Jan van de Kraats

The Pathways of Light Measured in Fundus Reflectometry

We measured the spectral reflectance of the fovea of ten normal subjects in four conditions, i.e. under dark-adapted and bleached conditions and at two retinal angles of incidence. The objective was to study optical pathways through the photoreceptor layer, resulting in a model that simultaneously explains spectral, directional and bleaching properties of the fovea. On theoretical grounds, we propose that small reflections from the stack of discs in the cone outer segments are the origin of the directional component of foveal reflection. Non-directional reflection occurs at the inner limiting membrane and at all layers posterior to the outer segments. With four reflectance spectra as input, the model allows determination of the density of the photostable absorbers, the lens, macular pigment, melanin and blood. Because of the simplified modeling of the layers posterior to the photoreceptor layer, the values for the density of melanin and blood are not necessarily comparable to physiological data. The density of the visual pigment calculated with this model is consistent with psychophysical data, with estimates for the ten subjects ranging from 0.41 to 0.80. The long wavelength sensitive cone fraction is calculated as 0.56.

Optical density of the aging human ocular media in the visible and the UV

We analyzed the literature on the absorption in the young and aging human eye media. Five templates were derived to provide an adequate description of the spectra from 300 to 700 nm for the lens, cornea, aqueous, and vitreous. Two templates were found in all media. They stand for Rayleigh scatter and the absorbance of tryptophan. Three additional templates for the lens represent absorbance in kynurenine derivatives, such as 3-hydroxykynurenine glucoside (3HKG), and absorbance in two substances found at older age. Except for Rayleigh scatter, all templates have a Gaussian shape. Aging-trend functions were derived that show a linear slope on an age-squared scale. The result can be used to correct for media losses in visual perception tasks, in fundus reflectometry, and in studies on light damage.

Simultaneous measurement of foveal spectral reflectance and cone-photoreceptor directionality

An instrument for simultaneous measurement of foveal spectral reflectance and cone-photoreceptor directionality is described. The key element is an imaging spectrograph (spectral range of 420-790 nm) with its entrance slit conjugate to the pupil plane of a human eye. A 1.9-deg spot on the retina is sampled in 1 s. Video observation of the retina and the pupil facilitates proper alignment. Measurements were performed on 21 healthy subjects. Model analysis of the spectra provided densities of photostable ocular absorbers. As an example, macular pigment and melanin are discussed in more detail. Spatial profiles exhibited the optical Stiles-Crawford effect, reflecting cone-photoreceptor directionality.

Retinal densitometer with the size of a fundus camera

Abstract This paper describes a small, user-friendly fundus reflection densitometer. All optics and part of the electronics are contained in a ☐ with the size of a fundus camera. A personal computer is used for control and on-line display of output. A single 30 W halogen lamp provides bleaching and measuring light. A chopper wheel generates 24 light pulses in 100 msec time frames: 16 pulses of measuring light at different wavelengths covering the spectrum, four pulses of bleaching light (optionally), and four dark pulses for assessing the dark current of the photomultiplier. The fundus can be viewed when the bleaching light is on. The measuring field has four widths ranging from 1.6 to 5.4 deg; the bleaching light is fixed at 25 deg. A fixation aid may be positioned anywhere in the bleaching field. A microprocessor sorts the quanta, detected by the photomultiplier after reflection from the fundus, in 16 channels labeled with wavelength information. Real-time changes in spectral reflection can be viewed on a monitor. Due to optimal design of entrance and exit pupils foveal density differences of up to 0.5 were recorded in human subjects. This is higher than ever reported before with retinal densitometry.

Lutein and Zeaxanthin Measured Separately in the Living Human Retina with Fundus Reflectometry

PURPOSE To separately measure the optical densities of lutein (L) and zeaxanthin (Z) in the human retina in vivo. L and Z are the basic constituents of the macular pigment (MP). METHODS Spectral fundus reflectance was measured in 23 subjects (group 1) at 0 degrees , 1 degrees , 2 degrees , 4 degrees , and 8 degrees eccentricity with a modified macular pigment reflectometer. A model generated the optical densities of L (LOD) and Z (ZOD), using their slightly different absorption spectra. Three other subjects (group 2) took 20 mg/d zeaxanthin for 6 months; they were measured approximately monthly for 18 months. RESULTS Mean LOD for group 1 at the central fovea was 0.200 +/- 0.061 (range, 0.085-0.305), mean ZOD was 0.494 +/- 0.169 (range, 0.169-0.806), resulting in a mean Z fraction [ZOD/(LOD + ZOD)] of 0.71. ZOD dropped faster toward the periphery than LOD, measuring 0.044 and 0.010 (Z fraction 0.18) at 8 degrees , respectively. Zeaxanthin supplementation in group 2 caused a significant increase in ZOD, and no or minor changes in LOD. ZOD further increased over a 10-month period after supplementation in all subjects. CONCLUSIONS LOD and ZOD had different spatial profiles that, apart from scaling factors, showed similarity to in vitro literature data. Supplementation with Z caused LOD to decrease and ZOD to increase. These results strongly suggest that the optical densities of L and Z can be assessed in vivo by fundus reflectometry, opening new ways of investigating the putative protective roles of L and Z in retinal disease.

Fast assessment of the central macular pigment density with natural pupil using the macular pigment reflectometer

We built a new macular pigment reflectometer (MPR) for fast and objective measuring of the optical density of macular pigment in the human eye, using the undilated eye. The design is based on the spectral reflectance from a spot of white light at the fovea. To evaluate its performance, we measured the macular pigment of 20 healthy subjects, ages 18 to 79 years, under four conditions: (1) natural pupil in the dark, (2) natural pupil with dim room light, (3) dilated pupil in the dark, and for comparison with a different technique, (4) heterochromatic flicker photometry (HFP) in dim room light with natural pupil. Condition 1 was repeated in a subset of 10 subjects after an interval of at least 3 days. Data analysis with a model of reflectors and absorbers in the eye provided the density of the macular pigment in conditions 1 to 3. Dim room light and pupil dilatation had no influence on measured density. Mean within subjects variation was typically 7%. Mean difference between test and retest after at least 3 days was 1%. Correlation between MPR and HFP was r=0.56 (p=0.012). Mean within subjects variation with HFP was 19%. The new instrument holds promise for specific applications such as epidemiological research.

Directional and nondirectional spectral reflection from the human fovea

A model of the directional and nondirectional reflection spectrum of the human fovea is developed, incorporating reflectors, absorbers, and a wavelength-dependent optical Stiles-Crawford effect OSCE. Data from 102 healthy subjects between 18 and 75 years obtained with the fundus reflection analyzer FRA, an imaging spec- trograph that measures the directional reflection profile of the human fovea in the pupil plane from 400 to 950 nm, were analyzed. Sub- groups of young 40 years and old 50 years observers were defined. Mean results of the young group defined a template for di- rectionality versus wavelength. For the whole group, mean reflection at 550 nm from the cones was 2.12%, from the retinal pigment epi- thelium 0.56%, and from the choroid 7.92%. Lens density, cone disc reflection, and blood layer thickness showed significant trends versus age. The model for the first time simultaneously describes the spectra of the directional and nondirectional reflection of the human fovea. Rayleigh scatter losses of the media and in preretinal layers were as- sumed zero in the nondirectional pathway. Mean density of the macu- lar pigment of a subgroup 53 subjects, 19 to 75 years correlated significantly with independent data from reflectance and autofloures- cence images obtained by scanning laser ophthalmoscope SLO and data from flicker photometry.

Sharp cutoff filters in intraocular lenses optimize the balance between light reception and light protection

PURPOSE: To facilitate the selection of the spectral filter in intraocular lenses (IOLs). SETTING: Department of Ophthalmology, University Medical Center Utrecht, Utrecht, The Netherlands. METHODS: The spectral transmission of commercially available IOLs was measured, and information from manufacturer data sheets and the literature was gathered. Illumination with sunlight filtered by the natural eye media (aged 20 and 70 years) and artificial eye media resulted in calculated values for the blue‐light damage and the signals from the sensory systems (rods, cones, melatonin suppression, and melanopsin). Results were presented as log differences with respect to the 20‐year‐old human lens. A single rating value for each lens was calculated to facilitate the selection. RESULTS: The 70‐year‐old lens showed a reduction in blue‐light damage of 0.45 log units compared to the 20‐year‐old lens, although a similar reduction occurred in the signal from the short‐wave‐sensitive cones. Intraocular lenses showed a range of nearly 1 log unit in blue‐light damage protection. A change in dioptric power in blue‐filtering IOLs, with a corresponding change in thickness, strongly influenced behavior. Dioptric power was less of a factor in IOLs with sharp cutoff filters. CONCLUSIONS: Blue‐filtering IOLs sometimes showed density spectra very different from those of the natural lens; however, their filtering generally caused only mild sensory losses. Nearly none of the IOLs offered an optimum tradeoff between protection and signal reduction. Sharp cutoff filtering near 445 nm provided better performance, removing limitations in optical design.

Spectral transmission of intraocular lenses expressed as a virtual age

Aim: Currently available intraocular lenses (IOL) have widely different spectral filters. This study aimed to calculate the virtual age of IOL with regard to photoprotection and photoreception, i.e. the age of the natural lens that has similar effects on these aspects. Methods: With diffuse solar radiation as a light source blue light damage was calculated for natural lenses at all ages, commercially available IOL and Schott steep cut-off filters in the wavelength range 300–600 nm. Similarly, the input to the short wavelength sensitive cone system was calculated for the range 380–600 nm. Results: The virtual age for photoprotection of IOL and steep cut-off filters varied from under 0 to 66 years. Most IOL had similar ages for photoreception, and thus show a reasonable resemblance to the spectral properties of the natural lens. Two IOL and all steep cut-off filters had a lower age for photoreception than for photoprotection, and thus outperformed the natural lens. Conclusion: The virtual age of an IOL relates its spectral filtering properties to what happens in the healthy crystalline lens. Many older IOL types offer less protection than the lens of a newborn. Middle age seems a reasonable choice for an IOL.

Foveal Cone-Photoreceptor Integrity in Aging Macula Disorder

PURPOSE To establish the relation between AMD stage and a quantitative measure for the integrity of foveal cone photoreceptors related to the optical Stiles-Crawford effect. METHODS Fifty-six AMD eyes and 57 control eyes were included in the final analysis. AMD was graded in accordance with the International Classification System into five mutually exclusive stages. Stages 0 to 1 were labeled no AMD, stages 2 to 3 were labeled early AMD, and stage 4 was labeled late AMD. Fundus reflectometry, together with a model-fit procedure, provided information on directional cone reflectance (Rd), a quantitative measure for the integrity of foveal cone photoreceptors. Optical densities of macular pigment (MPOD) and melanin (MOD) were also obtained. A general linear model analysis was used to compare Rd, MPOD, and MOD among the AMD stages. RESULTS Mean Rd was lower in early AMD (0.92%, P < 0.001) and late AMD (0.86%, P < 0.001) compared with mean Rd in the no-AMD stage (1.76%). Mean MPOD was not different in early AMD (0.53, P = 0.05), but it was lower in late AMD (0.19, P < 0.001) compared with mean MPOD in the no-AMD stage (0.42). Mean MOD was lower in early (1.09, P = 0.001) and late (1.01, P = 0.004) AMD compared with mean MOD in the no-AMD stage (1.23). CONCLUSIONS Foveal cones show signs of misalignment and/or outer segment deterioration in early AMD. Melanin rather than macular pigment may play a protective role against AMD, although loss of these ocular pigments can also be caused by AMD.