IN 1931, Daniel E. McGuire, a Shadyside, Ohio, farm lad then 17, hit on the use of a slit in place of a pinhole in the telescope mirror maker's Foucault knife-edge test and for this his name has been in the book "Amateur Telescope Making" (page 380) since the edition of 1933. Later, McGuire became a precision optical worker for several professional producers. In June, 1943 he hit on something which he now, by invitation, describes after using it a year and becoming closely familiar with it-the employment of spacers of scotch tape in connection with the air-spaced test plate for optical surfaces. From 15 Lincoln Ave., White Plains, N. Y., he writes:

IT is commonly believed by opticians that interference fringes appear between optical surfaces only when they are in close contact with each other. This is the earliest form of optical testing in which the interference of light waves reveals minute irregularities on the surfaces. This method prevails to the present day. It has many disadvantages, but they can be overcome to a great extent through the use of air-spaced test plates.

With the use of monochromatic light, a surface may thus be tested when removed many thousands of wavelengths from the test-plate surface. While there is no need for going to this extreme, there are many advantages in separating the two surfaces as much as 100 wavelengths (0.002"). This reduces the risk of scratching poorly cleaned surfaces, and increases speed many times over that of the conventional method.

The ideal material for separating the surfaces is scotch tape. This material is uniform in thickness and, when properly applied at three equidistant points on the test plate, it is easy to maintain parallelism between the surfaces. For 2" to 4" test plates the tape is cut to 1/4" to 3/8" width and about 1" lengths. The cleaner end is applied to the surface, and the other end, because it has been handled when cutting to size, is folded down over the edge of the test plate. For this critical work the scissors must be kept clean. Any dust particles that are sealed under the tape add to the thickness of the air space, hence cleanliness must be the rule when applying the tape. For the same reason air bubbles under the tape must be avoided.

To prevent curling of the edges and corners of the tape the test-plate surface must itself be cleaned with great care. When removing a grease film from the area near the tape rubbing must begin near the center of the tape and proceed outward to the glass. Dust is removed with a soft brush.

An ordinary test plate, whether flat or curved, cannot be used with the increased air space without proper collimation of the light. The rays can be focused upon a flat, or on a curved surface by a lens system (Figure 1) on the back of the test plate, so that all rays will have normal incidence upon the surface being tested. The rays return in the reverse direction and the collimating lens system refocuses them upon the light source.

Test plates of the usual type, optically modified by using the proper collimating systems but without spacers, are thereby decidedly improved. The fringes are more plainly visible on the steeper curves, and the readings are more dependable. Distortion due to oblique reflection angles of marginal rays does not exist in any collimating test plate unless there are serious errors in the collimating system.

An off-axis point of view distorts the fringes. To maintain alinement of the axis a small spot of paint is applied to the center of the back surface of the collimator. When its reflection is hidden by the spot itself, the point of view is on axis. Only one eye can see the undistorted pattern, since both cannot be on the axis together.

A beam splitter is essential to viewing the test plate on axis. This is a piece of glass, plane-parallel, polished on both sides, which reflects only a part of the light and transmits the remainder (disregarding the small portion that is absorbed by the glass itself). It works best with a thin coat of silver or aluminum on one side, but an uncoated beam splitter is satisfactory when the room is darkened. It makes no difference on which side it is coated, but the under surface is the more protected from dust. A piece of clear plate glass is good enough for this work. Figures 1 and 2 reveal the function of the beam splitter.

It is important that the light source be made large enough to allow for changes in the alinement of the test plate axis, the same applies to the size of the beam splitter. An excessive amount of spherical aberration in the collimator also requires a large light source- to provide even illumination of the test plate surface.

Any light source which radiates a small number of different wavelengths, concentrated into very narrow lines in the spectrum, is suitable for air-spaced test plates. Low-pressure mercury vapor lamps are better than high-pressure lamps, although the latter work well when the air space is not too great. Sodium vapor lamps are very good. Neon and fluorescent lamps can be used, but they are not recommended. Other types of lamps have not been used by the writer.

The separation of two flat surfaces tested in parallel, monochromatic light, has little effect upon the reading of the fringes but with all curved surfaces the readings go concave with a widening separation of two matched surfaces, the change in the reading being in direct proportion to the amount of separation. The steeper the curves, the greater the number of Newton's rings for a given separation.

When curved test plates are air-spaced, the test-plate surface must be altered from the true radius to a new curvature. In order to produce straight fringes, when testing the opposite curvature of the true radius, the change in radius is always made equal to the thickness of the spacers. Convex air-spaced test plates are made steeper, concave ones are made flatter, than the true curvature.

When the new curvature is established, the air space must remain constant in order that the test plate can be relied upon. In practice this cannot be guaranteed, but there is another way to be sure of the test-plate reading. Although the spacers are not reliable in their thickness, the master test plate is reliable in curvature. The air-spaced test plate for steep curves therefore must always be accompanied by the master test plate. Straight fringes are no indication of the true reading unless they are seen when testing the master. When the master appears to be wrong, due to an error in spacing, this "wrong" reading is the correct reading for the work.

The scotch tape spacers never grow thinner with use, although they may be compressed through excessive pressure and show temporarily a convex reading. It is quite normal for the tape to collect dust around the edges while in use; it becomes lodged underneath the tape and the air space is increased. The error in spacing may change the reading as much as three fringes before it is necessary to renew the tape. When the master shows a concave reading of three fringes, the work must read the same in order to be correct. Straight fringes on the work, in this case, are three fringes convex. An excessive amount of dust is avoided when the most accurate work is desired. In flat work, or with shallow curves, it is not so necessary to check with a master.

A sodium vapor lamp, radiating light in wavelengths of 0.00002", permits the convenient use of round numbers. A surface having 18 degrees of curvature shows a change in reading of one fringe for each 0.001" change in separation. (Collimated light, having normal incidence over the whole surface, is always used in this demonstration.) Spacers having 0.002" thickness change the reading two fringes from the contact reading. A further change in thickness, while the tape is in use, changes the reading in a direct proportion with the error in spacing. A 10 percent increase in spacing changes the reading only 0.2 fringe. When testing a surface having three times the curvature, or 54 degrees, the same change in spacing causes the reading to change nine () times as much. The 0.002" spacers change the reading 18 fringes from the contact reading, and a further change of 10 percent changes the reading 1.8 fringe.

The following procedure is used when correcting the test-plate surface. The spacing material is selected first, and a fair quantity is held in reserve for future replacements. The separation of the surfaces is established by the thickness of the spacing material at hand. Testing is done on the master test plate with spacers temporarily applied to its surface. The test-plate curvature is altered until the concave readings, due to separation, are reduced to zero.

Most of the alteration can be checked by contact readings when the number of fringes is predetermined. The test plate is made to read convex. The testing is done in the usual way, without collimation. The test plate is blocked for polishing by machine, and only the finishing touches are left for hand work. The back surface is cleaned to allow testing of the final work by collimated light. Spacers are used as described above.

Spherical aberration in the collimating lens system has some effect upon the straightness of the fringes. The air-spaced test plate cannot be made spherical unless the collimator is corrected for spherical aberration. It is easier to compensate for the aberration in a simple type of collimator by producing an aspheric test plate. Ordinarily the aspheric surface is produced, without awareness of it, in striving to duplicate the master curve and produce straight fringes; and there is no need for estimating its value.

The collimating lens system has an infinite variety of shapes. Every different test-plate curvature, diameter, thickness, and refractive index of test plate and auxiliary collimating lens requires a different design.

A steep, convex test plate requires one or more concave surfaces to diverge the rays and form a virtual image of the light source at the center of curvature. The turning point, from a concave to a convex collimator, is found where the radius of the convex test plate is n times the viewing distance, where n equals the refractive index of the test plate. A flat test plate requires a convex back surface to collimate the rays parallel to the optical axis. All concave test plates require one or more convex surface to converge the rays and form a real image of the light source at the center of curvature.

A constant viewing distance is maintained for all work of the same diameter, and it is used in the calculation of the curves for the collimating systems. The usual ratio of viewing distance to diameter of test plate is about six to one for the larger test plates. Ten to one, or longer, is better for the smaller surfaces. The viewing distance is never less than 10", on the smaller surfaces, unless a magnifier is used.

Variation of the viewing distance has some effect upon the reading of the test plate; but the tolerance for errors: in collimator design is not very exacting. A greater air space makes greater accuracy necessary. It is most practical to keep the air space to the smallest amount consistent with safety.

It is not necessary to test any of the collimating surfaces for figure, but flaws in the glass are more noticeable than with contact test plates.

McGuire's article will be concluded next month with a discussion of the design of the collimating lens system and of the methods of using the test plates.

To forestall possible objections that the general method of using separated test plates may have been used in several large precision optical industries for some time, and that McGuire therefore cannot claim it categorically, it is to the point to state that his main contribution is the scotch tape spacers-which, however, in actual shop production greatly speed up the test and therefore the work. You can put the work on the tester, test, remove it and go on working, quickly; so you will test oftener. He contributes some thing else -the article. It does the public little good to hear that a given method has already been in use in some shop if nobody who is qualified to do so by experience with it takes the trouble to write it up and publish it for the benefit of others. McGuire has greatly improved the method and he here makes it available, also, to all.

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