Interpretation of data obtained from psychophysical tests of visual function usually requires that the investigator know to what degree the subject achieves foveal fixation. This presents no challenge at moderate to high luminances, since several methods are available for direct observation of the retinal image of a fixation target (eg, Visuscope, Pleoptophore). In addition, von Noorden1 has designed a fixation device which, incorporated within the optical system of the Zeiss fundus camera, permits photographic recording of the image overlying the retina. But techniques which employ intensity levels sufficient for examination of fundus detail cannot be relied on to ascertain whether fixation is maintained at lower luminances. The unit to be described is a further modification of the Zeiss fundus camera which extends photographic determination of fixation to luminances near the absolute threshold of vision.
A single glass fiber, 75μ in diameter, was inserted into a length of 26-gauge hypochrome
SIEGEL IM, BRUNO B. Determination of Fixation in the Dark-Adapted Eye by Means of Fundus Photography. Arch Ophthalmol. 1964;72(5):670-671. doi:10.1001/archopht.1964.00970020670017