Human perception of visual stimuli modulated by direction of linear polarization

Abstract

This study explores both theoretically and experimentally the human perception of polarized light beyond that currently established. The radial analyser theory of Haidinger's phenomenon (HP) is used to predict the effect of observing visual stimuli comprising patterned zones characterized by orthogonal planes of linear polarization (linear polarization direction fields, LPD-fields). Any pattern can be represented as an LPD-field including optotypes and geometric forms. Simulated percepts differ from the original patterns although edges are mostly preserved. In edge-rich images a cross of attenuating contrast spanning the field of view is predicted. The mathematical model is verified experimentally using a liquid crystal display (LCD)-based polarization modulator imaged through a tangential (azimuthal) analyser with properties complementary to a radial analyser. The LCD device is then used in vivo to elicit perceptual responses in human subjects. Normal humans are found to readily detect spatially and temporally modulated isoluminant spatially-isochromatic, highly polarized LPD stimuli. Most subjects match the stimuli to corresponding images of theoretically predicted percepts. In particular edge perception and the presence of the contrast cross was confirmed. Unlike HP, static patterned LPD stimuli are perceived without difficulty. The simplest manifestation of human polarization perception is HP which is the fundamental element of an open set of stimulus-dependent percepts. This study demonstrates that humans have the ability to perceive and identify visual pattern stimuli defined solely by polarization state modulation.

The Clinical Application of Polarization Pattern Perception

“…As the novelty of our approach precluded the use of pre-existing clinical data in the development of appropriate test patterns, the design of the stimuli was largely based on the known perceptual responses to both equivalent luminance stimuli and polarization stimuli, as well as the radially symmetric geometry of human polarization perception. 3 , 5 The developed stimuli were intended to give a graded and quantifiable measure of polarization perception over a suitable response range for naïve individuals with either normal or abnormal eyes.…”

“…Full details of this display modification process are described elsewhere. 3 – 5 , 14 Filters positioned within the dLCD allowed for a uniform luminosity blue field with a peak wavelength of 460 nm, corresponding to the peak sensitivity of polarization pattern perception. 4 The dLCD enables the presentation of a spatial image that is modulated by angle of polarization but with constant luminance across the screen.…”

“…Non-uniform polarization-modulated fields of light yield percepts that can be made to vary in spatial complexity from simple one-dimensional patterns to complex optotypes. 3 – 5 This ability has been described as polarization pattern perception (PPP). 4 …”

Computational simulation of human perception of spatially dependent patterns modulated by degree and angle of linear polarization

“…As described previously [20], theory was tested in an optical model in which an LCD-generated polarization-modulated pattern was photographed through a polarizing filter with diattenuation that was radially symmetric about the centre point of the filter (Oriel Instruments, 'Polarization axis finder' 25328). The method was extended here to two types of LCD: a twisted nematic (TN) LCD (Dell P1913Sb) for the generation of an AoP stimulus, and an in-plane switching (IPS) LCD (Dell P1914Sc) for the generation of a DoP stimulus [11].…”

“…The method was extended here to two types of LCD: a twisted nematic (TN) LCD (Dell P1913Sb) for the generation of an AoP stimulus, and an in-plane switching (IPS) LCD (Dell P1914Sc) for the generation of a DoP stimulus [11]. Each of the monitors was converted into a polarizationmodulated display by the removal of the front polarizing filter [7,21,22] and insertion of a blue-transmitting polymer filter (Lee Filters Ltd, UK, #075, 'evening blue'; peak transmission 440-460 nm) behind the back polarizing filter of the LCD panel [6,20]. The luminance output of each monitor was the same, as determined using a photometer (model CS100-A, Minolta Co. Ltd., Japan,); similarly, the spectral output of each monitor was identical, as determined using a solid-state spectrometer (USB2000, Ocean Optics Inc., FL, USA).…”