It is generally recognized that with other factors constant the intensity of light incident on the retina varies with the size of the pupil and that consequently determinations of the sensitivity of the eye to light are influenced by the size of the subject's pupil. Opinions differ, however, both as to the proper method of controlling or correcting for this variable factor and as to the practical necessity for so doing in clinical studies of light sense.
Troland1 in 1917 suggested the employment of a special unit of retinal brightness called the "photon," defined by him as "that intensity of stimulation which accompanies the use of a pupillary area of one square millimeter and an external stimulus surface brightness of one candle per square meter." In order to obtain the photon value of any given stimulus, the external brightness in millilamberts is multiplied by10 π times the area
SLOAN LL. INSTRUMENTS AND TECHNICS FOR THE CLINICAL TESTING OF LIGHT SENSE: IV. SIZE OF PUPIL AS A VARIABLE FACTOR IN THE DETERMINATION OF THE LIGHT MINIMUM. Arch Ophthalmol. 1940;24(2):258–275. doi:10.1001/archopht.1940.00870020050006
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