International projectionist (Jan-Dec 1946)

all the glass as it comes from the furnaces but, in accordance with well established and generally recognized standards, selects only the better part of it to satisfy specific requirements of optical designers. As the matter stands now, the bubble count is generally one of the minor and least troublesome factors in glass selection. Optical manufacturers have to worry about and control many other things seldom heard about and practically never recognized even by "advanced amateurs" or professional projectionists and photographers, who otherwise easily detect and wonder about a few minute bubbles in a lens. Optical manufacturers continuously worry and struggle with such defects as striae, reams, cords, streaks, seeds, stones, cloudiness, coloration and strain in glass. They have to rigidly control their raw materials and their manufacturing processes in order to guard against all possible defects and to maintain the index and the dispersion of a given type of glass within strictly specified limits. In finished optical elements, they have to worry about the accuracy of grinding and polishing, about accidental scratches and chips, and about fogging of polished surfaces. Recently to this list of worries was added that anent the quality of anti-reflection coating. In their struggle against many unfavorable factors, optical manufacturers steadily have been adhering to extremely high standards. These standards have been so high that during the war the Army and the Navy became alarmed by the rules of the optical art under which manufacturers have been rejecting huge quantities of finished lenses only because of some "beauty defects." Subsequently, in order to accelerate production, optical manufacturers were instructed by the Army and the Navy to relax their "beauty standards" and to permit, within reasonable limits, any appearance defects which Inspecting an unusually large piece of optical glass, preliminary to actual work. do not measurably affect the performance of an optical instrument. The various specifications covering optical glass and lenses have been restudied, revised, and expanded several times during the last few years. As a typical example of a revised specification may be mentioned the joint Army-Navy Specification for Optical Glass (No. JAN-G-174, dated January 30, 1945). It has 15 pages, more than 70 numbered paragraphs and sub-paragraphs, 6 tables, and 4 sketches illustrating the government methods for glass inspection. In its text it covers a great variety of the matters pertaining to glass, including the tolerances on striae, reams, cord, seeds, stones, bubbles, light absorption, index, dispersion, color, strain, etc. This is a basic specification covering optical glass in general. There are a number of other specifications pertaining to the use of glass in lenses and other optical instruments. All of them recognize the fact that optical glass entirely free from bubbles cannot yet be economically produced in large quantities, and that some glasses [barium crowns] are apt to have more bubbles than others. Actual Import of Bubbles They definitely recognize that only in some special cases is it of primary importance to secure glass practically free from bubbles. These special cases cover the glass components placed in the focal plane of an instrument — for example, recticles in eyepieces; even then the specifications consider a glass piece as being free from bubbles if, in this particular application, it does not contain bubbles exceeding 0.0004 inch in mean diameter. Otherwise, the specifications are much more liberal, and in the case of photographic filters they permit bubbles as large as 0.06 inch in diameter. The specifications not only establish the maximum permissible size of bubbles, but they also clearly state how many bubbles can be tolerated per unit area or unit volume. For photographic optics, the specifications tolerate as many as 16 small bubbles per cubic inch of glass. What are the effects of bubbles in a lens? It is as easy to answer this question as the question about the straw that broke the camel's back. Every bubble acts as a small lens within the lens, and it diverts a certain portion of light from the image-forming beam. This light may suffer several reflections, refractions, and absorptions, and finally a part of it may reach the image plane of the optical instrument (photographic lens). If the bubbles are "too large" and if "too many" of them are present, they may seriously affect the performance of a lens. This statement does not mean anything unless we determine what portion of the Marking bubbles before cutting a piece of optical glass. light may be so diverted by a bubble and by "many" bubbles. To determine" this, we have to indulge in some mathematics. The useful light utilized by a projection or a photographic lens in the image formation is nearly proportional to the square of the lens diameter; or, to be more scientific, of the lens entrance pupil diameter. The light diverted by a bubble is proportional to the square of the bubble diameter. Hence the portion of the light diverted by a bubble from the useful light is equal to the square of the ratio of the bubble and the lens diameters. Suppose we have a bubble of as large as a 0.04 inch diameter in a lens of one inch diameter. This bubble will divert less than 0.0016 (less than two-tenths of one per cent) of the useful light. Sixteen bubbles of 0.01 inch diameter will have the same effect. Bubbles of this size are easily visible. If we start to deal with bubbles barely visible to the naked eye, i.e., with bubbles of about 0.004 inch diameter, we will need 100 such bubbles to divert the same portion (0.0016) of the useful light. The effect is so small that an applicable Army specification permits a loss more than three times as great, and a photographer or a projectionist would not be able to detect losses many times greater. Scientists, using suitable instrumentation, can measure even atoms and components of atoms. They would have no particular difficulty in detecting a microscopic bubble and measuring its "effect," nor in demonstrating that the air we breathe in cities contains the deadly carbon monoxide! City people continue to breathe, however, and do not drop dead. This is because the content of (Continued on page 31) 10 INTERNATIONAL PROJECTIONIST • December 1946