mirrors are alined within the assembly. The small item in the upper left is one of the two identical mirror mounts. Each is glued to an edge of its associated mirror, and the mirrors are locked in position with thumbscrews.

Figure C7 shows the holder for the 1.15 μm half-wave plate. The plate is first mounted in a metal ring which may then be rotated within the cut-out portion of the holder during system alinement and then locked with a thumbscrew.

Figure C8 is an assembly view of the clear plastic cover. The projections on the bottom fit the support frame top surface. The cover protects the alinement adjustments and it also prevents dust from settling on the optical surfaces thus relaxing the cleanliness requirements on the room in which the scanner is located.

Figure C9 is an overall view of the apparatus showing the proportions and the spatial relations of the various items comprising the scanner.

Figure C10 is a top view which includes all the items used to deflect the laser beams in the raster patterns. Also shown are the locations of the assemblies which are drawn in figures C-2 through C-7. The terminal strip in the upper left is used to anchor the electrical loads to the galvanometer.

Figure C11 is a closeup of the deflection system between the vertical deflection scanning mirror and the microscope camera tube. The latter extends through a hole in the support frame.

Figure C12 is a rear view of the support frame top. The halfwave plate is in the foreground, and mirror M2 is on its right.

Figure C13 is an end view from the left of the support frame top. The slide assembly is in the center, and the vertical mirror galvanometer and lens assembly L1 are in-line on the right. The circular item below the slide assembly is the holder for the rotatable analyzer used to adjust the intensity of the 0.6328 μm scan.

Figure C14 shows the clamps locking the microscope to the base plate, and the base itself (both supplied by the microscope manufacturer). Also shown is the screw arrangement used at all four corners of the support frame which allows precise alinement to be made of the scanning system as described in the main text.

Figure C15 is a rear view of the scanner apparatus. It shows most clearly the position of mirror M1. The reflected light circuit lens L2 and photodetector amplifier can be seen attached to the support frame to the right of the microscope.

Figure C16 is a closeup of the items used for the reflected light circuit. The 5 cm focal length lens L2 focuses the reflected light

to a pinpoint on the germanium photodiode mounted in the center of the 5 by 9 centimeter box which contains an amplifier for the photodiode. The BNC signal output cable was disconnected for photographic clarity.

Figure C17 is a view into the vertical illuminator port with the illuminator removed. A half-silvered mirror was mounted in a metal frame which fits snugly in a slot intended for an optical filter in the illuminator port. The mirror can be adjusted and locked into position with a thumbscrew. The frame on which the half-silvered mirror is mounted can also be rotated. The mirror is adjusted so that light reflected from the specimen during scanning is directed to a germanium photodiode where it is focused to a point. With proper adjustment of the 5 cm

lens in this light path, the point is stationary on the photodiode during scanning of the specimen.

APPENDIX D

Color Monitor

The color monitor was made by rebuilding an inexpensive color TV set. The TV set served as a convenient source of parts rather than serving as equipment to be modified. None of the original circuits in the TV were used, however, in starting with a TV, the advantage of having a color CRT mounted in a cabinet complete with degaussing coil, purity yoke, deflection yoke, convergence yoke, and radiation shielding is realized. The TV used for the project was a small-screen tube-type set with in-line guns in the picture tube. The in-line guns simplify color convergence. Three basic circuits are needed to make the color monitor. These include the deflection amplifiers, the video amplifiers, and the high voltage power supply for the anode of the CRT.

The deflection amplifiers were built on a separate chassis, and the remainder of the circuits were built in the cabinet that contains the CRT. The identical vertical and horizontal deflection amplifiers are essentially class B audio power amplifiers with current sensing rather than the usual voltage sensing for feedback control. Figure D1 is the schematic for a deflection amplifier and its associated power supply.

The deflection yoke is almost totally an inductive load so some special considerations had to be taken into account when designing these amplifiers. The most important consideration is that the beam deflection be proportional to the current passing through the yoke and not the voltage across it. Therefore, the amplifier should be a voltage to current converter. A low value resistor in series with the yoke provides a negative feedback signal that causes the high gain amplifier to perform the conversion. Since the yokes made for the various makes and models of TV sets are all different in terms of dc resistance, number of turns, and voltage required to achieve a given beam deflection, certain parameters of the amplifier given in figure D1 may need to be adjusted to match it to a particular yoke. It was experimentally determined what current was needed to provide about a 50% overscan at dc or low frequencies, and what voltage was needed to provide a similar overscan at the highest scanning frequency that would be used. When the yoke was connected to a dc power supply, a 2 A current was needed to deflect the spot on the CRT from the center to the edge of the screen. It was concluded that the amplifier should therefore have current limiting at plus or minus 3 A.

An audio power amplifier was used to determine the ac requirements of the yoke at 1.2 kHz which is the highest operating frequencies of the scanning mirrors. It was found that with a 1.2 kHz sine wave about 14 V peak-to-peak was needed to give a full screen deflection. It was concluded that 21 V peak-to-peak would provide enough overscan. An overall supply voltage of about 24 V should allow for circuit losses, so that plus and minus 12 V supplies were made to operate the amplifiers. The dc resistance of the horizontal winding of the particular yoke used

was 1.6, and the resistance of the vertical winding was 2. The lower frequency frame rate is used on the vertical winding, so that the current-deflection measurement should be done using this winding. Similarly, the higher frequency measurement should be performed on the horizontal winding. The 0.2 emitter resistors for the output transistors set the current limiting point. If these are increased in value, the amplifier will current limit at lower current values. The maximum voltage that can be delivered across the yoke is limited by the power supply. The circuit should work equally well with higher supply voltages. However, all resistors except those to the left of the diode bias string in figure D1 should be increased in value as the supply voltage is increased. For the particular set that was rebuilt to make the monitor, the horizontal deflection winding and a winding on the color convergence yoke were connected in series. This connection was not disturbed, and the deflection amplifier output goes through both yokes.

The high-voltage supply for the CRT used has to supply approximately 18 kV for the anode and 680 V for the focus element. The horizontal output stage for most black and white large screen TV sets made during the last fifteen years generates these voltages. An easy approach to obtaining such a supply is to use the parts from such a TV, since such a supply uses many special inductors and high voltage parts. Figure D2 shows a simplified schematic of such a circuit. Also shown in the figure is the power supply for the high-voltage circuit and the other circuits in the monitor. An inductor must be connected to the highvoltage transformer in the place where the deflection yoke is normally connected, as this is a tuned circuit. Due to the high voltage and current demands on the inductor, the deflection yoke from the black and white TV was actually used. The part was mounted as far away as possible from the CRT so that it would not deflect the electron beam in the CRT. The yoke must be mounted in such a manner that there are no conducting materials in the hole where the CRT normally goes. The power transformer also generates fields that can deflect the electron beam or cause color purity problems, so this part should also be mounted as far away as possible from the CRT.

The schematic for the video amplifiers and blanking circuit is given in figure D3. The dual-gate MOS transistors are depletion devices and are of the type that have integral protection diodes. A negative voltage bias is required for the input gate of the blanking circuit. The grid of the horizontal output tube conveniently provides a source for this bias. The 12 Vac required for the CRT heater must be provided from a separate winding on the power transformer (or a separate transformer) because the heater should be at a dc potential close to that of the cathode.

APPENDIX E

Circuit Schematics

The electronic circuits for the scanner were designed to handle a wide variety of specimen-measurement situations. All of the circuits feature dc coupling with temperature-drift compensation with the exception of the single-stage photodiode amplifier which does not have drift compensation. All of the circuits utilize regulated power supplies. These features make it possible to achieve a high degree of display accuracy even when slow scanning rates are used. Both the mixer and the preamplifier feature a wide dc offset nulling capability with indicator lamps to enable fast and accurate compensation for the dc voltages present on the specimen. The wide offset range and indicator lamps have been found to be indispensable.

Figure El is the block diagram and figure E2 is the schematic diagram of the mixer. High-voltage video-amplifier transistors are used extensively throughout the mixer. One reason for using such transistors is that the immunity to high-voltage transients accidentally applied to the input is improved. The particular transistors used are fast enough so that the amplifier has a flat response to about 2 MHz, but slow enough so that high frequency instabilities do not occur.

Figure E3 is the block diagram and figure E4 is the schematic diagram of the preamplifier. The preamplifier uses a tightly-regulated power supply for improved performance at low signal voltage levels. Actually, three voltage regulators are used for each voltage polarity. The first regulator for each voltage polarity is a pre-regulator which provides the other two regulators with a relatively clean, constant voltage. The second regulator provides a very clean voltage to operate the amplifier circuits and also provides a voltage reference for the third regulator. The third regulator supplies the specimen with the required operating power and can be adjusted anywhere between 0 and 20 V for the positive-polarity regulator, and 0 and -20 V for the negative-polarity regulator. A high-pass filter is included in the preamplifier. This filter is a double-pole type which inserts a coupling capacitor in the input circuit as well as inserts a capacitor in the feedback loop. A switch is also included in the preamplifier which allows the type of amplifier to be changed without disconnecting the specimen or disturbing the specimen voltage settings. Either a conventional bipolar transistor amplifier or a fast recovery FET amplifier with no loop ac feedback can be used. There are two sections on this switch and the section which switches the input signal is located physically toward the front of the chassis and the section which switches the output is located toward the rear of the chassis. A long connecting rod operates the two sections in unison. The physical separation ensures freedom from oscillation. Output attenuators are located on the rear of the preamplifier chassis. The gain-bandwidth product of the circuits in both the preamplifier and the mixer is quite large, and construction should be attempted only by persons familiar with such circuits.