Monochromatic Ocular Aberrations Research Articles

Defocused contrast sensitivity function in peripheral vision.

Human peripheral detection performance is affected by optical factors such as defocus and higher order aberrations. From optical theory, we would expect defocus to produce local depressions (notches) in the contrast sensitivity function (CSF). However, such notches have not been observed in peripheral vision, and it is unknown whether human peripheral vision can detect local depressions (notches) in the CSF, such as those produced by monochromatic defocus when all monochromatic ocular aberrations are corrected. The purpose of the study was to identify such notches. Participants were three adult emmetropes. Following full adaptive optics correction, on-axis and 20° nasal visual field detection CSFs in monochromatic light were measured for the right eye with a 7mm diameter pupil, both without and with ±2 D defocus, and with separate determinations for horizontal and vertical gratings. Defocused CSFs were compared with predictions based on theoretical modulation transfer functions. Notches in the monochromatic defocused CSFs were identified for peripheral vision at optically predicted spatial frequencies with other monochromatic ocular aberrations corrected, provided that there was adequate spatial frequency sampling. The spatial frequencies of notches were similar to those predicted from optical theory, but their depths (0.3 to 0.9 log unit) were smaller than predicted. With fine spatial frequency sampling, notches were identified in defocused monochromatic CSFs when all other monochromatic ocular aberrations were corrected, both on-axis and at 20° eccentricity. Unless recognised as such, notches may contribute to noise in through-focus detection measurements of peripheral visual performance.

AO adapted to SD‐OCT

Abstract Purpose Optical coherence tomography (OCT) has emerged as a leading technique in ophthalmic imaging due to its capability to non‐invasively resolve tissue morphology with high sensitivity and high axial resolution. Despite increases in axial resolution, monochromatic ocular aberrations limited the transverse resolution for retinal imaging to ~20 μm, which is too large for visualization of cellular structures. Adaptive optics (AO) may be used to correct such aberrations, leading to an improvement in image contrast and lateral resolution. Methods A successful combination of ultra‐high speed (120,000 depth scans/s), ultra‐high resolution optical coherence tomography with adaptive optics and an achromatizing lens for compensation of monochromatic and longitudinal chromatic ocular aberrations, respectively, allows for non‐invasive volumetric imaging in normal and pathologic human retinas at cellular resolution. Results The capability of this imaging system is demonstrated through preliminary studies by probing cellular intraretinal structures that have not been accessible so far with in vivo, non‐invasive, label‐free imaging techniques, including pigment epithelial cells, micro‐vasculature of the choriocapillaris, single nerve fibre bundles and collagenous plates of the lamina cribrosa in the optic nerve head. In addition, the volumetric extent of cone loss in two colour‐blinds can be quantified for the first time. Conclusion AO OCT might provide opportunities to enhance the understanding of retinal pathogenesis and early diagnosis of retinal diseases. Commercial interest

Dynamics of ocular aberrations in keratoconus

Background: The aim was to investigate the fluctuations in monochromatic ocular aberrations with accommodation and tear‐film changes in moderate keratoconic eyes.Methods: We measured the changes in ocular higher‐order aberrations in 10 moderate keratoconic and 10 visually normal eyes to accommodative stimuli ranging from zero to 5.00 DS using a Hartmann‐Shack aberrometer. In addition, the changes in ocular higher‐order aberrations were measured for up to 15 seconds after a blink in eight keratoconic and eight visually normal eyes.Results: These results show that ocular spherical (p = 0.68) and coma‐like (p = 0.71) aberrations did not change significantly with accommodation from zero to 5.00 DS in keratoconic eyes. In contrast to normal eyes, the ocular higher‐order RMS error tended to decrease in magnitude after a blink in keratoconic eyes. Vertical coma became less negative with time after a blink in the keratoconic group, therefore, reducing the manifest ocular higher order RMS error by counteracting the negative vertical coma of the cornea.Conclusions: Compared to the manifest monochromatic higher‐order aberrations, any dynamic fluctuations in ocular aberrations with accommodation and tear film changes are relatively small in moderate keratoconic eyes. This implies that the correction of monochromatic higher‐order aberrations in keratoconus using customised soft contact lenses will not be significantly hindered by such dynamic aberrational changes.

Repeatability of corneal and ocular aberration measurements and changes in aberrations over one week

Background: Both Shack‐Hartmann aberrometry (IRX3, Imagine Eyes, Orsay, France) and Scheimpflug photography (Oculus Pentacam, Oculus Inc Wetzlar, Germany) are known to provide repeatable measurements. The variability in measurements of corneal and ocular aberrations obtained with these instruments over one week had not been assessed. The aim was to study the variability in corneal and ocular aberrations in the human eye over one week and to determine the impact of age on corneal and crystalline lens aberrations and on the variability of these measurements.Methods: Monochromatic ocular aberrations were measured with the IRX3 and monochromatic corneal aberrations were measured using Scheimpflug photography on one eye of 23 normal subjects at periods of a few seconds, one hour and one week after the initial measurement.Results: No significant differences were found between the initial aberrations and aberrations measured at 59 ± 24-seconds, 1.10 ± 0.24-hours and 7.11± 0.31-days later. Analysis with Bland and Altman plots showed that measurements with both instruments were highly repeatable over the times studied. There was no relationship between age and the variance of corneal and ocular aberrations (higher order, spherical and coma aberrations). Corneal spherical aberration did not show a significant correlation with age, whereas the lens aberrations changed from being negative in the younger age groups to positive in the older age group, however, these differences failed to reach statistical significance (p > 0.05).Conclusion: The variability found in all the measurements was small and not clinically significant and could be attributable to instrument noise, changes in the tear film and to small fixational eye movements.

Volumetric retinal imaging with ultrahigh-resolution spectral-domain optical coherence tomography and adaptive optics using two broadband light sources

Ultrabroadband sources, such as multiplexed superluminescent diodes (SLDs) and femtosecond lasers, have been successfully employed in adaptive optics optical coherence tomography (AO-OCT) systems for ultrahigh resolution retinal imaging. The large cost differential of these sources, however, motivates the need for a performance comparison. Here, we compare the performance of a Femtolasers Integral Ti:Sapphire laser and a Superlum BroadLighter T840, using the same AO-OCT system and the same subject. In addition, we investigate the capability of our instrument equipped with the Integral to capture volume images of the fovea and adjacent regions on a second subject using the AO to control focus in the retina and custom and freeware image registration software to reduce eye motion artifacts. Monochromatic ocular aberrations were corrected with a woofer-tweeter AO system. Coherence lengths of the Integral and BroadLighter were measured in vivo at 3.2 microm and 3.3 microm, respectively. The difference in dynamic range was 5 dB, close to the expected variability of the experiment. Individual cone photoreceptors, retinal capillaries and nerve fiber bundles were distinguished in all three dimensions with both sources. The acquired retinal volumes are provided for viewing in OSA ISP, allowing the reader to data mine at the microscope level.

Ultrahigh resolution optical coherence tomography and pancorrection for cellular imaging of the living human retina

Cellular in vivo visualization of the three dimensional architecture of individual human foveal cone photoreceptors is demonstrated by combining ultrahigh resolution optical coherence tomography and a novel adaptive optics modality. Isotropic resolution in the order of 2-3 microm, estimated from comparison with histology, is accomplished by employing an ultrabroad bandwidth Titanium:sapphire laser with 140 nm bandwidth and previous correction of chromatic and monochromatic ocular aberrations. The latter, referred to as pancorrection, is enabled by the simultaneous use of a specially designed lens and an electromagnetically driven deformable mirror with unprecedented stroke for correcting chromatic and monochromatic aberrations, respectively. The increase in imaging resolution allows for resolving structural details of distal elements of individual foveal cones: inner segment zones--myoids and ellipsoids--are differentiated from outer segments protruding into pigment epithelial processes in the retina. The presented technique has the potential to unveil photoreceptor development and pathogenesis as well as improved therapy monitoring of numerous retinal diseases.

Aberrations of chick eyes during normal growth and lens induction of myopia

Understanding the control of eye growth may lead to the prevention of nearsightedness (myopia). Chicks develop refractive errors in response to defocusing lenses by changing the rate of eye elongation. Changes in optical image quality and the optical signal in lens compensation are not understood. Monochromatic ocular aberrations were measured in 16 chicks that unilaterally developed myopia in response to unilateral goggles with -15D lenses and in 6 chicks developing naturally. There is no significant difference in higher-order root mean square aberrations (RMSA) between control eyes of the goggled birds and eyes of naturally developing chicks. Higher-order RMSA for a constant pupil size exponentially decreases in the chick eye with age more slowly than defocus. In the presence of a defocusing lens, the exponential decrease begins after day 2. In goggled eyes, asymmetric aberrations initially increase significantly, followed by an exponential decrease. Higher-order RMSA is significantly higher in goggled eyes than in controls. Equivalent blur, a new measure of image quality that accounts for increasing pupil size with age, exponentially decreases with age. In goggled eyes, this decrease also occurs after day 2. The fine optical structure, reflected in higher-order aberrations, may be important in understanding normal development and the development of myopia.

Influence of ocular chromatic aberration and pupil size on transverse resolution in ophthalmic adaptive optics optical coherence tomography

Optical coherence tomography (OCT) enables visualization of the living human retina with unprecedented high axial resolution. The transverse resolution of existing OCT approaches is relatively modest as compared to other retinal imaging techniques. In this context, the use of adaptive optics (AO) to correct for ocular aberrations in combination with OCT has recently been demonstrated to notably increase the transverse resolution of the retinal OCT tomograms. AO is required when imaging is performed through moderate and large pupil sizes. A fundamental difference of OCT as compared to other imaging techniques is the demand of polychromatic light to accomplish high axial resolution. In ophthalmic OCT applications, the performance is therefore also limited by ocular chromatic aberrations. In the current work, the effects of chromatic and monochromatic ocular aberrations on the quality of retinal OCT tomograms, especially concerning transverse resolution, sensitivity and contrast, are theoretically studied and characterized. The repercussion of the chosen spectral bandwidth and pupil size on the final transverse resolution of OCT tomograms is quantitatively examined. It is found that losses in the intensity of OCT images obtained with monochromatic aberration correction can be up to 80 %, using a pupil size of 8 mm diameter in combination with a spectral bandwidth of 120 nm full width at half maximum for AO ultrahigh resolution OCT. The limits to the performance of AO for correction of monochromatic aberrations in OCT are established. The reduction of the detected signal and the resulting transverse resolution caused by chromatic aberration of the human eye is found to be strongly dependent on the employed bandwidth and pupil size. Comparison of theoretical results with experimental findings obtained in living human eyes is also provided.

Ocular wave-front aberration statistics in a normal young population

Monochromatic ocular aberrations in 108 eyes of a normal young population ( n=59) were studied. The wave-front aberration were obtained under natural conditions using a near-infrared Shack–Hartmann wave-front sensor. For this population and a 5 mm pupil, more than 99% of the root-mean square wave-front error is contained in the first four orders of a Zernike expansion and about 91% corresponds only to the second order. Comparison of wave-fronts aberrations from right and left eye in 35 subjects, showed a good correlation between most of the second- and third-order terms and a slight (but not clear) tendency for mirror symmetry between eyes.

Measurement of monochromatic ocular aberrations of human eyes as a function of accommodation by the howland aberroscope technique

Further development of the objective version of the Howland and Howland [1976] Science, 193, 580–582; (1977) Journal of the Optical Society of America, 67, 1508–1518] aberroscope technique for measuring ocular aberrations is described. Compensation for refractive corrections and calibration is discussed. The technique was used to investigate the effect of accommodation upon the monochromatic aberrations of the right eyes of 15 subjects. Coma and coma-like aberrations were the dominant aberrations for most people at different accommodation levels, thus confirming previous findings. Variations in aberrations were considerable between subjects. About half the subjects showed the classical trend towards negative spherical aberration with accommodation. Changes in spherical aberration with accommodation in this study were less than that found in previous studies where all monochromatic aberration was considered to be spherical aberration.