Image Intensifier or Flat Panel Detector?
The question of what’s better – a digital Flat Panel Detector (FPD) or an analog Image Intensifier (II) – is a good one and depends on the actual usage of the system. There are multiple factors to consider when designing an x-ray inspection system. Image Intensifiers are old school technology from the late 1950’s and were the standard (only option other than film) up until some where around 2003- 2004. The technology is a vacuum tube (electron multiplier) with input and output windows that are phosphor coated to convert photons/electrons into visible light.
This same technology is used in a smaller scale for night vision. The main advantage of this technology when used in an x-ray inspection system is the ability to image down to 5 or 10kV. There were other advantages to II based systems over FPD; one is the speed. Image Intensifier based systems operate or produce images at 30 FPS (frame per second) – this is considered real-time. Two is gain, 15000 to 36000 gain makes the Image Intensifiers very efficient at converting electron/photons to visible light at low x-ray or light levels. This is key if you are imaging paper or very light density samples but no so important for most Non Destructive Testing or SMT/PCB applications. Third is the easy ability to create magnification that is not pixel based, the magnification can be achieved in the Image Intensifier by reducing the input window size electronically. A four-inch input on a 2/4 Image Intensifier can be reduced to 2 inches and double the inherent magnification above and beyond the physical geometrical magnification. This technique also has disadvantages because as you reduce the input size you also reduce the Photon statistics resulting in a need to increase kV or mA to offset the loss of incoming photons / electrons / light.
Now for the down side of Image Intensifier, the vacuum tube is convex at the input window, there is always an inherent pin cushioning effect on the resulting image and makes measurements difficult without doing some type of correction algorithms. All output windows of Image Intensifier’s are somewhere around 25mm regardless of the input window size, the input can be electronically manipulated but the output remains the somewhere around the same 25mm. By using lenses and cameras that are focused on the output window we can transfer the image to a monitor or computer. Again this is an area that allows us to increase magnification by using a variable lens system ( 7X zoom is typical) or choosing a lens camera combination to maximize magnification. The problems arise from the mechanical camera lensing combinations, the coupling of the camera to the lens and the combination of the two to the output window results in light loss and degradation of the image.
In the old days we used CCD cameras that needed to be run through an A/D converter before the computer processing, today we would use a mega pixel digital camera and avoid the A/D conversion. The camera/lens portion of this set up is very susceptible to dust and vibration and can easily become unfocused and require frequent cleaning and or adjustment. Then we get to the analog portion of the Image Intensifier. No matter what mega digital camera and lens combination you attach to the Image Intensifier it is always going to be 256 levels of grayscale. In other words, you get 256 shades of gray. This was fine for old school visual inspection but is really under utilizing the computing power of the newest image analysis software packages. Then there is the size factor for the standard electronics inspection Image Intensifier, the weight is somewhere around 20 pounds and the physical size is around 18 inches in length depending on the camera combination, the use of the Image Intensifier will require a larger cabinet/x-ray system regardless of the sample size. Then there is the issue of moving the Image Intensifier on a stage, tilting the weight becomes much more difficult and also exposes the camera/lens combination to vibrations which lead to an out of focus condition and reduced resolution. Image Intensifiers (outside of the night vision ones) operate at 24000 volts DC, so there is a chance of the vibrations to contribute to the failure of the HV power supply that is physically attached in some cases to the Image Intensifier.
Flat Panel Detectors became commercially available somewhere around 2000-2001 when computing power became available and more affordable. This availability was also enabled by considerable improvements to the semiconductor fabrication techniques needed to build large tiles of sensors. FPD uses a couple of methods to convert the scintillating layer of visible light to electrical signals that are then converted to a image that can be displayed and analyzed with the latest software/computer advances. The two most prevalent technologies are photodiodes and CMOS. There are a couple other technologies available but they are very cost prohibitive when building general electronics inspection systems.
The advantages of FPD are the size of the physical package, the flat input window and the grayscale or spacial latitude (4096 minimum grayscale vs. 256). The abundance of grayscale has resulted in computer analysis software algorithms that can detect a single grayscale variance thereby producing test results that are impossible to achieve through visual analysis (the human eye can not really detect grayscale past 256 shades). Furthermore, the resultant flat image requires no corrections for accurate image analysis and measurement. The signals produced are also digital, so there is no loss of the signal A/D conversion or image degradation because of lensing or camera configurations.
Larger FPD’s can also be economically produced by connecting multiple photodiodes or CMOS panels together as opposed to large area detectors made from single sheets of amorphous silicon. There are no moving parts (focus – zoom – iris) on a FPD and the requirements to move (z-axis / tilt) are fairly simple as well as no dust or vibration concerns. FPD detectors had only two disadvantages or concerns when compared to Image Intensifiers. One is the speed; typically FPD’s will capture images at speeds of less than 30 FPS although the speeds are increasing as the cost for the increased speed is decreasing. Modern FPDs used in Creative Electron TruView X-Ray Inspection systems come standard with 30 FPS speeds. The second disadvantage is the FPD’s need for high flux or high photon statics. Typically a FPD will require a higher kV \ mA combination (wattage) to achieve a usable x-ray image vs. an image intensified system. However recent advances in FPD technology has greatly bridged this gap.
The original FPD’s were very expensive and painfully slow when compared to Image Intensifier/ camera systems but the trend has been that of larger FOV / Panel sizes running at faster speeds (30 FPS) while at the same time bring the costs in line with Image Intensified systems. The use of FPD’s is pretty much the standard in industrial cabinet x-ray systems today.
There were only a few reasons that an Image Intensifier based systems would excel over a FPD based system, that being a low density sample requiring very low penetration (paper) or speed/FPS and the speed issue is quickly becoming a non-issue. Please let us know what you think about this post by including your feedback in our comments area.