The dependence of pupil size upon external light stimulus under static and variable conditions

Abstract This investigation forms part of a general investigation of the variation of visual sensitivity with time in consequence of a change in the external light stimulus. Part of the variation in sensitivity of the eye as a whole is due to change in pupil size, and this pupil size factor may conveniently be considered separately, and afterwards combined with the variation of the intrinsic sensitivity of the retina of the eye; this is measured with fixed pupil size (artificial pupil). The investigation of the pupil size factor falls naturally into two parts. One must first know what is the pupil size for any static condition in the subject’s field of view, that is, for any primary conditioning stimulus. Then follows the determination of the variation of pupil size with time after the change in the external light stimulus. The case investigated here is the change from a high to zero stimulus, consideration being given also to the dependence of the variation on the initial stimulus level. 1—The Dependence of Pupil Size upon the Light Stimulus Applied to the Eye under Static Conditions Some aspects of this part of the subject have been investigated by a number of authors. Couvreux (1924), Lythgoe (1936), Reeves (1920), and Schroeder (1927) give results showing the dependence of pupil size upon the general background brightness, approximately uniform over a large part of the field of view. Holladay (1926), in addition, gives results for a single glare source in presence of a uniform background, the glare source being at various angular separations from the fixation point. The results of Schroeder may be ruled out of consideration in the present connexion, as his method automatically measures the effective, not the real, pupil size, allowing for the pupil efficiency effect (Stiles and Craw­ ford, 1933). Couvreux and Holladay used a double-pinhole pupillo- meter, which is an undesirable method as the subject has to concentrate on making the measurement and is not under the sole influence of the light stimulus to be tested. Lythgoe and Reeves photographed the pupil by flashlight exposure. This is the only really satisfactory method of universal application and is the one chosen for the present work. For most of this, infra-red photography has been employed, with Ilford infra­ red-sensitive cine film and Ilford infra-red lamp glass as filter in front of the illuminant. The latter was a 500-c. p. pointolite lamp focussed by a condenser on to the eye to be photographed. A certain amount of red light is visible through the Ilford filter, but this was found not to be dis­ turbing with a time of exposure of 1/20 second or less. The pupil diameters given in what follows are those measured on the photographs directly, without making any allowance for corneal refraction, that is, the apparent pupil diameters as seen from outside the eye, not the actual diameters. Pupil size is influenced by the brightness of the general field of view, by its size and by glare sources in it, both in respect of their intensities and their positions relative to the direction of fixation. It would be desirable to be able to represent all these effects in a single formula, or at least in a consistent family of curves. The results obtained do not so far render this possible, and the best one can do is to investigate the more complex conditions individually.