Abstract
Purpose: :
In vision the Stiles-Crawford, SC, function is integrated across the pupil to estimate retinal images. The validity of this approach is however questionable for the following reasons: (i) the SC effect originates in the optics of the retina despite being characterized at the pupil, and (ii) the Maxwellian illumination used to characterize the effect differs fundamentally from vision through the natural pupil. Earlier studies have affirmed an integrated SC function using quasi-monochromatic light (B. Drum, Vis. Res. 1975) but it is questionable if this conclusion holds valid also for highly coherent light. A hypothetical integrated SC function for coherent light will either reproduce the traditional SC effect or it will not. If the latter proves true, it will confirm that vision is sensitive not only to the amplitude (and intensity) of light at the retina but also to phase variations. Here we examine the validity of an integrated SC function for highly coherent light.
Methods: :
We measured the integrated SC function using narrow annular and half-annular apertures in the eyes of both authors. The chosen light source was a highly coherent HeNe laser source (632.8 nm). The central visual field of 1.5 degrees was compared for a reference beam entering through the pupil centre with the corresponding light that entered through the annular or half-annular aperture. The results were compared with SC distributions obtained using a traditional bi-partite field test to confirm the validity of the approach. Also, for comparison the tests were repeated using incoherent white light from a tungsten-halogen lamp.
Results: :
For coherent light, the half-annular apertures show a marked reduction in the integrated SC and for the complete annular apertures the integrated SC effect is completely absent. Both of these observations are in agreement with predictions based on cone photoreceptor waveguiding. When the experiments were repeated with highly incoherent light the traditional SC effect reappeared confirming the integration of an incoherent SC function.
Conclusions: :
We have experimentally proven that an integrated SC effect for coherent light is completely absent. This is in agreement with waveguide theory for the cone photoreceptors confirming that both the amplitude and phase distribution of light being incident on the retina matters for the visual sensation produced. If incoherent light is used for the same experiments, the traditional integrated SC effect reappears for the unobstructed pupil. The results have important implications for the use of lasers in vision tests, in retinal imaging, and in simulating effective retinal images with refractive optics.
Keywords: photoreceptors: visual performance • optical properties