Reduction In Retinal Illuminance Research Articles

Effect of pre‐receptor filters on chromatic sensitivity and other visual functions in the eye

Spatial and spectral information carried by light emitted from visual displays is filtered as it passes through spectacle lenses in front of the eye as well as by filters within the eye. The image forming and the spectral transmittance properties of the elements involved in image formation in the eye cause changes in both the luminance and colour contrast in retinal images. In addition, the amount of light that reaches the retina is always reduced by pre‐receptor filters and this reduction in retinal illuminance can cause diminished retinal sensitivity to both luminance and colour contrast. Of particular interest are the use of coloured lenses in front of the eye to reduce the short‐wavelength content of the light that reaches the retina and the use of ‘notch’ filters to absorb light selectively in certain wavebands, with inevitable consequences on both the luminance and chromatic contrast of retinal images. The presence of variant cone‐pigments in some subjects or the complete absence of one class of cone pigment in others, can also produce large deviations in both luminance and colour contrast signals that can either enhance or diminish the conspicuity of coloured objects (doi: 10.1364/josaa.22.000017).The purpose of this study was to develop and validate a model of red/green (RG) and yellow/blue (YB) colour discrimination that can also predict how chromatic sensitivity is affected by the use of spectrally‐selective, spectacle lenses or by filters internal to the eye, when viewing visual displays. We examined how RG and YB colour thresholds relate to the corresponding cone contrasts needed for detection of colour differences. The results reveal an invariance of cone contrasts at threshold for different states of chromatic adaptation. In other words, colour contrast signals at threshold follow Weber's law (doi: 10.1111/j.1475‐1313.2010.00773.x). A combination of RG and YB colour signals is needed to predict the colour threshold ellipse. The model relies on these findings and predicts how coloured filters external to the eye as well as pre‐receptor filters within the eye alter the measured RG and YB colour thresholds. Model predictions have been compared against measured colour thresholds using the CAD test (doi: 10.1093/bmb/ldx007). A number of experiments were carried out when the visual display was viewed with no lens in front of the eye or through a number of tinted lenses designed to absorb progressively more blue light.The effects of pre‐receptor filters within the eye such as the lens and the macular pigment have also been investigated. The results show that ‘blue‐blocking’ filters employed in typical spectacle lenses used with daylight illumination cause only small shifts, approximately along the daylight locus, without any significant loss of either RG or YB chromatic sensitivity. ‘Orange’ lenses that absorb greater amounts of short‐wavelength light cause a significant increase in YB thresholds, but have little or no effect on RG thresholds. More surprisingly, the model also predicts that the optical density of the macular pigment has little or no effect on the size of the colour threshold ellipse.Keywords: colour, cone contrasts, blue light, CAD test.

Perceived brightness with small apertures

To quantify the relative perceived brightness when viewing through a small aperture as that used by presbyopic patients with small-aperture corneal inlays or intraocular lenses with an embedded aperture. Laboratorio de Óptica, Universidad de Murcia, Murcia, Spain. Prospective case series. The relative perceived brightness when 2 apertures (3.0mm and 1.6mm diameters) were presented monocularly to the participant was determined. With equal transmittances in both apertures, a flickering effect caused by the different retinal illuminance for each condition is perceived. The participant's task was to modify the transmittance of the 3.0mm pupil until the flickering was minimized. This transmittance value indicates the relative perceived brightness reduction. The measurements were performed under 3 average luminance levels. The perceived relative luminance measured ranged between 38.5% and 46.9%. This represents an increase in brightness perception with a small aperture of 1.24 and 1.51, respectively, compared with what would be expected. This trend was consistent for all the participants in the study. The perceived brightness with a small aperture was less pronounced than what would be predicted by the reduction in retinal illuminance. Under real visual conditions, this effect could be even more significant because binocular effects and temporal adaptation might further increase the perceived brightness with the small aperture.

Study on the equivalent luminance of luminous fibers in mesopic vision

Luminous fiber is a kind of new functional material which is prepared by melt spinning process adding colored pigments and luminous material into polyamide fiber. Based on the experimental data of the emission spectra and the test luminance of the luminous fibers, the effect of emissive colors of luminous fibers on the equivalent luminance in mesopic vision is analyzed. The results show that the effect of emissive colors of white, yellow, blue, and green luminous fibers on equivalent luminance increases firstly with the reduction of retinal illuminance, and then decrease. The effect is maximum when retinal illuminance is .1Td. The emissive color of the red luminous fiber has evident effects on the equivalent luminance, and the factor of emissive color should be considered fully to evaluate the actual visibility.

Optical simulation of reduced retinal illuminance caused by senile miosis

In order to simulate elderly vision for young observers, we developed an optical goggle using filters whose transmittance optically simulates the senile miosis: reduction in retinal illuminance as a function of ambient illuminance. First, we measured the pupil size of young and elderly subjects (mean age were 21.8 and 67.1 years, respectively) under six illuminance conditions (from 0.03 to 4850 lx), and formulated the relationship between illuminance and pupillary area functions in both age groups. Next, the transmittance formulae for the filter simulating the senile miosis were derived based on the properties of the pupillary area and visual functions of the elderly and the young subjects including the effect of mydriasis induced by the reduction of light due to the filter in young observers. Finally, we developed a goggle-type simulator using active optical devices which enables young people to experience senile miosis in any environment in real time.

Senescent Changes in Color Discrimination and Color Appearance

Age-related anatomical and physiological changes in the human visual system have been reported such as an increase in the density of the lens, a reduction of pupil size and loss of retinal ganglion cells. Although the retinal stimulus and neural signals are strongly affected by such age-related changes, color appearance is stable throughout the life-span because of parallel losses of cone sensitivities and other compensation mechanisms related to color constancy. On the other hand, there is a loss of sensitivity in color discrimination due, in part, to the reduction of retinal illuminance. The results of recent experimental studies by the author and colleagues indicate that this loss in discrimination is not only caused by the reduction of retinal illuminance, but also by a decrease in the signal-to-noise (S/N) ratio in neural pathways processing signals from S-cones and antagonistic signals from L- and M-cones. This change can be considered as a sort of a trade-off between maintaining a constant signal level from the cones through the life-span and decreasing the signal-to-noise ratio in the human visual system. These results suggest some possible methods for making safer and more comfortable visual environments for the elderly.

The effect of red and neutral density filters on the degree of eccentric fixation

The reported shift of fixation to a more central position when a red filter is placed before an eye which fixates eccentrically, has never been satisfactorily explained. To further investigate this phenomenon, 14 subjects with eccentric fixation were recruited. A Wratten No. 92 filter was placed before the eccentrically fixating eye, and any change of fixation was assessed by the after-image transfer test. A neutral density (ND) filter was then used, to mimic the reduced retinal illumination caused by the red filter. The results suggest that there is a significant reduction in the eccentricity of fixation, when placing the red filter before an eccentrically fixating eye, and also when placing the ND filter. No significant difference in the effects produced by either filter was found. This suggests that it is the reduction in retinal illuminance caused by the red filter which contributes towards such improvements in fixation.

The effect of red and neutral density filters on the degree of eccentric fixation

The reported shift of fixation to a more central position when a red filter is placed before an eye which fixates eccentrically, has never been satisfactorily explained. To further investigate this phenomenon, 14 subjects with eccentric fixation were recruited. A Wratten No. 92 filter was placed before the eccentrically fixating eye, and any change of fixation was assessed by the after-image transfer test. A neutral density (ND) filter was then used, to mimic the reduced retinal illumination caused by the red filter. The results suggest that there is a significant reduction in the eccentricity of fixation, when placing the red filter before an eccentrically fixating eye, and also when placing the ND filter. No significant difference in the effects produced by either filter was found. This suggests that it is the reduction in retinal illuminance caused by the red filter which contributes towards such improvements in fixation.

Reduction in stereoacuity with age and reduced retinal illuminance

Stereoacuity has been reported to decrease significantly in the seventh decade of life. Could this decrease be due to a reduction of retinal illuminance in the ageing eye? In this study, we used a more rigorous psychophysical procedure than that used previously, on 35 subjects, aged 21-67 years, to determine if the reported effect of age on stereoacuity could be replicated. We also compared the stereoacuity of 15 healthy young adults under normal and reduced retinal illuminance. A forced-choice algorithm was used to improve the accuracy of measurements. Results suggest that the reduced retinal illuminance resulting from normal ageing is not the cause of the decreased stereoacuity found with ageing.

Aging-Related Foveal Flicker Sensitivity Losses in Normal Observers

Photopic flicker sensitivity for healthy subjects over 60 years of age was compared with that of subjects in their 20s and 30s. The stimulus was achromatic and viewed foveally through a 2.0-mm artificial pupil which assured a constant pupil size in all subjects. As a group, older subjects had lower flicker sensitivity than younger subjects. This was found even when the retinal illuminance difference between age groups that arises from increased lens absorption in older adults was taken into account. The results indicate that a reduction in retinal illuminance with age cannot account for all the flicker sensitivity losses in older adults.

Effect of pupil size on kinetic visual field measurements

We determined visual field size as a function of pupil size by repeated measurements of Goldmann fields with an I2 target in the same subjects using contact lenses with different sized artificial pupils. Increasing pupil size from 3 to 6.8 mm increases visual field size from 2.79 to 3.34 steradians, an average increase of 19.7 per cent; a control experiment suggested that the major part of this effect can be explained by reduction of retinal illuminance produced by the variation in pupil size.

Mesopic spectral luminous efficiency functions: Final experimental report.

Spectral luminous efficiency functions in the mesopic range were measured for 24 subjects by the direct brightness matching method in a foveal 10° field covering a range from 100 to 0.01 Td of the adaptation level. The mean functions based on the 24 subjects were presented as the fundamental data set for the development of a mesopic photometric system. The mean functions showed a smooth and continuous change from the photopic-type function to the scotopic-type with the reduction of retinal illuminance. The Individual differences were fairly large throughout the level investigated. The empirical formula proposed in an earlier report was found to be useful to describe the mesopic function at any level.

The influence of age on the electroretinogram and visual evoked potential.

Changes in the ERG and VEP across the life span were investigated. The dark adapted and scotopic ERGs both showed a progressive increase in the implicit times of the A and B waves and a reduction in the amplitude of the AB configuration. There was also an increase in the implicit times of the oscillatory potentials of the photopic ERG. The flash and pattern onset-offset VEP both showed changes in waveform with age whilst the waveform of the pattern reversal VEP was constant. The amplitudes of the components of the flash and pattern reversal VEP were very high in the teenage group, but once reduced, were constant from the twenties onwards, showing no further consistent age changes. The latencies of the components of the pattern VEPs showed an increase with age which could be accounted for by the reduction in retinal illuminance due to the decrease in pupil diameter with age. However, the increase in the latency of the flash major positive (P2) component was greater than that expected from the decrease in retinal illuminance alone, suggesting that this is due to neural factors.

The influence of age on the spatial and temporal contrast sensitivity function.

A clinical technique for sampling the spatial and temporal CSF at low, intermediate and high frequencies is described. Reductions in contrast sensitivity with age were shown with the highest spatial and temporal frequency stimuli. The reductions can be almost completely accounted for by the reduction of pupil area with age, causing a reduction in retinal illuminance. The results show a significant deterioration in vision with age that is not revealed by standard visual acuity testing.