Wave-front aberrations in the anterior corneal surface and the whole eye

In order to investigate the sources of wave-front aberrations in the human eye, we have measured the aberrations of the anterior cornea and the whole eye using a topographic system and a psychophysical wave-front sensor. We have also calculated the aberrations for the internal optics of both eyes of 45 young subjects (aged 9 to 29 years). The mean rms for the anterior cornea was similar to that for the internal optics and thewhole eye when astigmatism was included, but less than that for both the internal optics and the whole eye with astigmatism removed. For eyes with low whole-eye rms values, mean rms for the anterior cornea was greater than that for the whole eye, suggesting that the anterior corneal aberration is partially compensated by the internal optics of the eye to produce the low whole-eye rms. For eyes with larger whole-eye rms values, the rms values for both the anterior cornea and the internal optics were less than that for the whole eye. Thus the aberrations for the two elements tend to be primarily additive. This pattern exists whether or not astigmatism was included in the wave-front aberration rms. For individual Zernike terms, astigmatism and spherical aberration in the anterior cornea were partially compensated by internal optics, while some other Zernike terms showed addition between the anterior cornea and internal optics. Individual eyes show different combinations of compensation and addition across different Zernike terms. Our data suggest that the reported loss of internal compensation for anterior corneal aberrations in elderly eyes with large whole-eye aberrations [J. Opt. Soc. Am. A 19, 137 (2002)] may also occur in young eyes.     

Comparison of two scanning instruments to measure peripheral refraction in the human eye

To better understand how peripheral refraction affects development of myopia in humans, specialized instruments are fundamental for precise and rapid measurements of refraction over the visual field. We compare here two prototype instruments that measure in a few seconds the peripheral refraction in the eye with high angular resolution over a range of about ±45 deg. One instrument is based on the continuous recording of Hartmann-Shack (HS) images (HS scanner) and the other is based on the photorefraction (PR) principle (PR scanner). On average, good correlations were found between the refraction results provided by the two devices, although it varied across subjects. A detailed statistical analysis of the differences between both instruments was performed based on measurements in 35 young subjects. Both instruments have advantages and disadvantages. The HS scanner also provides the high-order aberration data, while the PR scanner is more compact and has a lower cost. Both instruments are current prototypes, and further optimization is possible to make them even more suitable tools for future visual optics and myopia research and also for different ophthalmic applications.     

Eye rotation and vignetting in visual instruments

The eye can rotate to accommodate the angular position of an object and the distance of the object from it. The rotation of the eye inside its socket to align its visual axis in the direction of an off-axis image may introduce full or partial vignetting when one is looking through a visual instrument with a real exit pupil. We analyze the effects of vignetting owing to rotation of the eye in visual instruments with real exit pupils.     

Simple dispersion law for arbitrary sequences of dispersive optics

We give a simple general formula for the total angular dispersion due to multiple arbitrary dispersive elements in a series. It is simply the sum of the individual elements' angular dispersions but with each divided by the total spatial magnification afterward (or, equivalently, multiplied by the total angular magnification afterward).     

Depth resolution in stereoscopic systems

In television stereoscopic systems the magnification and the effective eye separation can be varied at will. This study shows that, under carefully chosen experimental conditions, depth resolution can be closely predicted by the use of geometrical optics and can be enhanced by increases in magnification and effective eye separation. The limited spatial resolution of conventional television or other photoelectronic systems may, however, prevent high levels of stereoacuity, comparable with naked-eye viewing, from being achieved in a wide-field system. In more complex environments the inevitable conflict of perceptual cues might be expected to reduce depth resolution further.     

Design of Achromatic Anamorphotic Telescopes

Thin lens equations are given for the correction of the longitudinal and transverse chromatic aberrations of an anamorphotic telescope consisting of four thin lenses, and it is shown how a thin lens solution for such a system having a given Petzval sum and a given angular magnification in the first principal section, may be obtained. From this, two conditions, which the optical constants of the glasses of the lenses have to satisfy for simultaneous correction of the two chromatic aberrations and the two aberrations of astigmatism and distortion, resulting from the anamorphotic effect, are then derived. Finally a method for the selection of glasses which makes achromatic correction possible, is given.     

Parabola-doublet aplanat becomes anastigmatic when second doublet is inserted near focus

A doublet of choice glasses may be located in the converging focal cone of the infinity-focused parabola to yield an aplanatic telescope or camera. The resulting angular field is limited by high astigmatism but is significantly larger than that of the coma-limited parabola. The spherical and chromatic aberrations are so well corrected and the coma so well balanced that the doublet may be used unaltered with a parabola of arbitrary focal length and speed with excellent results for the unvignetted rays. A second doublet nearer to the focus and designed independently of the first corrects the system's astigmatism while preserving its aplanaticism. It may also be designed for flattening the field. This arrangement may allow for greater flexibility in the placing of optical elements than does Wynne's triplet for modest-aperture systems. Equations are presented for choosing candidate glasses for the first doublet from the very limited manifold of solving glasses.     

Active optics: single actuator principle and angular thickness distribution for astigmatism compensation by elasticity

Active optics techniques providing high quality large astigmatism compensations to be used in a fixed or variable optical path, are presented in three ways: analytical calculations, finite element analysis, and experimental validation. The use of an angular thickness distribution allows improving the optical quality of toroidal deformations. We emphasize the case of a single actuator design, in which the principle is proven with a prototype.     

Improved field scanner incorporating parabolic optics. Part 2: Experimental verification and potential for volume scanning

We investigated the experimental performance of an afocal scan engine employing two off-axis parabolic reflectors and it was found not to introduce astigmatism when compared to a freely propagated beam. The performance of the new afocal engine is very similar to an ideal single-mirror scan engine in terms of spot size and beam spot profile (or point spread function) and has an improved flatness of field over other two-dimensional laser scan engines. The parabolic scan engine is contrasted with a comparable spherical mirror arrangement and found to produce superior performance at the intermediate image plane when focused through a scan lens. Further modeling and experimentation point toward volume scanning applications. The significant performance improvement provided by this design, now verified experimentally, will result in superior image quality for fast scanning confocal and two-photon microscopy in particular.     

Automated multifilter rotating shadow-band radiometer: an instrument for optical depth and radiation measurements

The multifilter rotating shadow-band radiometer is a ground-based instrument that uses independent interference-filter-photodiode detectors and the automated rotating shadow-band technique to make spectrally resolved measurements at seven wavelength passbands (chosen at the time of manufacture between 350 nm and 1.7 μm) of direct-normal, total-horizontal, and diffuse-horizontal irradiances. This instrument achieves an accuracy in direct-normal spectral irradiance comparable with that of tracking radiometers, and it is more accurate than conventional instruments for the determination of the diffuse and total-horizontal spectral irradiances because the angular acceptance function of the instrument closely approximates the ideal cosine response, and because the measured direct-normal component can be corrected for the remaining angular acceptance error. The three irradiance components are measured with the same detector for a given wavelength. Together with the automated shadow-band technique, this guarantees hat the calibration coefficients are identical for each, thus reducing errors when one compares them (as opposed to measurements made with independent instruments). One can use the direct-normal component observations for Langley analysis to obtain depths and to provide an ongoing calibration against the solar constant by extrapolation to zero air mass. Thus the long-term stability of all three measured components can be tied to the solar constant by an analysis of the routinely collected data.     

Liquid surface oscillations in a viscous liquid column induced by temperature fluctuations

It was found that timewise fluctuating temperature gradients induce, due to the variation of the surface tension, oscillations of the free surface of a liquid column, which may in or close to resonance lead to large amplitudes, able to disingtegrate the liquid system. An analytical solution leading to the magnification functions and their phases is presented for an arbitrary angular temperature field.     

Selection of Glasses of Achromatic Anamorphotic Telescopes

A graphical method for the selection of glasses which makes possible achromatic correction of a telescopic system of four cylindrical lenses which has a given Petzval sum and a given angular magnification in the first principal section, and which is to be corrected for the two aberrations of astigmatism and distortion, resulting from the anamorphotic effect, is described. The method is simple and much more effective than a previous calculation method. An illustrative example is given.     

Surface light scattering: integrated technology and signal processing

The miniaturization of surface-scattering instruments for measuring viscoelastic properties is investigated. The concepts are based on the use of holographic optical elements and integrated optics. Compact forms of optics that provide the necessary spatial and angular selections are devised. Four systems representing increasing levels of integration are considered. It is demonstrated that efficient signal and data processing can be achieved by evaluation of the statistics of the derivative of the instantaneous phase of the detector signal.     

Lau optometer: A new subjective instrument to measure the astigmatism of the human eye

Abstract

We have developed an optometer to measure the ocular astigmatism of human eyes and tested its performance. In this instrument we make use of an optical field that has coherence only along specific meridians to form interference images of different orientations on the subject's retina. The subject sees fringes of different orientations sequentially, at a fixed vergence, by rotating an eyepiece consisting of a probe cylinder. From the angular positions of the eyepiece at which each of the fringes are seen best by the subject, his or her astigmatism is calculated. The instrument has the potential to probe meridional vision and accommodation characteristics of human eyes.     

Improving solar ultraviolet irradiance measurements by applying a temperature correction method for Teflon diffusers

To establish trends in surface ultraviolet radiation levels, accurate and stable long-term measurements are required. The accuracy level of today's measurements has become high enough to notice even smaller effects that influence instrument sensitivity. Laboratory measurements of the sensitivity of the entrance optics have shown a decrease of as much as 0.07-0.1%/deg temperature increase. Since the entrance optics can heat to greater than 45 degrees C in Dutch summers, corrections are necessary. A method is developed to estimate the entrance optics temperatures from pyranometer measurements and meteorological data. The method enables us to correct historic data records for which temperature information is not available. The temperature retrieval method has an uncertainty of less than 2.5 degrees C, resulting in a 0.3% uncertainty in the correction to be performed. The temperature correction improves the agreement between modeled and measured doses and instrument intercomparison as performed within the Quality Assurance of Spectral Ultraviolet Measurements in Europe project. The retrieval method is easily transferable to other instruments.     

Correlation-based phase space beam characterization

A generalized correlation-based definition for moments of arbitrary order is introduced that can also accommodate mixed spatial and angular moment. Moreover, a transformation law forthese moments for propagation through linear optical systems is derived. This law has the same form as the corresponding propagation law of the moments defined in terms of the Wigner distribution function. The correlation-based moments can be used to fully characterize beams of arbitrary states of coherence.     

Sunset Science. II. A useful diagram

We present diagrams that show how layers in atmospheric thermal structure are related to the altitudes at which they are seen tangentially. These dip diagrams show that the inferior mirage greatly magnifies the apparent angular size of the lowest few centimeters of atmosphere. Conversely, inversion layers below eye level are compressed-even to zero apparent thickness, in ducts. The diagrams show that, even when distant objects are miraged, the ray crossings occur beyond the lowest point on each ray where the line of sight is tangent to a horizontal surface in the atmosphere. Therefore the apparent altitudes of these tangent points are a monotonic function of their actual heights in the atmosphere. This monotonicity explains an apparent paradox in low-Sun images.     

Compact double focusing mass spectrometer design for partial pressure measurements

A double focusing mass spectrometer with Wien filter and magnetic deflector (both located in the same pole-gap) is proposed as a partial pressure analyzer. The magnetic field area to accommodate a beam of given radial angular aperture and the partial pressure sensitivity are compared with those of the single focusing instruments. The numerical examples given show a mass resolution increase of more than two compared with a single focusing instrument.     

Automatic calibration of analog and digital measuring instrumentsusing computer vision

This paper presents an automatic calibration system capable of calibrating measuring instruments that do not have a digital interface. Image analysis algorithms are used to automatically determine the instrument reading. It can be used with analog and with digital displays. The maximum uncertainty in the detection of the pointer's position in analog instruments is less than the human eye can discriminate     

Adaptive optics and vision

It has long been recognised that the optical quality of the human eye is far from diffraction limited. This affects our visual acuity and severely limits the resolution at which images of the living retina can be obtained. Adaptive optics is a technique that can correct for the eye's aberrations and provide diffraction limited resolution. The origins of the technique lie in astronomy, but it was successfully adapted to the human eye just over 10 years ago. Since then there have been rapid developments in the field of adaptive optics and vision science. In vivo images of the retina can now be routinely achieved with unprecedented resolution. Sophisticated experiments can be performed to gain a deeper knowledge of the interaction of neural retinal architecture and visual perception. This article presents the theory behind adaptive optics for the human eye and reviews the developments in this field to date.