Journal of the Society of Motion Picture and Television Engineers (1950-1954)

Desired T Number 0.5 0.71 1.00 1.41 2.00 2.83 4.00 5.66 8.00 11.31 16.00 22.63 32.00 Table 2 Value of Q = Cosec-i (2 X T number), Degrees 90 45 30 20.708 14.478 10.183 7.181 5.072 3.583 2.533 1.791 1.266 0.895 Diameter of Aperture — 100 tan Q, mrr 00 100 57.74 37.80 25.82 17.96 12.60 8.88 6.26 4.42 3.12 2.21 1.56 telephotometer, or a small hole in an opaque screen can be moved around in 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. 13.3.7 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. 13.3.8 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 * Suitable systems are the "Electronic Photometer" model 500 (Photovolt Corporation, 95 Madison Ave., New York, N. Y.), and the "Magnephot" (W. M. Welch Scientific Co., 1515 Sedgwick St., Chicago, III.). It is felt that a barrier-layer cell, although desirable for reasons of simplicity, has insufficient sensitivity for accurate determinations of the smaller apertures unless a galvanometer of exceptionally high sensitivity is employed. Page 7 of t pages photocell reading is noted at axial and corner positions, and the corresponding light ratio found from a calibration curve of the photocell meter* 13.4 Colllmated Source Method of Lens Calibration 13.4.1 This method has been described by Daily n and Townsley,14 the underlying theory being embodied in Section 5 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 disk of light in its focal plane. This circle of light will be less than the prescribed limit of 3-mm diameter for all lenses under 9 inches in focal length. Uniformity of the collimated beam can be checked by moving a small hole in an opaque screen across the beam, and any variations in the photocell reading noted. 13.4.2 For the comparison unit, an open aperture is used of diameter equal to the focal length of the lens divided by the desired T number. This aperture is first mounted in front of an integrating sphere with the usual photocell detector, and the light from the collimator is allowed to enter the aperture. The aperture plate is now replaced by the lens, the iris diaphragm is closed down to give the same photocell reading, and the T-stop number is engraved on the iris ring. The intermediate thirds of stops can be added by using 0.1 or 0.2 density filters, as described in Section 13.3.3. 13.4.3 To guard against drift and linevoltage variations which might occur between the readings on the comparison aperture and on the lens, it is convenient to leave the known standard aperture in place in front of the sphere, and to insert the lens into the beam in such a position that the little image of the source falls wholly within the standard aperture. The meter reading should then remain the same no matter whether the lens is in or out of the beam. A second plate with a 3-mm aperture should be placed over the compari PH22.90-1953 February 1954 Journal of the SMPTE Vol. 62 181