T. J. T. P. Van Den Berg

The intraocular straylight function in 129 healthy volunteers; Dependence on angle, age and pigmentation

The direct compensation method allows for an accurate (standard deviation below 0.05 log unit) determination of intraocular light scattering between 3.5 and 25 deg of scattering angle and is suitable for untrained subjects. The method was used to study population behaviour and individual variation in 129 volunteers between 20 and 82 yr of age, visual acuity equal to or better than one and no apparent eye pathology. The results indicate straylight to increase with the 4th power of age, doubling at 70. In addition to the age dependence, there was great variation between individuals. Part of this is due to negative correlation with pigmentation.

Importance of pathological intraocular light scatter for visual disability.

For healthy eyes intraocular light scatter was investigated several decades ago. For pathological eyes, however, little is known. As clinical test several techniques have been proposed but none has gained general acceptance. A disadvantage of these tests was that quantities were estimated that related only indirectly to the amount of light scatter. We propose a method that gives a direct estimate of the light scatter.We studied patients with cataract, corneal dystrophy, iris and fundus hypopigmentation, etc. A remarkable finding was that visual acuity correlates rather weakly with the amount of scatter. Since, however, the amount of scatter causes a considerable loss of visual function, the results show that for these patients the visual acuity test gives a rather limited impression of their visual handicap. More attention to the problems associated with intraocular light scatter is needed.

Dependence of intraocular straylight on pigmentation and light transmission through the ocular wall

The straylight function of the human eye depends on eye color, especially at larger angles of scattering. As a potential cause for this dependence, transmission of light through the ocular wall was measured, using a psychophysical method. For a light-blue eye effective transmission of the iris was 1% for red and 0.2% for green light. Also the eyewall around the iris transmits a significant amount of light. For the dark-brown eyes of pigmented individuals transmission is lower by two orders of magnitude. Although important, transmission proved to be only partly responsible for the pigmentation dependence, the other cause probably being reflection from the fundus.

Near infrared light absorption in the human eye media

Available literature on the spectral transmittance of the eye media in the infrared (IR) is insufficiently accurate to estimate the deposited doses, e.g. in view of light damage risks. Analysis of the data of Boettner and Wolter [Investigative Ophthalmology and Visual Science, 1, 776-783 (1962)] for cornea, anterior chamber, lens and vitreous separately, shows that they can be explained on the basis of the pure water content. The transmittance spectrum of Geeraets et al. [Archives of Ophthalmology, 64, 606-615 (1960)] for all eye media combined is found to be impossible. It is concluded that in the near-IR, light losses in the eye media are best estimated with the absorption coefficient for pure water. A table is given from 700-2500 nm in steps of 10 nm.

On the relation between glare and straylight

An overview is given of the basic phenomena that may lead to glare complaints in patients. Prominent among them is increased intraocular straylight; this can be measured. Other causes may include: increased sensitivity to normal straylight, the length of (increased) light and dark adaptation times, (small angle) neuronal lateral interaction. Distinction must be made between disability glare and discomfort glare. Tests have been proposed to determine glare-induced loss of various visual functions. Often the test results are thought to be directly related to straylight but this may be untrue.

Light transmittance of the human cornea from 320 to 700 nm for different ages

In vivo relative corneal transmittance was estimated using Tans [(1971) Vision in the ultraviolet, thesis, University of Utrecht, The Netherlands] data on scotopic spectral sensitivity in aphakic eyes. This was combined with in vitro corneal transmittance data and in vivo light scattering data to arrive at absolute data. All data combined, the following function (no age dependence is found) resulted: log(transmittance) = -0.016-c*lambda-4 (lambda = wavelength in nm, lambda > 310 nm). c = 85*10(8) nm4 for direct transmittance (acceptance angle of the order of 1 deg) and c = 21*10(8) nm4 for total transmittance (acceptance angle close to 180 deg).

Measurement of stray light and glare: comparison of Nyktotest, Mesotest, stray light meter, and computer implemented stray light meter

Aim: To evaluate the properties of devices for measuring stray light and glare: the Nyktotest, Mesotest, “conventional” stray light meter and a new, computer implemented version of the stray light meter. Methods: 112 subjects, divided in three groups: (1) young subjects without any eye disease; (2) elderly subjects without any eye disease, and (3) subjects with (early) cataract in at least one eye. All subjects underwent a battery of glare and stray light tests, measurement of visual acuity, contrast sensitivity, refraction, and LOCS III cataract classification. Subjects answered a questionnaire on perceived disability during driving. Results: Repeatability values were similar for all glare/stray light tests. Validity (correlation with LOCS III and questionnaire scores), discriminative ability (ability to discriminate between the three groups), and added value (to measurement of visual acuity and contrast sensitivity) were all superior for both stray light meters. Results of successive measurements are interrelated for the conventional but not the new stray light meter. This indicates a better resistance to fraud for the latter device. Conclusions: The new computer implemented stray light meter is the most promising device for future stray light measurements.

Interaction between rod and cone signals studied with temporal sine wave stimulation

If a temporal stimulus is presented through the cones, the threshold for a rod stimulus may be influenced even if the cone stimulus remains subthreshold. We have used sinusoidal stimulation to study the process underlying this rod-cone interaction. A first-order model is proposed consisting of a single threshold mechanism preceded by linear summation of rod and cone signals with a pure latency difference. Two deviations from this model are described. First, occasional additivity failures appear; second, if the phase difference characteristic is corrected for latency, the remaining part cannot be explained in terms of the de Lange curves of the rod and cone systems.

An improved mathematical description of the foveal visual point spread function with parameters for age, pupil size and pigmentation

An analytical description of the point spread function (PSF) for human foveal vision, together with its closed form two-dimensional Fourier transform, the modulation transfer function (MTF), is proposed. It also possesses an analytical line spread function (LSF) counterpart. It fits to both PSF and MTF experimental data and holds over the full angular and spatial frequency domain. Variation in the PSF and MTF descriptions with pupil size, age and iris/skin pigmentation are accounted for by analytical expressions in the parameters.

Blue-yellow perimetry in the detection of early glaucomatous damage

A perimetric method using blue stimuli on a yellow background was compared with perimetry using white stimulion on a white background as a method of detecting glaucomatous damage. Meridian perimetry was used with an adapted Tübinger perimeter. The difference between the blue-on-yellow meridian and the white-on-white meridian was subdivided into two parts: the general blue sensitivity loss (GBSL), probably due to optical factors, and the corrected blue sensitivity loss (CBSL), probably due to glaucoma. Nine normals, fourteen primary open angle glaucoma (POAG) patients and nine ocular hypertensives (OHT) were tested. All POAG patients and some of the OHT group showed higher CBSL values than the controls. The blue-yellow meridian showed broader and deeper defects than the white-white meridian in all of the POAG group; some of the OHT group had defects in the blue-yellow meridian that were not present in the white-white meridian.In conclusion, blue on yellow perimetry shows promise as a method for more sensitive detection of early glaucomatous damage.