We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
1949
LENS-CALIBRATION REPORT
375
A single set of apertures is sufficient to calibrate lenses of all focal mgths, since the only factor involved is sin 0, and that is fixed by the
>erture used.
The extended source should be uniformly bright over its useful area
within ±3 per cent. (This could be tested with a suitable telephoteter, or a small hole in an opaque screen could be moved around
front of the source, and any consequent variations in photocell reading noted.) The source conveniently may be a sheet of ground glass covering a hole in a white-lined box containing several lamps mounted around the hole and shielded so that no direct light from the lamps falls on the ground glass itself. The photocell receiver conveniently may be of the phototube type with a simple direct-current amplifier.* Care must be taken to ensure that the phototube sensitivity and the line voltage do not change between making readings on the open aperture and on the lens itself; to guard against this, some convenient turret arrangement is desirable with the lens on one side and the open aperture on the other so that the two may be interchanged and compared immediately with each other by merely turning the turret.
To measure the corner-to-center illumination ratio, the lens is set in position and the 3-mm hole and the photocell are displaced laterally by the desired amount. The photocell reading is noted at axial and corner positions, and the corresponding light ratio found from a calibration curve of the photocell meter.
Reference is made to the RCA patent to Sachtleben,9 U. S. 2,419,421, claim 3 of which covers this method of lens calibration.
XIV. COLLIMATED SOURCE METHOD OF LENS CALIBRATION
This method has been described by Daily11 and Townsley,14 the underlying theory being embodied in (1) above. Light from a small source (a 5-mm hole covered with opal glass and strongly illuminated from behind) is collimated by a simple lens, or an achromat if preferred, of about 15 inches focal length and 2 inches aperture. This gives a collimated beam which will be focused by the test lens to form a small circle of light in its focal plane. This circle of light wiJl be less than the prescribed limit of 3 mm diameter for all lenses under 9 * Suitable systems are the "Electronic Photometer" model 500, (Photovolt Corporation, 95 Madison Avenue, New York, New York), and the "Magnephot" (W. M. Welch Scientific Co., 1515 Sedgwick St., Chicago, Illinois).
It is felt that a barrier-layer cell, although desirable for reasons of simplicity, has insufficient sensitivity for accurate determinations of the smaller apertures.