Biomedical subjects

Luuk Franssen

Publications and source records attributed to Luuk Franssen.

5 recordsLinked to original sources

PubMed2006

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.

Cataract↗

PubMed2006

Compensation comparison method for assessment of retinal straylight.

PURPOSE: Presently, no instrument or method exists that is generally accepted for routine clinical assessment of (functional) retinal straylight. Yet retinal straylight is the cause of major patient complaints, such as hindrance from glare and loss of contrast. It results from disturbances in the optical media that increase light-scattering over angles of 1 degrees to 90 degrees . Its assessment would help to decide whether to perform surgery for (early) cataract and would help in the evaluation of corneal or vitreal turbidity. METHODS: The psychophysical technique of the "direct compensation" method was adapted to make it suitable for routine clinical assessment. In the new approach, called "compensation comparison, " the central test field is subdivided into two half fields: one with and one without counterphase compensation light. The subject's task is a forced-choice comparison between the two half fields, to decide which half flickers more strongly. A theoretical form for the respective psychometric function was defined and experimentally verified in a laboratory experiment involving seven subjects, with and without artificially increased light scattering. The method was applied in a separate multicenter study. Its reliability was additionally tested with a commercial implement (C-Quant; Oculus Optikgeräte, Wetzlar-Dutenhofen, Germany). RESULTS: A repeated-measures SD of 0.07 log units was achieved, to be compared with differences in the young normal population of 0.4 log units and an increase with healthy aging by 0.5 log units at 80 years and by 1.0 or more log units with (early) cataract or corneal disturbances. Reliability was further found to be high when using the commercial version of the METHOD CONCLUSIONS: The compensation comparison method for measuring retinal straylight is suited for clinical use to diagnose patients with complaints caused by large angle light scattering in the eye such as early cataract.

Adult↗

PubMed2005

Wavelength dependence of intraocular straylight.

Wavelength dependence of retinal straylight has been a mystery since Stiles in 1929 [Stiles,W.S., 1929. The scattering theory of the effect of glare on the brightness difference threshold. Proc. R. Soc. Lond. (Biol.) 105, 131-41.] supposed it to have the strong Rayleigh type lambda(-4) dependence, typical for small particle light scattering, but which was never found. Using the accurate 'compensation comparison' approach, retinal straylight was measured from 625 to 457 nm. Subjects with a large variety of ocular pigmentation were included. Straylight was found to depend strongly on pigmentation of the eye, in addition to age. Young and well-pigmented eyes (young negroids) show nearly perfect lambda(-4) dependence. With less pigmentation (blue-eyed Caucasians), a red dominated component is added, negating the lambda(-4) dependence.

Adult↗

PubMedDate not supplied

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.

Europe↗

PubMedDate not supplied

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 subject's 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.

Computer Simulation↗