Directional light scanning laser ophthalmoscope

Abstract

The cone photoreceptor mosaic of the living human eye has in a limited number of cases been imaged without the use of wavefront-correction techniques. To accomplish this, the directionality of the photoreceptors, as manifested by their waveguiding properties, may be used to advantage. In the present paper we provide a model of our recently proposed directional light scanning laser ophthalmoscope [Opt. Lett. 29, 968 (2004)] together with a detailed numerical analysis of the device. The outcome is compared with experimental results.

Ultrasmall spot size scanning laser ophthalmoscopy

“…That only few cones are identifiable with annulus 2 suggests that most of the incident light is contained outside of their acceptance angle which may be estimated to be on the order of ~3° ( NA cone ≈0.06). Finally, some flickering of the cones may be appreciated in the videos that may be a consequence of interferences or reflective changes as well as changes in the coupling efficiency between the incident light and the cones for each raster scan [14]. …”

“…Selected magnified regions are shown below the images from which the reduced spot size with annular apertures may be appreciated. (and associated videos) shows images recorded at the fovea centralis by means of the aforementioned 75 μm pinhole and may therefore not fully resolve the individual foveal cones. Indeed, the pinhole size matters directly since the images (like other confocal techniques) ultimately suffer from a convolution with the projected pinhole image [14]. Imaging with a smaller 50 μm pinhole (equal to 4.1 μm if projected onto the retina) has also been carried out but had lower signal and is not shown here.…”

“…To ensure that the cones appear as bright as possible in retinal images it is essential that the focused scanning beam is incident along the photoreceptor axes with a minimum of aberrations thereby increasing the light-coupling efficiency [2,10–14]. Cones appear bright in retinal images predominantly due to backscattering from the elevated refractive index of mitochondria located at the ellipsoid transition from inner- to outer segments [15,16] although temporal and outer-segment changes can also affect their visibility [17–19].…”

“…Cone photoreceptor alignment with respect to the incident illumination has previously been studied with SLO [2,10] and with flood illumination fundus photography [11]. The individual cone photoreceptors can be consistently resolved with an SLO provided that the scanning spot is comparable to or smaller than the cone diameter (and spacing) and that a small confocal pinhole is used to prevent unwanted stray light from masking the imaged cones [14]. Thus, in the waveguide picture of cones, it becomes a task of optimizing the radiative transfer and coupling efficiency of incident radiation to permitted modes of the retina receptors [9,20].…”

Second‐harmonic microscopy of ex vivo porcine corneas

“…Likewise, SH imaging has been successfully used to image the ex vivo optic nerve head (Brown et al ., 2007) with important implications for glaucoma analysis. Eventually, the combination of multiphoton microscopy with adaptive optics (Vargas‐Martín et al ., 1998; Fernández et al ., 2001) could permit to increase the range of imaging possibilities, including the living eye and retina (Hermann et al ., 2004; Vohnsen et al ., 2005a).…”