The directionality of photoreceptors from the human retina may be measured by psychophysical or reflectometric methods. Oddly the directionality factor ζ provided by the phororeceptor alignment reflectometer (PAR) is four times the shape factor ρ of the Stiles-Crawford function, instead of twice as we could guess from the PAR design. We have thus developed a model to understand better the mechanisms involved in the measurement of ζ. The model predicts that: first the factor ζ is determined much more by photoreceptor characteristics than by the retinal pigment epithelium properties, second the high value of ζ is due to the poor excitation of the modes TE01, TM01 and HE21 backwards, and third the instrumental anisotropy of the PAR is small. Consequently the PAR affords a tool for characterizing photoreceptors and allows us to determine the principal directions along which photoreceptors are stretched.
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Gorrand et al. (1997) studied this question.