Module 2: Cathode Ray Tube
2.1. Introduction
The cathode ray tube is by far the dominant display technology. Products that utilize CRTs include television and computer screens in the consumer and entertainment market and high-end applications include electronic displays for medical and military applications. By any objective account, the CRT is a mature technology. It originated in 1879 when Crookes made many pioneering contributions to a discharge tube.
In general, a cathode-ray rube (CRT) is an element which, operating in a display mode configuration, converts an electrical signal into visual imagery using an electron beam intensity modulated and deflected to impinge on a phosphor screen in a glass enveloped under vacuum.
2.2. Positive Attributes of a CRT
Mature technology (i.e. reliable, robust, typically outcast product lifetime), amenable to modern production techniques and inexpensive to manufacture.
Versatile design (i.e. same design can be used in dozens of product categories).
Satisfactory luminous efficiency and capable of full-color.
Relatively simple construction with few connections.
More than adequate temporal response and resolution.
2.3. Negative Attributes of a CRT
Requires a large volume of space (i.e. 'rule of thumb' - tube depth is comparable to screen diagonal.
Tube is very heavy and therefore not useful in portable product categories.
High voltages required which also limits its utility in portable products.
In today's society where portability dominates in the consumer electronics industry, the CRT is losing ground. The development of flat panel technologies for portable
products computer screens (LCDs), television (plasma) makes the CRT very vulnerable to the market penetration of these new technologies.
2.4. Fundamental CRT Operation
There are essentially four major components of a CRT: 1) the vacuum tube; 2) the electron source or "gun"; 3) the deflection mechanism and 4) the phosphor screen.
The Vacuum Tube
The vacuum tube or bulb is maintained at a very high level to facilitate the use of an electron beam. The front surface of the tube defines the area of the display. A thin layer of phosphor is deposited on the front surface of the tube. The tube is typically made of glass to sustain high vacuums and has three main sections - the front surface, the funnel and the neck. Most tubes are manufactured from glass but in some cases the funnel and neck can be fabricated from metal or ceramic. For demanding applications that require additional robustness (i.e. military), an implosion proof faceplate may be adhered to the front tube surface for durability. This typically comes at the expense of optical throughput, but anti-reflection coatings are often used to improve contrast to affect the transmission losses. In some CRT designs, conducting coating is employed to minimize electromagnetic interference.
The Electron Source
The electron source generates a high density electron beam whose current can be modulated. The electron beam can be focused and/or reflected via electrostatic methods.
Deflection Mechanisms
There are various deflection mechanisms used to steer the electron beam to designated positions of the front surface to create visual imagery. Electrostatic or magnetic methods are the most commonly used.
Viewing Screen
The phosphor screen on the inside front surface of the tube provides the conversion of the electron beam into visible light. The screen is coated with a thin layer of phosphor particles. In addition, a thin layer of a conducting material, usually
aluminum, is utilized to create an electric potential at the surface of the screen so the electrons accelerate toward it.
2.5. The Basic Nuts and Bolts of a CRT
The simplest description of a CRT display is as follows - the electron beam bombards the phosphor screen to create visible light. To create a real image, the beam is systematically scanned over the phosphor screen by means of a deflection mechanism (magnetic or electromagnetic) when the appropriate potentials are applied. The information content, contained within the video signals, is applied to the electron beam current controlling the electrode of the electron source which synchronizes with the signals delivered to the beam deflection mechanism. Grid 1 or the cathode ray can be utilized for this purpose. Grid 1 is a better choice for this function, especially when resolution is of utmost importance because modulating the cathode varies beam velocity and deflection sensitivity of the tube. The serial electrical information contained in the electron beam is responsible for the final visual imagery output on the 2-D screen. Most CRTs are updated or refreshed at 60 Hz which eliminates flicker and is capable of video images.
At this point, it is worthwhile to go into a little more detail on the cathode and deflection mechanisms.
2.6. Cathode
The oxide-type cathode is used in most CRT applications and consists of a nickel (Ni) substrate coated with barium, strontium and calcium oxides. The cathode peaks at
~800 degrees Celsius and provides a dc emission density up to ~0.2 amp/cm2 peak current density. When higher current densities are necessary, a tungsten- impregnated cathode is used. The tungsten-impregnated cathode is a porous tungsten matrix heated to temperatures exceeding 1000 degrees Celsius which is impregnated with barium and calcium aluminates. The tungsten cathode peaks at higher temperatures than the more conventional oxide cathode and therefore requires more complex processing techniques. The tungsten cathode is typically used in applications where the beam current rises above 1.5 mH, such as with projection tubes.
2.7. Electrostatic Deflection System
An electrostatic deflection arrangement consists of two sets of metal plates of length L systematically located with respect to the electron beam axis at a distance D of each other. At the anode outlet aperture (held at potential V0), the beam enters the deflection plates whose potentials are V0 + VD and V0 - VD, respectively. The deflection angle a at the exit of the plates is given by the expression:
There are games one can play with the deflection sensitivity; for example, to increase the deflection sensitivity, plates can be flared to have an optimum contour. For high frequency deflection systems, delay lines must be incorporated to sufficiently match electron beam speed in the deflection zone with the signal propagation speed in the delay line.
2.8. Electromagnetic Deflection System
An electromagnetic deflection coil is composed of two perpendicular windings generating electromagnetic fields perpendicular to the trajectory of the electron beam in the vertical and horizontal beams.
Remember Lorenz's fundamental law:
Fm = -e(n x B)
where e is the electric charge, n is the velocity of the electron, B is the magnetic field and F is often referred to as the Lorentz force.
The figure shows the basic operation of electromagnetic deflection where a field of length l is applied perpendicular to the electron beam accelerated at VB. Assuming the field to have uniform intensity along l, the beam is deflected onto a circular path of radius R. The deflection angle is given by
where Ni is the number of ampere turns responsible for generating the magnetic field, D is the diameter of the cylindrical winding generating the field, L is the length of the field region and VB is the accelerating voltage. The deflection angle is related to the coil diameter. The coil diameter is limited by the tube neck diameter. It is preferable to use a smaller neck thereby increasing sensitivity to reduce deflection power. However, this a catch 22 situation since a small neck diameter cannot accommodate the large electrostatic focusing lenses required to reduce spherical aberration and spot size. Therefore, for optimal spot size and deflection power, a compromise must be determined for best performance when this method of electrostatic focusing is required.
2.9. Color CRTs
Up to this point, we have only been concerned with the fundamental operation of a CRT. Practically speaking, one of the most important aspects of modern display is generating full color images. There are many ways to create full-color and they will be briefly reviewed below.
2.10. Shadow Masking CRTs
Shadow masking configurations are by far the most successful in creating full color images. The shadow mask CRT typically uses three electron beams deflected by one coil (simplest configuration). The electron beams traverse a perforated metal mask (shadow mask) before impinging on the selected luminescent screen material. The shadow masks apertures are typically configured as stripes, circles or slots.
The use of the word shadow mask is obvious. A color center is located in space from which the point source of light casts highlights (the beam traverses through) or shadows. The arrangement of the electron optics and the deflection system is such that three electron beams converge onto the screen after passing through the shadow mask, each beam impinging on one phosphor which when bombarded with electrons, emits red, green or blue visible light. The red, green and blue phosphors that are spatially arranged on the viewing screen are manufactured by photolithographic techniques.
The Trinitron TM design shown in the figure uses vertical stripe arrays, rather than circular or slotted apertures, that repeat red, green, blue when viewed from the faceplate side of the tube. The electron source or gun is located in-line rather than the other configurations shown in the figure (Delta configuration). The Trinitron TM has superior resolution in the vertical direction since it is not aperture limited in that direction and the in-line electron gun configuration is simpler in the sense of beam convergence. The only negative attribute of the Trinitron TM is that the mask is not self-supporting which ultimately limits the size of the vacuum tube.
2.11. Beam Index CRTs
A method to improve the performance of shadow mask CRTs is the beam index approach. It can be argued that this is simpler than the shadow mask approach since no mask is needed, but addressing schemes significantly increase in complexity. The beam index CRT consists of alternating red, green and blue triplets with a grid structure in front to focus and deflect the beam into the appropriate color position on the screen. The basic configuration of the beam index CRT contains an electron gun of a single cathode and a mechanism to split up the electrons into two beams - one beam which is named the pilot beam and is the scanning beam used to determine the position of the color beams. In the figure, a single color beam is used which converges at the deflection center with the pilot beam; this enables a single deflection system to be used. Magnetic deflection and focus are typically used and the color phosphors are laid down in a parallel stripes, somewhat like the Trinitron TM configuration.
These vertical stripes are most common with the electron beam scanning orthogonal to the stripes. The stripes alternate red, green and blue. The synchronizing signal can be generated by providing either a strip of material which is a relatively good secondary emitter or a UV phosphor behind one of the phosphor stripes. The UV index signal generation is generally preferred because this provides a high signal-to- noise ratio and tends to be more reliable.
In the figure, the electron beam is scanned at some low level across the phosphor. A photomultiplier tube is employed to pickup the indexing signal, the circuits controlling the electron beam modulate it in synchronization with its position determined by the index signal. Conceptually, the initial beam senses where the phosphor lines are,
informs the controller of the electron beam which in turn is synchronized to position the beam in the correct position on the screen to create visible light.
Although there are significant advantages to this approach, such as lower cost, lower power, higher resolution possibility and lighter weight, addressing is more complicated and focusing onto only the strip to excite the phosphor is challenging.
Therefore, this mode has seen little application.
2.12. Penetration CRT
Operation of a penetration CRT is based on the penetration depth of an electron beam in vertically stacked successive layers of phosphors. It is based on the principle that the penetration depth of electron increases as the electron beam velocity increases. Let us take a red-green example. At low voltages, the first layer of luminescent material is excited and a red color is obtained. At high voltages, the electrons penetrate the red layer without losing much energy and excite the green layer to produce green. At intermediate beam energies, the colors produced are mixtures - desaturated red in this example. With a barrier layer intermediate, the phosphor layers tend to improve color purity. The concept of color penetration can be achieved in several ways with vertical stacking or mixed phosphors. Typically, only two layers are used so it is only applicable to simple color displays.
2.12. CRT Resolution and Contrast
Defining and measuring the resolution of a CRT is a complex task. According to the Society for Information Display, resolution is a measure of the ability to delineate picture detail; also, the smallest discernible and measurable detail in a visual presentation. Resolution can be expressed in terms of the modulation transfer function, spot diameter, line width, raster lines or television lines. By any objective account, measuring CRT resolution is complex. This is in fact an understatement.
According to Norman H. Lehrer, an authority in the CRT field, defining CRT resolution is a complicated task. He states that no matter which term is selected, a meaningful and reproducible statement of resolution must include both the operating parameters of the tube and the specifications of the display to avoid the ambiguities that often occur because of the large number of tradeoffs possible in CRTs.
Unfortunately, there is no prescription for defining resolution, but one thing is for certain, pixel density (number of pixels per linear distance such as pixels/inch or pixels/cm) is certainly one of the most important resolution parameters. The following figure shows some simple CRT definition as a rule of thumb.
2.13. Contrast Ratio
Contrast ratio of CRTs is no different than other display technologies. It is defined as the ratio of luminance Lon of the pixel to the luminance of the pixel when it is not excited, Loff
Brightness contrast is sometimes used which includes background luminance into the equation. This sometimes is very important for CRTs because phosphor scatters light in the pixel off state.
In order to improve contrast, the usual technique is to use an absorbing faceplate (absorbing ambient light impinging on the front of the tube) or a spectrally matched filter. In addition, anti-reflection coatings on the front face are used to reduce reflections from the ambient environment. There is a tradeoff between optical throughput and contrast and depending on ambient lighting conditions, contrast improvement techniques may or may not be employed.
2.14. Line Width and Modulation Transfer Function
The visible emission of a CRT phosphor dot usually presents a Gaussian energy distribution. The measurement is usually performed at full-width-half-maximum of the Gaussian curve which will be denoted as L1/2.
The spot size is related to the modulation transfer function (MTF) by the expression:
where MTF is fractional, L is the neperian logarithm, N is the number of cycles per millimeter and L1/2 is usually expressed in millimeters. The spot image is typically measured with a microscope equipped with a fiber optic probe coupled to a photomultiplier or LCD array.
Another method employed is the Schrinking raster technique. After the raster of N horizontal lines is scanned in the CRT screen, the vertical size of the screen is reduced until the line structure disappears and produces a uniform luminance to the observer. The number of raster lines is divided into the raster height:
2.15. Brightness
Brightness is measured as area brightness (raster luminance) LR or peak line brightness LP. Peak line brightness is inversely proportional to writing speed, a direct function of the refresh rate of the displayed line and of the beam current. Raster luminance LR is related to the raster emitting surface (S), to the beam current (I), to the screen efficiency (E), transmission (T) of the faceplate and to the screen voltage (VS) by the expression
where E is in lumens/watt, I is amperes, S is in m2 and T is fractional. There is not a simple relation between peak line brightness and raster brightness.
2.16. Final Note
The worldwide market for CRTs is large, exceeding 50 million in 1996. High resolution CRTs with good color performance are mainstream because of the synergy between multimedia and high resolution color television and the upcoming digital television and the Internet. The growth of the CRT is expected to exceed 100 million units yearly, even with progress of flat panel displays that are beginning to capture market share.
