Interferometric countercheck in precision cementing G. K. Sharma and G. P. Dimri

Instruments Research & Development Establishment, Dehra Dun, 248008, India. Received 9 October 1982. 0003-6935/83/081132-02$01.00/0. © 1983 Optical Society of America. It is possible by observing the number of Newton's rings

in the air gap between the surfaces to be cemented to a doublet and comparing them with the ring pattern after cementing to deduce the nature of cementing and the uniformity or thick­ness of the cement layer.

Since the air gap between the surfaces to be cemented to a doublet system should be uniform, they must be worked to the same curvature, and since the observations are normal, the air film thickness could be defined as taking into account the phase reversion at the reflecting surface, 2μT + λ/2 = (2n + l)λ/2 for dark fringes, where T is the thickness of the air film, n is the number of rings at a particular distance from the center, μ is the refractive index of the film (1 for air), and λ is the wavelength of light under which the fringes are being ob­served. This can be simplified to give

Each Newton's ring indicates an increase in the thickness of the air gap by λ/2 and a check on the regularity of the two surfaces.

Once the surfaces are cemented, the air gap is filled in by the cement, and the application of Eq. (1), keeping in mind that the cement, e.g., cellulose caprate, would have an index of 1.48, indicates that for the given air gap the number of fringes that should now appear would be 1.48 times or maybe 1.5 times.

In other words, if two circular Newton's fringes had been observed at regular intervals before cementing [Fig. 1(a)], one should observe three such fringes after cementing at regular intervals [Fig. 1 (b)]. Any deformity in the shape of the fringes indicates an uneven thickness, and crowding toward the center or the edge indicates a sudden increase in cement thickness. The formation of straight line fringes indicates the formation

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Fig. 1. (a) Fringes before cementing; (b) fringes after cementing with cellulose caprate.

Fig. 2. Fringes obtained in a cemented doublet. The crowding of fringes near the edges shows the nonuniformity of cement

thickness.

of a wedge in the cement layer. Necessary adjustments for the uniformity of the cement layer follow.

As the difference between the refractive index of glass and the optical cement is small, the fringes produced by a doublet generally are of poor contrast. Figure 1 is a representative sketch of fringes which would appear in a well-cemented doublet, although not with the same degree of contrast. Figure 2, however, is a photograph of the fringes while ce­menting a doublet with cellulose caprate under sodium light and is a representation of the level of contrast that is usually obtained.

Considerable advantage is obtained if this procedure is followed to reduce subsequent failures due to improper ce­menting in the system as a whole. Although several authors mentioned1 have the uniformity of the cement layers in dou­blets, any information regarding the quantitative estimation of cement thickness using Newton's rings has not been pro­vided in the literature.

The authors are grateful to R. Hradaynath for his valuable guidance and constant encouragement during the progress of this work.

Reference 1. D. F. Home, Optical Production Technology (Adam Hilger,

London, 1972), Chap. 6.

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