Medical Electronics Manufacturing
Magazine
MEM Article Index
Medical Electronics Manufacturing Fall 1998
Display Technologies
Cathode Ray Tubes
Ken Compton, director of strategic marketing, Clinton Electronics Corp.
Cathode ray tube (CRT) technology has evolved over the years from early television to today's large-screen formats and high-resolution computer displays with photorealism. The CRT continues to address these needs because it is a flexible device built from relatively low-cost materials and well-defined manufacturing processes. Whether the CRT is a Trinitron aperture grill, slot mask, or delta triad, the fundamental architecture is to accelerate an electron beam from the source (cathode) to the phosphor screen.
The high-voltage (anode) potential at the screen face provides the energy to accelerate the electrons to impact velocity. The impact energy is converted to light output by the phosphor. In a raster-scanned device, the electron beam current sent to the screen is controlled to vary the image content for brightness and color balance. A monochrome CRT operates the same way but with just one phosphor and a single electron gun (color having a gun and phosphor for red, blue, and green). A monochrome CRT displays brightness (luminance) from black (off pixel) to peak white (on pixel) and in the incremental steps provided by a video graphics source to render gray scale.
The advent of teleradiology, picture archival and communications systems (PACS), and digital image sensors (direct radiology) are changing the topography of medical imaging and the use of soft copy displays. The needs of medical imaging are driving changes in the basic CRT to improve the visual appearance and replicate hard copy film. The fundamentals for the CRT remain the same, so the challenge in developing medical electronics that use CRTs is to extract greater performance while staying within the boundaries of the glass envelope (CRT bulb).
The shadow mask (or aperture grill) that determines the dot pitch or distance between like phosphor dots limits color CRTs. A quality color CRT with a 0.25-mm dot pitch can resolve the equivalent of 100 dpi (25.4/0.25). A monochrome CRT, which does not have a shadow mask, is limited by the ability of the electron optics and video bandwidth to form a pixel. Unfortunately, nature and physics do not mix well with electrons being forced together in high concentrations to meet the demands of soft copy.
Medical film has a wide dynamic range with peak whites determined by the light box intensity (approximately 5000 fL). Although a fraction of this passes through the film, it is still as much as five times brighter than today's diagnostic-grade CRT displays. The inherent technical problem is that high luminance output requires phosphor current densities (measured in microamps per square centimeter) that by nature inhibit small pixels. Monochrome has a 3:1 advantage over color because a single gray-scale pixel can be formed directly, as opposed to the eye's having to integrate the red, green, and blue pixels on a color CRT. This difference allows monochrome CRTs to display more resolvable pixels than color for the same size CRT. A 21-in. diagonal bulb rated at 1600 x 1280 (2-megapixel addressable) for a color CRT can also be manufactured as a monochrome at 2560 x 2048 (5-megapixel addressable). For this format, the pixel size needs to be at or below 0.16 mm (0.0061 in.) if the industry standard measurement is taken at the 50% point of the Gaussian spot/luminance distribution of the pixel. A cross section of medical displays may all be rated as 5-megapixel addressable, but their ability to resolve 5 megapixels can be very different.
A typical CRT monitor.
The color CRT shadow mask also absorbs approximately 80% of the electron-beam energy, making it difficult to provide high luminance beyond that of commercial office products. The advantage of not having a shadow mask is greater efficiency in delivering electrons to the phosphor screen for increased luminance. The increase in luminance for medical applications starts at three times that of commercial requirements for primary diagnostic reads in controlled ambient lighting—that is, darkened rooms. For medical procedural use in bright lights, the luminance requirements are greater—five to six times that of commercial office products. High luminance levels require phosphor-current densities that exceed standard electron optics' ability to control the shape and concentration of the beam at the point of impact. What appears as out of focus or washed out is the merging of adjacent and out-of-round pixels. This parameter, which is controlled by the electron optics and the video amplifier that drives the CRT, can be quantitatively measured as modulation transfer function (MTF). The electron optics controls the width of a horizontal line; the video amplifier controls the width of a vertical line. The MTF is measured on each axis separately.
The MTF is more important than available peak luminance because it provides the ability to resolve alternate pixels as on and off. It is a performance attribute that must be designed into the display at the outset of the design process. In practice, users can control the ambient lighting as needed to enhance the contrast ratio. As noted earlier, the industry standard for measuring a pixel is the 50% point, which means the lower 50% is overlapping with adjacent pixels. Some overlap is needed to yield a smooth gray tonal field; otherwise, the image would be similar to newsprint images made up of separated dots. The pixel on a CRT has skirts that can be two to four times the diameter of the 50% point, depending on the quality of the electron optics. Writing the image to the edge by deflecting the beam also contributes to changing the shape and focus characteristics of a pixel. The subject of deflection systems and CRT interaction is beyond the scope of this piece, but nonetheless a critical consideration in the design of a quality medical display.
The eye can see the luminance well below the 50% point of the pixel. How well the pixel size is controlled from the 50% point down to 5% determines the quality of a medical image. Factors such as halation, light energy bleeding from an on-pixel into an off-pixel area, dramatically influences the MTF. This means that controlling the ratio of the 5-50% point of the pixel is at least as important as peak luminance. Electron optics with more than a 3:1 ratio have a difficult time resolving one-on, one-off pixels.
In today's 5-megapixel displays, the target should be a 2:1 ratio. Using the 0.16-mm pixel size at the 50% point means the 5% point should be no greater than 0.32 mm (0.0125 in.). By controlling this factor with the electron optics, spot growth from deflection is also minimized. The combination of off-center deflection spot growth and excessively large skirts at the 5% point are the primary reasons the MTF falls off away from screen center.
Custom electron optics and dynamic spot shaping circuits improve the MTF, yielding more useful screen area and better reading conditions. Recent advances have shown dramatic improvements in the performance of this mature technology.
