Michael Engles

The influence of filtering by the macular carotenoids on contrast sensitivity measured under simulated blue haze conditions

PURPOSE Distant objects are often obscured as a result of wavelength-dependent scattering in the atmosphere. This scattered light, which is mostly short-wave, effectively forms a veiling luminance (or background light) against which a target must be detected and discriminated. The macular pigment (MP) carotenoids could reduce the effective background intensity by selectively filtering out short wavelengths which would increase the contrast of the object in the retinal image, thus improving visibility. This Visibility hypothesis was originally posited by Wooten and Hammond (2002). This study represents a first empirical test of the hypothesis. METHODS Five young healthy subjects were evaluated. MP optical density (OD) was measured using HFP. Visibility was assessed by measuring contrast sensitivity thresholds at 8 cycles/deg (CST) using an optical system that passed xenon-light through the sine-wave grating. Blue haze was simulated using an ecologically valid broad-spectrum filter. Changes in MP density were simulated using a variable path length filter with an oil-based carotenoid solution that mimicked the absolute absorption spectrum of MP. RESULTS The average baseline CST was 0.004. Adding 0.25OD of simulated MP lowered the average threshold to 0.003 (25%). An additional 0.25OD decreased thresholds an additional 10% and the effect reached a plateau at about 0.50. DISCUSSION The largest improvement (about 25%) in contrast occurred with the initial, and relatively modest, addition of 0.25OD units of simulated MP suggesting that the largest improvements may be linked to initial increases in MPOD.

Visibility through atmospheric haze and its relation to macular pigment.

Purpose A major factor limiting the visibility of distant targets is veiling attributed to atmospheric scattering, known commonly as haze. It has been suggested that yellow filters (in this case, the macular pigments, MPs) could selectively absorb this haze, thus extending visual range. This study is an empirical test of the visibility hypothesis. Methods Twelve subjects had their full contrast sensitivity function (CSF) (represented by six spatial frequencies) assessed in the presence of two background conditions: simulated blue haze and short wave–deficient light. Contrast sensitivity at the peak of the CSF (7.5 cycles per degree) was measured in the presence of the same two backgrounds as the full CSF, with the addition of a broadband xenon background condition in a separate sample of 25 subjects. Results Simulated blue haze was found to uniformly reduce the CSF in the 12 subjects for whom the full CSF was assessed. Therefore, only the peak was measured in the larger sample of 25 subjects. Macular pigment density was significantly related to energy at threshold for both the haze and xenon backgrounds but not the short wave–deficient background. Thus, subjects with higher levels of MP could withstand more light before losing sight of the target. Conclusions This result is consistent with previous modeling of the visibility hypothesis as well as visual range improvements seen when using an artificial MP filter.