After conversion of radiation to visible light, the light image must be changed to a video signal so that an image can be produced for viewing on a monitor. In most instances, a closed-circuit TV camera (CCTV) is used. The cameras can be very small are typically housed in an enclosure on the back of the image intensifier. Several camera types are available, and each type has unique capabilities. One camera feature is the dynamic range, or the range of light levels that it can resolve. A high dynamic range is needed if part thicknesses differ and bright and dark areas must be imaged at the same time. Some cameras have very high dynamic ranges and would be especially suitable for RTR. Another feature of cameras is the type of transducer changing light energy to electrical energy. Some cameras have pickup tubes while others have solid-state electronics.
Charged couple device (CCD) cameras are very popular in RTR systems and have solid-state electronics consisting of a very small semiconductor chip. The chip is made up of discrete elements (pixels), which receive light and create a voltage and a current directly corresponding to the amount of light received. This same technology is used in home video camcorders and digital cameras. See figure 11 for an example of a CCD camera. The image is not recorded on film by means of grains of silver; rather, the voltage produced creates an RS170 signal either to produce an image on a monitor or to record the image on a VCR tape, CD or other means.
Figure 11. Picture showing one of the many commercially available CCD cameras that can be used for RTR. A lens would be attached to the right side of the camera and focused on the image intensifier output screen. The signal from the camera is then fed to a monitor for viewing.
If the CCD camera is not used, another type of pickup device (a transducer) is used to convert the visible to an electronic signal. This usually involves a vacuum tube containing a light-sensitive material allowing the conversion of light to an electronic signal. Some common devices using this system are the image isocon, videcon, newvicon, and plumbicon videcon. These systems give better resolution than the CCD camera but usually have a lower dynamic range (1/m), which means that the range of light levels able to be shown is smaller. Vacuum tube devices usually have longer lag times, which can cause blurring if the part is to be inspected on the fly, or while moving.
Whichever transducer the camera uses, the RS170 signal produced creates the image on a monitor in the same way an image is produced on a TV screen. The CRT tube emits an electron beam controlled by two electromagnetic fields. One field causes the beam to move horizontally on the screen; the other to move vertically. The TV image or frame is made up of 525 lines (625 lines in Europe), and a new frame is generated every 1/30th of a second. The 525 lines are scanned rapidly with every other line (odd lines first, then even lines) being scanned and interlaced with the adjacent line. Each new frame starts in the upper left-hand corner. Vertical resolution (visible details lying horizontal) is limited by the space between lines. In summary, the RS170 signal creates the image by means of an electromagnetic field moving the beam across the screen horizontally and then vertically to make another line. After 525 lines are created, a new frame is produced. Thirty frames per second is the standard that fools our eyes into believing we are looking at a steady picture. If fewer frames per second are produced, the eye may detect flicker in the image.
The monitor image is produced by an analog signal, and its increase or decrease is analogous to the value it represents. Therefore, the signal is or can be constantly differing in intensity. Examples of analog information are the column of mercury in a thermometer, or a cars speedometer when a needle in front of a scale of numbers indicates speed. For more information on digital and analog information, refer to the appendix of this book.
The image on the monitor produced by the analog RS170 signal can be used without enhancement. Equipment needed in this case consists of X-ray source, image intensifier, camera, and monitor. If the combination of components yield the spatial resolution and sensitivity needed, then the system is adequate.