A new, simple, inexpensive means of testing functional vitamin A status: the night vision threshold test (NVTT). A preliminary field-test report.

Vitamin A deficiency is the leading cause of preventable childhood blindness in developing countries. Each year, an estimated 13.5 million children world-wide are unable to adapt to the dark and half a million children progress to complete blindness annually from lack of vitamin A. Most of the currently available methods for assessing vitamin A status are expensive, require sophisticated instrumentation and are not efficacious in field conditions. A simple, inexpensive method was developed to identify children with defective dark-adaptability, thereby providing a reflection of marginal vitamin A stores. The purpose of this preliminary study was to test the field-efficacy of the Night Vision Threshold Tester (NVTT). Thirty-nine middle-school children with a mean age of 13.5 +/- 1.37 years were initially tested for their ability to adapt to the dark using the

Lateral-shift variable aberration generators.

We describe and analyze an optical device capable of generating tilt, defocus, astigmatism, coma, and spherical aberration wave-front deformation. This optical device consists of a pair of aspheric plates that produce aberrations by laterally shifting one plate relative to the other. The surface descriptions of these optical plates are provided, and their aberration-inducing properties are verified with ray-tracing software. In addition, we examine the versatility and the limitations of using variable aberration generators and provide insight into how aberrations may be controlled by a simple lateral shift. The device may find application in aberration control in lens systems that are nonrotationally symmetric.

Using corneal height maps and polynomial decomposition to determine corneal aberrations.

PURPOSE: To review the use of corneal videokeratoscopic height date, elaborate on the advantages and disadvantages of such data, describe techniques for overcoming the limitations of height data, and demonstrate its use in quantifying the optical properties and aberrations of the cornea. METHODS: The steep sag of the cornea hides fine variations in corneal height that arise naturally or due to disease or surgery. The dynamic range, or ratio of the overall sag to the feature height, is the primary limitation of videokeratoscopic height data. Techniques for removing single or multiple reference surfaces are described in detail, and applications of the methodology to wavefront and raytracing analysis of corneal aberrations arising from radial keratotomy (RK), photorefractive keratectomy (PRK), and keratoconus are described. RESULTS: Removing a single reference surface from the raw corneal height data begins to reveal subtle variations in corneal height. However, expansion of surface height data into a complete set of basis functions provides a sophisticated method for extracting high-order corneal variations. Choosing an orthogonal basis set provides a robust least-squares fit and forms unique expansions of the surface. The resulting coefficients are uncorrelated and form a simple measure of the optical quality. CONCLUSION: Videokeratoscopic height data are useful for analyzing and quantifying corneal deformity arising from disease or refractive surgery and they provide a sophisticated alternative or complement to dioptric power maps.

High accuracy, wide range, rotation angle measurement by the use of two parallel interference patterns.

Based on the idea of measuring small rotation angles with a parallel interference pattern (PIP), a method is developed to measure large rotation angles accurately. Two parallel PIP's that have different periods are used to measure a rotation angle of an object. The measurement made with a small-period PIP provides a high accuracy, and the measurement made with a large-period PIP provides a wide range. An accurate measurement for wide-range angles is made by combining the two measured values. The accuracy of the phase detection is determined by the periods of two PIP's. Rotation angles from approximately -30 to 30 arc min can be measured with an accuracy of 0.2 arc sec. Analytical results are supported by experimental results.

Optical modeling of radial keratotomy incision patterns.

PURPOSE: To determine the optical effects of higher-order corneal shape variations resulting from radial keratotomy. METHODS: Videokeratoscopic height data were obtained postoperatively from several patients who had undergone radial keratotomy. For each of clear central zone sizes 3.00 mm, 4.00 mm, and 4.75 mm, two patients were chosen randomly from the larger study group. Data obtained 2 weeks postoperatively from these six patients were decomposed into the Zernike polynomials, and the low-order expansion terms were removed to disclose corneal height variations (the radial keratotomy artifact). The artifact was applied to a schematic eye model, and exact ray-tracing was used to evaluate visual performance, which was defined as a function of pupil diameter, optical zone (central clear zone) size, and radial keratotomy artifact centration. RESULTS: The radial keratotomy artifact degrades visual performance at midspatial frequencies more than it does at high spatial frequencies. This effect is most pronounced for smaller optical zones and for a pupil diameter of 4 mm. Visual performance remains nearly constant for small decentration (0.5 mm or less) of the radial keratotomy optical zone from the corneal apex. CONCLUSIONS: Residual refractive error, corneal asphericity, and the radial keratotomy artifact all affect visual performance after radial keratotomy. Isolated effects of the radial keratotomy artifact degrade visual performance, with the level of degradation dependent on pupil size, optical zone size, and centration of the procedure. More research is necessary to combine the radial keratotomy artifact with changes in corneal asphericity and to further quantify the optical effects of radial keratotomy.

Keratoconus detection based on videokeratoscopic height data.

PURPOSE: To develop a videokeratoscopic-based keratoconus detection scheme that avoids the ambiguity of dioptric power definitions and videokeratoscope design. METHODS: Corneal height data obtained with a commercial videokeratoscope are decomposed into the set of orthogonal Zernike polynomials. Expansion coefficients of a "normal" group and a keratoconus group are compared to find significant differences. Elevated Zernike terms are used to detect the disease in these populations. The performance of this detection scheme is compared to other videokeratoscopic keratoconus Indices. RESULTS: Two low-order Zernike polynomial terms are identified as being elevated in keratoconus patients and combined to form a new detection index. This index performed at least as well as keratoconus detection schemes based on the inferior-superior (I-S) value, the steepest radial axes (SRAX), and the Surface Asymmetry Index (SAI) for the samples studied. CONCLUSION: The proposed Zernike scheme offers a potentially viable algorithm for detecting keratoconus that avoids the ambiguities of dioptric power definitions and is independent of videokeratoscope design.

Comparison of three videokeratoscopes in measurement of toric test surfaces.

PURPOSE: We compared the accuracy of the Computed Anatomy TMS-1 (1.41), the EyeSys Laboratories Corneal Analysis System (2.1), and the Visioptic EH-270 (3.0) videokeratoscopes in measuring toric surfaces. These non-rotationally symmetric aspheric surfaces served as models of corneal astigmatism. METHODS: Precision diamond-turned toric surfaces modeling 0.00 diopter (D) to 7.00 D of astigmatism were fabricated. A three-dimensional contact profiler was developed to calibrate the aspheric surfaces. Videokeratoscopic data taken at "best focus" were compared to the theoretical shape to quantify device measurement errors. RESULTS: The Computed Anatomy system measurement accuracy shows no statistically significant correlation between measurement error and surface toricity (r2 < 0.13). Measurement error increased linearly with surface astigmatism for the EyeSys Laboratories system (0.12 D rms error per D of astigmatism, r2 > 0.96, p < 0.001 and the Visioptic system (0.03 D error per D of astigmatism, r2 = 0.88, p < 0.001). CONCLUSIONS: This study found systematic performance differences among the three machines. Under ideal alignment conditions, the Computed Anatomy TMS-1 is more accurate at detecting astigmatism. The EyeSys Laboratories Corneal Analysis System apparently underestimates the amount of surface astigmatism because of excessive data smoothing. The Visioptic EH-270 errors are primarily in the central zones and may be due to ring localization errors.

Measurement of two-dimensional small rotation angles by using orthogonal parallel interference patterns.

Based on measuring one-dimensional small rotation angles by using a parallel interference pattern (PIP), a method for measuring two-dimensional (2D) small rotation angles by using two different PIP's that are orthogonal to each other is proposed. We simultaneously measure the 2D small rotation angles Δθ and Δφ by detecting the phases of the orthogonal PIP's reflected by an object at two detection points. A sensitivity of 4.9 mrad/arcsec and a spatial resolution of 1.5 × 1.5 mm(2) are achieved in the measurement. Theoretical analysis and experimental results show that error ε(1) in the measurement of Δφ is almost equal to -0.01Δθ and error ε(2) in the measurement of Δθ is almost equal to -0.01Δφ. For small rotation angles of less than a few tens of arcseconds, the random errors whose standard deviations are 0.6 arcsec are dominant.

Interferometer errors due to the presence of fringes.

Phase-shifting interferometry permits analysis of complex interferograms. However, the measurement accuracy is reduced as the number of fringes is increased. The wave-front from a defocused spherical surface is used to demonstrate this degradation for several different transmission reference objectives.

Representation of videokeratoscopic height data with Zernike polynomials.

Videokeratoscopic data are generally displayed as a color-coded map of corneal refractive power, corneal curvature, or surface height. Although the merits of the refractive power and curvature methods have been extensively debated, the display of corneal surface height demands further investigation. A significant drawback to viewing corneal surface height is that the spherical and cylindrical components of the cornea obscure small variations in the surface. To overcome this drawback, a methodology for decomposing corneal height data into a unique set of Zernike polynomials is presented. Repeatedly removing the low-order Zernike terms reveals the hidden height variations. Examples of the decomposition-and-display technique are shown for cases of astigmatism, keratoconus, and radial keratotomy.

Visual acuity modeling using optical raytracing of schematic eyes.

PURPOSE: We developed a methodology to predict changes in visual performance that result from changes in the optical properties of the eye. METHODS: Exact raytracing of schematic eyes was used to calculate the point spread function and the modulation transfer function of the visual system. The Stiles-Crawford effect, photopic response, diffraction, and the retinal contrast sensitivity are included in the model. Visual acuity was predicted by examining the modulation of the resultant retinal image of a bar target and by determining when the modulation falls below a threshold value. Visual acuity was predicted for refractive errors ranging from 0 to 5 diopters and for pupil diameters ranging from 0.5 to 8 mm. RESULTS: Visual acuity predictions were compared to clinically found Snellen visual acuities and were found to be highly correlated (r2 = .909). CONCLUSIONS: This modeling technique shows promise as a means of evaluating clinical and surgical procedures before undertaking clinical trails.

Quantification of the Brückner test for strabismus.

PURPOSE: To measure quantitatively the change in the coaxial fundus reflex with varying degrees of ocular misalignment. METHODS: The coaxial fundus reflex was imaged with a charge coupled device camera under conditions of simulated ocular misalignment ranging from 0 degrees to 7 degrees of fixation eccentricity. The effects of refractive error and pupil size were controlled. Average gray scale brightness values were calculated for each bright pupil image after some image processing was performed on the raw images. RESULTS: A reliable, sharply delineated, minimum brightness at foveal fixation was observed. CONCLUSIONS: It is estimated that this technique can be automated to detect the presence of 2 degrees to 3 degrees of ocular misalignment based on the difference in brightness of the bright pupil images between the two eyes.

Measurement of small rotation angles by using a parallel interference pattern.

We propose a method for measuring rotation angles by using a parallel interference pattern. At two points on a parallel interference pattern reflected by an object, we detect phase changes in the reflected parallel interference pattern caused by rotations of the object. A high sensitivity, or a high ratio of the phase change to the rotation angle, 17 mrad/arcsec, can be achieved by determining the positions of two detection points. A high spatial resolution of ~0.5 mm is also obtained. We analyze the measurement error caused by the alignment of the parallel interference pattern and a random measurement error caused by the phase detection. The theoretical analyses and the experimental results make the characteristics of the method clear and show that the method has an accuracy of 0.2 arcsec for small rotation angles.

Modeling soft contact lenses in raytrace code.

The surface interferogram feature of raytrace codes is used to model the optical performance of soft contact lenses, especially in situations where the lens is decentered. The contact lens is totally defined by a thickness data file, which can easily be shifted relative to the corneal vertex of a schematic eye model.

Modulation transfer function measurement of sparse-array sensors using a self-calibrating fringe pattern.

A simple method for the measurement of the pixel modulation transfer function (MTF) of sparse-array (extended MTF) sensors has been developed. We use a phase-shifting Twyman-Green interferometer to generate a series of single spatial-frequency fringe patterns incident on the sensor The resulting signal modulation is measured. We achieve self-calibration by restricting the measured spatial frequencies to multiples of the Nyquist frequency. The aliased patterns at these frequencies are unique and easily identifiable. Spatial frequencies of 480 cycles/mm are generated and measured. This frequency value is more than ten times that of the sensor sampling frequency. The expected MTF shape is obtained at multiples of the sampling frequency. At odd multiples of the Nyquist frequency, the MTF's are affected by the electronic bandwidth and cross talk in the charge-injection device sensor.

Color dependent optical prefilter for the suppression of aliasing artifacts.

Optical prefilters, which make use of the double-refraction effect in crystalline quartz, reduce the amount of aliasing artifacts in the displayed image of a video system. This paper describes anew class of optical prefilter that produces different amounts of blur for different colors. The chromatic variation is obtained by placing a polarization retarder between two quartz crystals. This type of filter is useful with sensors that have unequal pixel densities in the three sensed colors.

Sub-Nyquist interferometry.

The primary limitation of conventional phase-shifting interferometry (PSI) is its inability to measure surfaces with large aspheric departures. A new method of data analysis, sub-Nyquist interferometry (SNI), is described and demonstrated to overcome this problem. SNI is an extension of PSI, and it preserves the measurement precision that is inherent to PSI. For some types of wavefronts, measurement range improvements of more than 2 orders of magnitude are shown, and these improvements result from the utilization of a priori knowledge about the wavefront. Simple and reasonable assumptions are found to be very powerful for improving the aspheric measurement capability of the interferometer system.

A low-cost computed tomographic scanner.

The authors describe a film-recorded computed tomographic (CT) scanner that uses optical processing methods for reconstruction of a sectional image. Results are comparable to commercial CT methods using the same patient dose. Consequently, clinically satisfactory CT imaging can be performed at a substantially reduced cost.

Incoherent optical processor for x-ray transaxial tomography.

A single-channel incoherent optical processor that reconstructs an object from its 1-D x-ray projections has been developed. The projection data are recorded directly on x-ray film, which, after development, is used as the input transparency for the processor. The same computing algorithms used by commercial computed tomography units are optically implemented to reconstruct the object from these projections. The totally analog processor employs time-modulated OTF synthesis to achieve the necessary bipolar filtering operations. This technique involves a time-varying pupil plane mask in conjunction with synchronous demodulation of the detector output to obtain the desired response. Results are presented that are similar in quality to reconstructions produced by commercial scanners. The advantage of this system is that it should offer equivalent performance at a reduced cost.