Effects of contrast, cut-off spatial frequency and phase of the OTF on visual acuity and subjective image quality score
| Autor: | Legras, R., Sansot, A. P., Vandermeer, G., Nochez, Y., Navarro, Rafael |
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| Rok vydání: | 2012 |
| Předmět: | |
| Zdroj: | Digital.CSIC. Repositorio Institucional del CSIC instname |
| Popis: | Póster presentado al 6th EOS Topical Meeting on Visual and Physiological Optics celebrado en Dublin (Irlanda) del 20 al 22 de agosto de 2012. We evaluated the separated effects of a loss of contrast, sharpness, phase and a pure defocus on high-contrast tumbling-E visual acuity (VA) and subjective quality of image, by means of an adaptive optics visual simulator. [Introduction]: Retinal images suffer from optical blur, given by the Optical Trans-fer Function (OTF). Its modulus (the MTF) expresses the loss of contrast and sharpness (cut-off spatial frequency, COF), whereas its phase describes spatial shifts between frequencies (phase shift). These are the main factors affecting optical image quality, and hence they induce a loss of visual acuity and quality of vision. The aim of this experiment was to evalu-ate the effects of these three factors (contrast, cut-off frequency and phase) separately. The COF is a scalar variab-le, but contrast and phase are 2D functi-ons in general. To somehow convert them to scalar variables, the contrast was assumed constant within the frequency interval 0 < f < COF; the phase of the OTF was that of a pure defocus, given in diopters. In this way a synthetic OTF was obtained combining that phase with a MTF, which was a cylinder with radius COF, and height given by the contrast (plus a delta function at f = 0). We calculated the appearance of images on 5 -mm pupil diameter, degraded either by a loss of contrast (i.e. 1; 0.5; 0.25; 0.12 and 0.06) or by a lower cut-off spatial frequency (i.e. 50; 25; 12 and 6 c/deg) or by the phase shifts (i.e. 0.05; 0.10; 0.25; 0.5; 0.75; 1; 1.25; 1.5 diopters of pure defocus for 5 mm pupil). As a reference, these synthetic degradations were compared to that caused by pure defocus (modulus and phase), for the same diopters. [Discussion]: We measured these effects on degraded high-contrast tumbling-E visual acuity and on subjective score. Five subjects, aged between 22 and 40 years, scored three times the quality of each simulated image (three 0.4 logMar letters) using a continuous 5-items grading scale according to the ITU recommendations. Their visual acuities under the various conditions were also measured three times. To limit the degradation due the observer’s eye optics, the simulated images were viewed through a dynamic correction of aberrations (CRX1TM, Imagine Eyes) and an artificial pupil of 3mm. They were displayed on an EmaginTM micro display coupling with a colour filter ensuring a monochromatic light (i.e. 550 ± 50nm). The averaged intra-individual standard deviation (SD) was 0.03 logMar and 0.15 grade, the larger difference of SD between sub-jects and type of degradation was observed with image quality score. The averaged inter-individual SD was 0.03 logMar and 0.27 (dimensionless) grade. These SD were largely under clinical signifi-cant difference (i.e. 0.1 logMar and a difference of grade of 1). We normalized the results to obtain respectively the best and worst VA corresponding to the best and worst score for the defocus condition (see figure 2a). When lowering the COF, only the sharpness of the letter is degrading. [Conclusions]: The cut-off frequency induces a comparable loss of VA and subjec-tive score. However, a loss of contrast or phase shift appeared to have a more detrimental effect on VA than on subjective score. Considering a given loss of VA, we are subjectively more affected by a phase shift or loss of contrast than by a loss of sharpness (lower COF) or a pure defocus error. |
| Databáze: | OpenAIRE |
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