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
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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
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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 tungstenimpregnated 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 V 0), 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-tonoise 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 m 2 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.