The term, scale, is used when rulings are spaced relatively far apart, requiring some type of interpolating device to make accurate settings. The term, grating, is used when rulings are more closely spaced, producing a periodic pattern without blank gaps. Of course, gratings cannot be either generated or read manually. They require special readout systems, usually photoelectric. The only element that makes a microscope a measuring instrument is the reticle.
Fig.1 Types of reticles (a) Type A (b) Type B (c) Type C (d) Type D, and Checking the height of the slip gauge
Scales are often used in optical instruments. It typically involves a read-out system in which an index point is moved mechanically until it frames the scale line and then reads the amount of movement that has taken place. The preferred choice of material for a scale is stainless steel. It takes good polish, is stable, and lasts longer. However, its higher thermal coefficient of expansion compared to other materials limits its use. Glass is another popular material used for making scales. Scale graduations can be produced by etching photo- resistive material.
Scales are meant to be read by the human eye. However, the human eye is invariably aided by an eyepiece or a projection system, which not only reduces the fatigue of the human operator but also improves reading accuracy to a large extent. In more advanced optical instruments, photoelectric scale viewing systems are preferred. They enable more precise settings, higher speed, and remote viewing. The reading of the scale is accomplished electronically. Photodetectors sense the differing light intensity as the scale divisions are moved across a stationary photodetector. While the number of such light pulses indicates the distance moved, the rate of the pulses enables the measurement of speed of movement.
Scales with a continuously repeating pattern of lines or groves that are closely spaced are called reticles. The line spacing may be of the order of 50–1000 per millimetre. They are invariably sensed by photo-electric read-outs. There are two types of gratings: Ronchi rulings and phase gratings. Ronchi rulings consist of strips that are alternatively opaque and transmitting, with a spacing of 300–1000 per millimetre. Phase gratings consist of triangularly shaped, contiguous grooves similar to spectroscopic diffraction gratings.
When two similar gratings are placed face to face, with their lines parallel, a series of alternating light and dark bands known as moire fringes will appear. When one scale moves in a direction perpendicular to the lines with respect to a stationary index grating, the fringes are seen to move at right angles to the motion. These fringes are largely free from harmonics. Two photocells in the viewing optics spaced 90 fringe-phase degrees apart are capable of generating bidirectional fringe-counting signals.
Fig.2 optical linear encoders
As already pointed out, the main element that makes a microscope a measuring instrument is the reticle. It provides a reference in the form of cross-wires for taking measurements. The cross-wires (sometimes also called ‘cross-hairs’) are usually etched on glass and fitted to the eyepiece of the microscope. A variety of reticles are used with microscopes for precise setting to measure part features. Figure1 illustrates the four types of reticles that are normally used. Type A is the most common but does not provide high accuracy. The cross-wire thickness usually varies from 1 to 5 μm. This is usually used for microscopes that have a magnification of 5× for the objective lens and 10× for the eyepiece.
Better accuracy can be achieved if the lines are broken, as in reticle B. This is useful when the line on the feature is narrower than the reticle line. For precise measurement along a scale, reticle C is convenient. Parallel lines spaced slightly wider than the scale lines enable precise settings to be made. In this case, the eye averages any slight irregularities of the edges of the scale lines when seen in the clear spaces along each side. This is known as bifilar reticle.
Type D provides the highest accuracy of reading. It is preferred in measurements involving a high degree of precision like photo-etching jobs. The cross-wires are at 30° to each other. The eye has the ability to judge the symmetry of the four spaces created between the cross-wires and position the centre at the precise location for taking readings.