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J.L. Barbur, G. Walker, C. Chisholm; Assessment of Contrast Acuity in the Mesopic Range . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3611.
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Purpose: We examined how visual performance in the mesopic range is affected by changes in the spatial and temporal properties of the retina and / or changes in the quality of the retinal image as a result of increased aberrations and scattered light when the pupil size is large. The aim was to establish the extent to which retinal and / or optical factors set the limits of visual performance at low light levels. Methods: Pupil size, ocular aberrations, scattered light, chromatic sensitivity and "functional" contrast acuity (Aviat.Space.Environ.Med., 74,551–559, 2003) were measured as a function of retinal illuminance in the range 2 to –1 log trolands. A new, closed–loop system was developed for maintaining a constant retinal illuminance, independent of pupil size. The P_SCAN 200 system (Clinical Vision Science, 2, 131–141, 1987) was used to measure the area of the pupil every 20 ms. This signal was low–pass filtered and then used to adjust the luminance of the display monitor to maintain the specified retinal illuminance. The experiments were carried out on a calibrated visual display using spectrally calibrated neutral density filters. rms wavefront aberrations were measured using a WASCA wavefront analyser (Asclepion–Meditec Ltd.). Results: Both the amount of scattered light and the rms wavefront aberrations were found to increase rapidly with decreasing light level in the mesopic range. This was paralleled by a massive increase in contrast acuity thresholds and a rapid loss of both red–green and blue–yellow chromatic sensitivity. There was, however, little correlation between rms wavefront aberration and contrast acuity thresholds in the low mesopic range suggesting that, at least in the subjects investigated, the limit of visual performance over most of the range was set by the poor resolving power of the retina and not the optics of the eye. Conclusions: The size of the pupil affects contrast acuity thresholds, significantly, in at least two different ways. For stimulus conditions that generate a large pupil size, but where vision is still dominated by cone photoreceptors signals, increased aberrations can cause a large reduction in contrast acuity. These restricted conditions benefit from reduced higher order aberrations at large pupil size since the limiting factor is the quality of the retinal image. When vision is dominated by rod signals, contrast acuity thresholds are much increased, but the limiting factor becomes the resolving power of the retina. Visual performance in this range is strongly affected by even small changes in retinal illuminance caused by inter subject differences or within subject fluctuations in pupil size.
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