Luuk Franssen

Straylight in the human eye: testing objectivity and optical character of the psychophysical measurement.

The point spread function or PSF of the human eye encompasses hugely different domains: a small‐angle, high‐intensity domain, called the ‘PSF core’, and a large‐angle, low‐intensity domain, usually referred to as ‘straylight’. The first domain can be assessed by available double‐pass or other optical techniques. For the second domain psychophysical techniques have been developed, in particular the Compensation Comparison or CC technique, recently made available for clinical application in the C‐Quant instrument. We address the question of whether the psychophysical technique gives measures of straylight that are compatible with those made by optical methods. With a small adaptation the CC method can be used to assess straylight from physical light scattering samples, instead of straylight in the eye, using the same psychophysics, but without interference from the ocular straylight. The light scattered by each of seven light‐scattering samples, encompassing the range of straylight values observed in human eyes, was measured by two optical methods and by the psychophysical technique. The results showed that the optical and psychophysical measurements for the seven samples were almost identical.

Intraocular straylight after implantation of the multifocal AcrySof ReSTOR SA60D3 diffractive intraocular lens.

PURPOSE: To measure intraocular straylight (as a measure of glare) after cataract surgery and implantation of an AcrySof ReSTOR SA60D3 multifocal or AcrySof SA60AT monofocal intraocular lens (IOL) (both Alcon Laboratories). SETTING: University Hospital Maastricht, Maastricht; Isala Clinics, Zwolle; Netherlands Institute for Neurosciences, Amsterdam, The Netherlands. METHODS: In a prospective open observational case series, a newly developed straylight meter was used to objectively measure straylight 6 months postoperatively in 66 eyes with a diffractive AcrySof ReSTOR SA60D3 IOL (multifocal group) and 40 eyes with a monofocal AcrySof SA60AT IOL (monofocal group). A comparison of straylight levels in an age‐matched population without cataract (control group) was performed. RESULTS: The straylight level was 1.20 log units ± 0.16 (SD) in the multifocal group and 1.10 ± 0.19 log units in the monofocal group. When the difference in mean level of straylight was adjusted for age, mean straylight levels were 0.078 log units lower in the monofocal group than in the multifocal group (P = .026). Straylight levels in both pseudophakic groups were lower than in the control group without cataract (P<.0001). CONCLUSIONS: Levels of intraocular straylight log(s) were significantly lower for both types of IOL than in age‐matched subjects from the normal population. The mean level of intraocular straylight 6 months postoperatively was higher in patients with an AcrySof ReSTOR SA60D3 IOL than in patients with a monofocal AcrySof SA60AT IOL. Implantation of the former IOL would therefore result in a smaller gain in contrast sensitivity and a smaller reduction in glare and halos than implantation of the latter IOL.

Simulating the straylight effects of cataracts

PURPOSE: To study the additional straylight falling on the retina (retinal straylight) caused by cataract and find commercially available filters to simulate the cataract straylight effects. SETTING: Research laboratory. METHODS: The retinal straylight addition of cataract was derived from straylight parameter data in the literature. The scattering characteristics of cataract‐simulating filters were measured using a scatterometer. RESULTS: The straylight addition due to cataract follows a power law as a function of angle with power of −2.12 and straylight parameter log values of up to 1.6 for relatively mild cataract cases. Of the commercial filters that were tested, the Tiffen Black Pro Mist (BPM) filters resembled the straylight characteristics of cataracts fairly well. The filters had a limited effect on visual acuity and contrast sensitivity, which was also found for early cataracts. The BPM 2 followed a power law as a function of angle with power of approximately −2.21 and straylight log values of 1.12. CONCLUSIONS: The BPM 2 filter is a good early‐cataract‐simulating filter. Stacking such filters is a good way to increase the cataract density. A drawback is that the BPM 2 filter has a transmission of 66% so stacking filters reduces the overall transmission significantly.

Reliability of the compensation comparison stray-light measurement method

The compensation comparison (CC) method is a psychophysical technique to measure retinal stray light. It uses a two alternative forced choice (2AFC) measurement paradigm. The 25 binary (0 and 1) responses resulting from the 2AFC test are analyzed using maximum likelihood estimates. The likelihood function is used to give two quantities: the most likely stray-light level of the eye under investigation, and the accuracy of this estimate [called expected standard deviation (ESD)]. The CC method is used in 2422 subjects of the GLARE study. Each eye is tested twice to allow analysis of measurement repeatability. Furthermore, the large amount of responses is used to evaluate the shape of the psychometric function, for which a mathematical model is used. The shape of the psychometric function found by averaging the 0 and 1 responses fit well to the model function. Data sorted according to ESD show differences in the shape of the psychometric function between good and bad observers. These different shapes for the psychometric function are used to reanalyze the data, but the stray-light results remain virtually identical. ESD proves to be an efficient tool to detect unreliable measurements. In clinical practice, ESD may be used to decide whether to repeat a measurement.

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.

Ocular Media Clarity and Straylight

The optical media of the human eye contain several errors, and light scattering is one of them. Light scattering results in the spreading of light perceived around bright light sources and complaints about glare, hazy vision, etc. This phenomenon is called straylight and, by international standards, is used as the definition for disability glare. Straylight is recognized as an important aspect of quality of vision, assessed functionally by objective psychophysical means (compensation comparison, C-Quant instrument). Normal aging causes straylight to increase twofold by the age of 65. In pathology, straylight can be increased by a factor of 10 or more, constituting a highly disabling condition.

Reliability of the compensation comparison method for measuring retinal stray light studied using Monte-Carlo simulations.

Recently the psychophysical compensation comparison method was developed for routine measurement of retinal stray light. The subjects responses to a series of two-alternative-forced-choice trials are analyzed using a maximum-likelihood (ML) approach assuming some fixed shape for the psychometric function (PF). This study evaluates the reliability of the method using Monte-Carlo simulations. Various sampling strategies were investigated, including the two-phase sampling strategy that is used in a commercially available instrument. Results are given for the effective dynamic range and measurement accuracy. The effect of a mismatch of the shape of the PF of an observer and the fixed shape used in the ML analysis was analyzed. Main outcomes are that the two-phase sampling scheme gives good precision (Standard deviation = 0.07 logarithmic units on average) for estimation of the stray light value. Bias is virtually zero. Furthermore, a reliability index was derived from the responses and found to be effective.

Imaging of Forward Light-Scatter by Opacified Posterior Capsules Isolated from Pseudophakic Donor Eyes

PURPOSE Posterior capsule opacification (PCO) degrades visual function by reducing visual acuity, but also by increasing intraocular light-scatter. An in vitro model was used to elucidate the effect of PCO-morphology on light-scatter and its functional aspect, as can be assessed with straylight measurement. METHODS Forward PCO-scatter by opacified capsular bags was recorded with a goniometer and camera. The camera position mimicked the anatomic position of retinal photoreceptors; the camera recorded the scattered light that the photoreceptors would sense in an in vivo situation. Scattered light was recorded at different wavelengths and scatter angles, which were divided into a near (1° < θ ≤ 7°) and far (θ > 7°) large-angle domain. Using scattered light, the camera produced grayscale PCO images. The nature of the angular dependence of PCO-scatter was compared with that of scatter in the normal eye, by rescaling PCO images relative to the normal eyes point-spread function. RESULTS The scattered light images closely followed PCO severity. The angular dependence of PCO-scatter resembled that of scatter in the normal eye, irrespective of severity and PCO type. PCO shows the type of wavelength dependence that is normal for small particles: monotonically decreasing with increasing wavelength. At the near large-angle domain, the angular dependence of PCO scatter resembled the angular dependence of scatter in the normal eye less closely. CONCLUSIONS Surprisingly, PCO scatter and scatter in the normal eye have similar underlying scattering processes. However, data obtained at the near large-angle domain demonstrates that, apart from scatter, PCO may also have a refractile component, which is most pronounced in pearl-type PCO.

Effects of glistenings in intraocular lenses

Glistenings consist of multiple microvacuoles in intraocular lenses (IOLs) that cause retinal stray light and may affect quality of vision. For four IOL types, the microvacuole particle size distribution and particle volume density was measured using confocal light microscopy and dark field microscopy, and the corresponding extinction coefficient γ was determined. The light scatter contribution induced by microvacuoles was measured as function of both angle and extinction, and was verified by calculations using Mie theory. Two IOL types possessed significant glistenings having stray light levels higher than that of a healthy 20 year old crystalline lens corresponding to γ ≥ 0.08 mm(-1).

In-vitro recording of forward light-scatter by human lens capsules and different types of posterior capsule opacification.

The purpose of the present study was to elucidate the effect of posterior capsule opacification (PCO) on the straylight domain of visual function. PCO is heterogeneous with regard to morphology and severity; both aspects contribute to its functional effect. The isolated impact of capsule areas with specific morphology and severity on straylight was studied in-vitro by recording forward light-scatter. Forward light-scatter by four different capsule types, i.c. anterior capsule (AC), clear posterior capsule (PC), fibrotic and regeneratory PCO, was recorded at several visual angles with a goniometer, using different wavelengths. Angular (θ(a)) and wavelength dependencies (λ(b)) were studied by determining exponents a and b. Recorded straylight values of isolated capsule areas varied between 10× below to 10× above the value normal for the human eye, depending on the capsules condition (clear to opacified). The angular dependence of light scattered by clear PCs was weaker, whereas in the other capsule types it was stronger than in the normal eye. On average, the wavelength dependence of light scattered by different capsule types was similar, but the variation was considerable. At the smallest visual angles, increased angular and decreased wavelength dependence was found, especially in fibrotic and regeneratory PCO. It was concluded that the range of straylight values found in-vitro in lens capsules properly corresponded to that found previously in in-vivo pseudophakics. Surprisingly, the wavelength dependence of PCO indicated that small-particle light-scattering is important in PCO. Refractile effects were more important at small visual angles, as indicated by the combined stronger angular and weaker wavelength dependence.