring a computer interface typically operate with a black and white camera, since a color image requires more processing time and does not yield significantly more information about the object. In some cases, color filters can be used in conjunction with monochrome cameras to differentiate colored objects. For all these reasons, monochrome cameras are generally better for measurement and machine vision applications where resolution is more important, or color is not necessary. When a high resolution color image is necessary, it is beneficial to use a 3-chip (also called 3-CCD or RGB) camera. By utilizing three CCD sensors, these cameras offer the best of both worlds, yielding greater spatial resolution and dynamic range than single chip color cameras. The image is directed to each sensor using a prism and is then filtered to provide independent red, green and blue signals. The RGB output from a three-chip camera is considered to be superior to the standard NTSC/PAL and Y-C formats, because the RGB color information is on three separate signals.
