Off-axis Measurements Research Articles

Commissioning and validation of BrainLAB cones for 6X FFF and 10X FFF beams on a Varian TrueBeam STx

Small field dosimetry is a challenging task. The difficulties of small field measurements, particularly stereotactic field size measurements, are highlighted by the large interinstitution variability that can be observed for circular cone collimator commissioning measurements. We believe the best way to improve the consistency of small field measurements is to clearly document and share the results of small field measurements. In this work we report on the commissioning and validation of a BrainLAB cone system for 6 MV and 10 MV flattening filter‐free (FFF) beams on a Varian TrueBeam STx. Commissioning measurements consisted of output factors, percent depth dose, and off‐axis factor measurements with a diode. Validation measurements were made in a polystyrene slab phantom at depths of 5 cm, 10 cm, and 15 cm using radiochromic film. Output factors for the 6xFFF cones are 0.689, 0. 790, 0.830, 0.871, 0.890, and 0.901 for 4 mm, 6 mm, 7.5 mm, 10 mm, 12.5 mm, and the 15 mm cones, respectively. Output factors for the 10xFFF cones are 0.566, 0. 699, 0.756, 0.826, 0.864, and 0.888 for 4 mm, 6 mm, 7.5 mm, 10 mm, 12.5 mm, and the 15 mm cones, respectively. The full width half maximum values of the off‐axis factors agreed with the nominal cone size to within 0.5 mm. Validation measurements showed an agreement of absolute dose between calculation and plan of ≤ 3.6%, and an agreement of field sizes of ≤ 0.3 mm in all cases. Radiochromic film validation measurements show reasonable agreement with beam models for circular collimators based on diode commissioning measurements.PACS numbers: 87.53.Ly, 87.53.Bn, 87.56.nk, 87.55.D‐, 87.55.km

Structural and electrical anisotropies of Si-doped a-plane (11–20) GaN films with different SiNx interlayers

The effects of different SiNx interlayers on the structural and electrical properties of nonpolar Si-doped a-plane (11–20) GaN films grown on r-plane (1–102) sapphire were investigated. The surface roughness depends strongly on the SiNx coverage, deposition temperature and number of SiNx layers. The in-plane anisotropy of on-axis x-ray rocking curves (XRCs) (full width at half-maximum) was significantly decreased by the introduction of multiple SiNx-treated GaN interlayers, indicating coherently scattering domains of uniform size. Off-axis XRC measurements were also employed to investigate the effects on the mosaic twist corresponding to edge dislocation and the I1-type basal-plane stacking fault (BSF) density. Hall effect measurement showed that the electrical conductivity was the highest when multiple SiNx/GaN interlayers were employed. The measured sheet resistances (Rsh) along the c-axis were higher than those along the m-axis. These anisotropic conductivities could be explained by BSFs acting as carrier scattering centers. The ratios of Rsh along the two in-plane orientations also correlated well with the BSF densities.

Efficient reduction of defects in (1120) non-polar and (1122) semi-polar GaN grown on nanorod templates

(1120) non-polar and (1122) semi-polar GaNs with a low defect density have been achieved by means of an overgrowth on nanorod templates, where a quick coalescence with a thickness even below 1 μm occurs. On-axis and off-axis X-ray rocking curve measurements have shown a massive reduction in the linewidth for our overgrown GaN in comparison with standard GaN films grown on sapphire substrates. Transmission electron microscope observation demonstrates that the overgrowth on the nanorod templates takes advantage of an omni-directional growth around the sidewalls of the nanostructures. The dislocations redirect in basal planes during the overgrowth, leading to their annihilation and termination at voids formed due to a large lateral growth rate. In the non-polar GaN, the priority 〈0001〉 lateral growth from vertical sidewalls of nanorods allows basal plane stacking faults (BSFs) to be blocked in the nanorod gaps; while for semi-polar GaN, the propagation of BSFs starts to be impeded when the growth front is changed to be along inclined 〈0001〉 direction above the nanorods.

Repeatability of On- and Off-Axis Eye Length Measurements Using the Lenstar

To determine the repeatability of eye length measurements made centrally and off-axis using a low-coherence interferometer. Eye length was measured (left eye) in 29 adults using the Haag-Streit Lenstar LS 900. Five measurements were made centrally, 10 degrees temporally, and 10 degrees nasally on the retina by the same examiner at two separate visits. Intervisit repeatability was assessed by plotting the difference versus the mean of each pair of measurements (bias) and calculating the 95% limits of agreement (LoA). Within-session repeatability was determined by calculating the within-subject SD (Sw) of five consecutive measurements at a visit. Because of variability noted 10 degrees nasally, the Sw was also determined on a subset of 10 subjects using five measurements at one visit 30 degrees nasally and 30 degrees temporally. The mean ± SD age, spherical equivalent refractive error, and central axial length (at visit 1) were 24.0 ± 1.4 years, -3.46 ± 2.69 diopters, and 24.91 ± 1.10 mm, respectively. There was no significant bias for central, 10 degrees nasal, or 10 degrees temporal measurements between visits (all p > 0.09). The 95% LoA were ± 0.05 mm central, ± 0.12 mm 10 degrees nasal, and ± 0.05 mm 10 degrees temporal. The Sw (visit 1) were 0.025 mm central, 0.045 mm 10 degrees nasal, and 0.028 mm 10 degrees temporal. The Sw in the subset of subjects with 30-degree measurements were 0.023 mm (30 degrees nasal) and 0.030 mm (30 degrees temporal). Lenstar central and off-axis eye length measurements are very repeatable, although repeatability was not as good 10 degrees nasally on the retina as indicated by the larger 95% LoA between visits and larger Sw. The Sw for measurements centrally and 30 degrees off-axis were similar, suggesting that the reduced repeatability 10 degrees nasally is anatomical in nature. Despite greater variability 10 degrees nasally, Lenstar off-axis repeatability is still superior to the repeatability of on-axis ultrasonography.

Off-Axis Measurements on a mm-Wave Optically Controlled Lens Antenna

We present initial experimental data demonstrating the off-axis scanning performance and beam fidelity of a range of optically controlled transmission-type photo-injected Fresnel zone plate antenna (tx-piFZPA) designs at 94 GHz, based on a novel architecture. Several tx-piFZPA designs were characterized at a range of off-axis beam locations, up to 45 ° from boresight. Initial results indicate scan losses up to 5 dB at 10 ° deflection and increasing up to 12 dB at 45 ° deflection, across several tx-piFZPA designs. In addition, measured tx-piFZPAs have demonstrated highly repeatable and precise beam control, achieving a minimum pointing resolution of Δθ = 0.005°. The piFZPA method proves to be a promising cost-effective nonmechanical beam-steering solution for high millimeter- and terahertz-frequency applications.

An automatic compact Schlieren imaging system for ultrasound transducer testing

The current standard used for the characterization of ultrasonic transducers is the hydrophonic technique, which is able to measure the acoustic pressure profile. This technique allows a quantitative analysis, but is marred by several problems. A scan of the region of interest is a time-consuming operation, especially when measuring a long acoustic beam. Furthermore, a hydrophone placed near the radiating surface is certainly an impediment to the free propagation of the field. Off-axis measurements can be inaccurate because of the angular response of the hydrophone. These problems, together with the costs, have encouraged the search for an alternative quick and inexpensive test system. The well-known Schlieren technique allows a real-time visualization of the whole pressure range of the transducers, but to display the entire beam emitted by the transducers it is necessary to use very large-diameter lenses, with focal lengths of several meters. Such systems are very cumbersome and difficult to use. The system developed in this paper allows the imaging of an acoustic beam up to 200 mm in length, but the system is compact, being only about 1 m long and 0.30 m wide. A similar system based on the classic Schlieren effect would be several meters long, with lenses of 200 mm in diameter. Finally, the system can reconstruct the section of the beam at any height, using an acoustic tomography technique, and can also implement a quantitative analysis. Because it uses only commercial components, the developed and fabricated ultrasonic beam analyzer is a very low-cost imaging system. The goal of this work is to create a compact, low-cost system based on this technique to test a wide range of ultrasonic transducers up to 40 MHz and above.

Spectral contaminant identifier for off-axis integrated cavity output spectroscopy measurements of liquid water isotopes

Developments in cavity-enhanced absorption spectrometry have made it possible to measure water isotopes using faster, more cost-effective field-deployable instrumentation. Several groups have attempted to extend this technology to measure water extracted from plants and found that other extracted organics absorb light at frequencies similar to that absorbed by the water isotopomers, leading to δ(2)H and δ(18)O measurement errors (Δδ(2)H and Δδ(18)O). In this note, the off-axis integrated cavity output spectroscopy (ICOS) spectra of stable isotopes in liquid water is analyzed to determine the presence of interfering absorbers that lead to erroneous isotope measurements. The baseline offset of the spectra is used to calculate a broadband spectral metric, m(BB), and the mean subtracted fit residuals in two regions of interest are used to determine a narrowband metric, m(NB). These metrics are used to correct for Δδ(2)H and Δδ(18)O. The method was tested on 14 instruments and Δδ(18)O was found to scale linearly with contaminant concentration for both narrowband (e.g., methanol) and broadband (e.g., ethanol) absorbers, while Δδ(2)H scaled linearly with narrowband and as a polynomial with broadband absorbers. Additionally, the isotope errors scaled logarithmically with m(NB). Using the isotope error versus m(NB) and m(BB) curves, Δδ(2)H and Δδ(18)O resulting from methanol contamination were corrected to a maximum mean absolute error of 0.93 [per thousand] and 0.25 [per thousand] respectively, while Δδ(2)H and Δδ(18)O from ethanol contamination were corrected to a maximum mean absolute error of 1.22 [per thousand] and 0.22 [per thousand]. Large variation between instruments indicates that the sensitivities must be calibrated for each individual isotope analyzer. These results suggest that the properly calibrated interference metrics can be used to correct for polluted samples and extend off-axis ICOS measurements of liquid water to include plant waters, soil extracts, wastewater, and alcoholic beverages. The general technique may also be extended to other laser-based analyzers including methane and carbon dioxide isotope sensors.

Peripheral aberrations in the human eye for different wavelengths: off-axis chromatic aberration

The interest in the eye’s off-axis aberrations has increased strongly. On-axis the conversion of the aberration magnitude between different wavelengths is well known. We verified if this compensation is correct also for off-axis measurements by building a wavelength tunable peripheral Hartmann–Shack sensor and measuring 11 subjects out to ±30° in the horizontal visual field. At the fovea, an average longitudinal chromatic aberration of 1 D between red (671 nm) and blue (473 nm) light was found, and it increased slightly with eccentricity (up to 1.2 D). A similar trend was measured for astigmatism as a function of wavelength (increase ∼0.15 D). Computational ray tracing in model eyes showed that the origin of the small increase of chromatic aberrations with eccentricity is the change of the oblique power of the refractive surfaces in the eye. Factors related to increase of axial length and refractive index of the eye were found to have a very small influence.

Synthetic diamond X-ray dosimeter for radiotherapy

Abstract The performance of a synthetic diamond X-ray detector during typical clinical beam characterisation procedures was compared to the performance of standard clinical detectors; this diamond detector used a single crystal diamond film synthesised using chemical vapour deposition as its sensitive element. Measurements were performed using 6 MV photons from a Varian 600C linear accelerator. The procedures measured the dose profile with depth along the central axis in a phantom (tissue maximum ratio) for a 10 × 10 cm 2 field, variation in dose at the isocentre with field size (output factor), and dose profile across and beyond the X-ray beam (off-axis ratio) for 10 × 10 cm 2 and 1 × 1 cm 2 fields. Tissue maximum ratio values were within 0.8% of the values from a standard ion chamber, over a depth range of 1–15 cm. Output factors were measured for field sizes from 0.6 × 0.6 cm 2 to 15 × 15 cm 2 and agreed well ( 2 . Off-axis measurements showed reduced penumbral width when the lateral size of the detector was reduced by holding the diamond detector in an edge-on orientation; values were comparable to those obtained using a diode detector. Overall, these results demonstrate the potential of synthetic diamond detectors for clinical beam characterisation, particularly for small beam sizes.

Clinical investigations of a CVD diamond detector for radiotherapy dosimetry

X-ray detectors based on single crystal diamond film made via chemical vapor deposition were investigated to evaluate their performance under clinically relevant conditions for radiotherapy dosimetry. Studies focused on repeatability, dose rate dependence, tissue phantom ratios, output factors and beam profiling. Repeatability experiments revealed a temporary loss in sensitivity due to charge detrapping effects following irradiation, which was modeled to make corrections that improved short-term precision. Dose rate dependence was observed (Fowler fitting parameter Δ = 0.96 ± 0.2) using dose rates up to ∼2 Gy min^−1. The detector statistically distinguished (n = 5, P < 0.05) between dose values separated by 7.7 × 10^−3 Gy (1 MU). Depth dose measurements from 1 to 15 cm and output factors using 3 × 3 to 10 × 10 cm^2 field sizes compared well with a Farmer ion chamber (<1.3% difference). Output factor measurements indicate encouraging results for fields sizes <4 × 4 cm^2. Off-axis measurements showed that perturbation of the beam could be reduced when the detector is used in the edge-on orientation due to its thin-film sandwich configuration and ∼200 nm thick Ag contacts. This relatively inexpensive detector has potential to be used for routine dosimetry using conventional radiotherapy instrumentation.

Physical human model eye and methods of its use to analyse optical performance of soft contact lenses

A bench-top physical model eye that closely replicates both anatomical and optical properties of an average human eye was designed and constructed. The cornea was sourced from a flouro-polymer with refractive index (RI) of 1.376 and crystalline lenses were made of Boston RGP polymers, EO and Equalens II, with an equivalent RI of 1.429 and 1.423 respectively. These materials served to make crystalline lens components of different age groups and accommodative states. De-Ionized water, with RI of 1.334 represented both aqueous and vitreous humor. The complementary metal-oxide sensor of a PixelLink digital camera with a resolution of 5MP and a 2.2 microm pixel pitch, hosted on a motor-base, served as the 'acting' retina. The translation and rotary functions of the motor-base facilitated the simulation of different states of ametropia and assessment of peripheral visual function, respectively. We validated one of its configurations to suit normal viewing conditions and results from the on and off-axis optical quality measurements are presented. As a demonstration of potential practical uses, several corrective soft contact lenses were placed on the model eye and their optical performance evaluated.

Microdosimetry measurements characterizing the radiation fields of 300 MeV/u 12C and 185 MeV/u 7Li pencil beams stopping in water

In order to characterize the complex radiation field produced by heavy-ion beams in water, in particular the lateral dose fall-off and the radiation quality, microdosimetry measurements were performed at GSI Darmstadt using pencil-like beams of 300 MeV/u 12C and 185 MeV/u 7Li ions delivered by the heavy-ion synchrotron SIS-18. The ion beams (range in water about 17 cm) were stopped in the center of a 30 × 30 × 30 cm3 water phantom and their radiation field was investigated by in-phantom measurements using a tissue-equivalent proportional chamber (TEPC). The chamber was placed at 35 different positions in the central plane at various depths along the beam axis and at radial distances of 0, 1, 2, 5 and 10 cm. The off-axis measurements for both 12C and 7Li ions show very similar distributions of the lineal energy, all peaking between 1 and 10 which is a typical range covered by secondary hydrogen fragments and neutrons. The radiation quality given by the dose-mean lineal energy was found to be at a constant level of 1–2 at radial distances larger than 2 cm. The relative absorbed dose at each position was obtained by integration of the measured spectra normalized to the number of incident primary beam particles. The results confirm that the lateral dose profile of heavy ions shows an extremely steep fall-off, with relative values of about 10−3, 10−4 and 10−5 at the 2, 5 and 10 cm distance from the beam axis, respectively. The depth–dose curves at a fixed distance from the beam axis slowly rise until they reach the depth of the Bragg peak, reflecting the build-up of secondary fragments with increasing penetration depth. The measured 12C dose profiles were found to be in good agreement with a similar experimental study at HIMAC (Japan).

Repeatability of Peripheral Aberrations in Young Emmetropes

The purpose of this study is to assess the intrasession repeatability of ocular aberration measurements in the peripheral visual field with a commercially available Shack-Hartmann aberrometer (complete ophthalmic analysis system-high definition-vision research). The higher-order off-axis aberrations data in young healthy emmetropic eyes are also reported. The aberrations of the right eye of 18 emmetropes were measured using an aberrometer with an open field of view that allows peripheral measurements. Five repeated measures of ocular aberrations were obtained and assessed in steps of 10° out to ±40° in the horizontal visual field (nasal + and temporal -) and -20° in the inferior visual field. The coefficient of repeatability, coefficient of variation, and the intraclass correlation coefficient were calculated as a measure of intrasession repeatability. In all eccentric angles, the repeatability of the third- and fourth-order aberrations was better than the fifth and sixth order aberrations. The coefficient of variation was <30% and the intraclass correlation coefficient was >0.90 for the third and fourth order but reduced gradually for higher orders. There was no statistical significant difference in variance of total higher-order root mean square between on- and off-axis measurements (p > 0.05). The aberration data in this group of young emmetropes showed that the horizontal coma (C(3)(1)) was most positive at 40° in the temporal field, decreasing linearly toward negative values with increasing off-axis angle into the nasal field, whereas all other higher-order aberrations showed little or no change. The complete ophthalmic analysis system-high definition-vision research provides fast, repeatable, and valid peripheral aberration measurements and can be used efficiently to measure off-axis aberrations in the peripheral visual field.

A diagnostic tool for basic daily quality assurance of a tomotherapy Hi•Art machine

To investigate and evaluate the use of an in‐house developed diagnostic software tool using the imaging detector data for a quick daily quality assurance check of the output (dose) and lateral profile (cone) of a tomotherapy Hi•Art system. The Hi•Art treatment system is a radiation therapy machine for delivering intensity modulated radiation therapy (IMRT) in a helical fashion with an integrated CT scanner used for improved patient positioning before treatment. Since the system was developed specifically for IMRT, fat fields can be obtained by modulating the beam and therefore the fattening filter could be omitted. Because of this, the field has a cone‐like profile in both lateral and transversal directions. Patients are treated in a helical fashion with a tight pitch and a constant gantry rotation speed, while modulation is performed by a binary MLC. Consequently dose output per time‐unit (dose rate) as well as the shape of the cone‐profile are very important for correct patient treatment and should be closely monitored. However, using the company‐provided initial tools and conventional dosimetry, this can be a time consuming daily procedure. The aim of this work is to develop a fast, automated method of quality assurance based on the detector signal. A software tool called “tomocheck” running on the operation station has been developed to evaluate the output (dose rate) and the lateral cone profile (energy) of the Hi•Art system, comparing actual output and cone profile with a reference (previously approved against ionization chamber measurements). This is done by using the data of the 640 on‐board detector array that are directly retrieved and processed after a specific QA procedure. The detector file consists of the CT detector data and the three monitoring dose chamber readings over a time period of 200 sec. To evaluate the method, the system was benchmarked against ionization chamber measurements and classical IMRT QA methods. Action levels (final status “NOT ACCEPTED”) for dose ratio as well as the cone ratio are set to ±2%. The QA tool was introduced for daily QA in May 2007. For the following 24 months, a total of 931 morning checks was made on both tomotherapy machines. In 42 cases the check status was “NOT ACCEPTED”. In 34 cases the dose ratio (DR) was out of tolerance. The corrected cone ratio (CCR) was outside of specification tolerance in 8 cases. The tomocheck data was related to the ionization chamber measurements for the IMRT plan indicating a close relationship between the CCR and the off‐axis measurements. Average dose ratio against the mean value of the on‐ and off‐axis IC measurement indicates that this parameter is a good interpretation of the dose output. This tool makes it possible to perform an easy‐to‐use and fast basic daily quality assurance check featuring an output as well as an energy evaluation. Ideally this tool should offer also the combined dosimetry check of jaw width, couch speed, leaf latency, output, leaf/gantry synchrony, and lasers. This will be investigated in the future.PACS 87.55Qr

A Review of Peripheral Refraction Techniques

The recently discovered link between myopia development and peripheral refraction has triggered a stream of clinical and animal investigations to confirm the theory and to understand the underlying mechanisms. For this, precise peripheral refractometry has now gained importance in myopia research. For more than 70 years, many researchers have measured off-axis refraction of the eye in horizontal and sometimes vertical meridians over a range of angles, using several modified refraction techniques. These techniques varied not only with respect to the instrumentation used, but also the modifications performed to enable off-axis refraction. Modifications included either head or eye turn of the participant with respect to the peripheral angle tested or rotation of the instrument itself around the center of the eye. The main focus of this study is to review and compare all refraction techniques for off-axis measurements including necessary modifications made to equipment or procedures. Because a difference in instrumentation and techniques potentially limits the comparability of reported results, it is of particular importance to understand all the details of the particular refractometric technique chosen and any potential problems. Difficulties relating to all the methods are highlighted to provide information on preference and usefulness of certain peripheral refraction techniques for future technology and research work. All refraction techniques exhibited similar drawbacks, such as off-axis fixation, protraction when many peripheral angles were tested, and difficulties to obtain reliable measurements at large peripheral angles. Yet, from all the methods reviewed, the Shin-Nippon NVision K5001 open field autorefractor and the Hartmann-Shack wavefront sensor technique seem to be the most useful commercially available instruments to measure peripheral refraction.

Verification of Varian enhanced dynamic wedge implementation in MasterPlan treatment planning system

This paper investigates the accuracy of the two available calculation algorithms of the Oncentra MasterPlan three‐dimensional treatment planning system (TPS) – the pencil beam method and collapsed‐cone convolution – in modeling the Varian enhanced dynamic wedge (EDW). Measurements were carried out for a dual high energy (6–15 MV) Varian DHX‐S linear accelerator using ionization chambers for beam axis measurements (wedge factors and depth doses), film dosimetry for off‐axis dose profiles measurements, and a diode matrix detector for two dimensional absolute dose distributions. Using both calculation algorithms, different configuration of symmetric and asymmetric fields varying the wedge's angle were tested. Accuracy of the treatment planning system was evaluated in terms of percentage differences between measured and calculated values for wedge factors, depth doses, and profiles. As far as the absolute dose distribution was concerned, the gamma index method (Low et al. (13) ) was used with 3% and 3 mm as acceptance criteria for dose difference and distance‐to‐agreement, respectively. Wedge factors and percentage depth doses were within 1% deviation between calculated and measured values. The comparison of measured and calculated dose profiles shows that the Van Dyk's acceptance criteria (Van Dyk et al. (14) ) are generally met; a disagreement can be noted for large wedge angles and field size limited to the low dose‐low gradient region only. The 2D absolute dose distribution analysis confirms the good accuracy of the two calculation algorithms in modeling the enhanced dynamic wedge.PACS number: 87.53.Bn, 87.55.kn, 87.56.ng

Modeling the eye's optical system by ocular wavefront tomography

Ocular wavefront tomography (OWT) is the process of using wavefront aberration maps obtained along multiple lines-of-sight (LoS) to determine the shape and position of the major refracting elements of an eye. One goal of OWT is to create a customized schematic model eye that is anatomically similar and functionally equivalent to the individual eye over a large field of view. Wavefront aberration maps along multiple LoS were used as design goals for configuring a generic, multi-surface model eye with aberrations that match the measurements. The model was constrained by gross anatomical dimensions and optimized to mimic the measured eye. The method was evaluated with two test cases: (1) a physical model eye with a doublet lens measured with a clinical wavefront aberrometer along six LoS between -31 deg and +29 deg eccentricities, and (2) a mathematical model of the myopic eye for which wavefront aberrations were computed by ray tracing. In case 1, the OWT algorithm successfully predicted the structure of the doublet model eye from the experimental on- and off-axis aberration measurements. In case 2, the algorithm started with a symmetric five surface model eye and optimized it to generate the on- and off-axis aberrations of a GRIN myopia model eye. The adjusted model closely mimicked the physical parameters and optical behavior of the expected myopia model eye over a large field of view. The maximum discrepancy between aberrations of the OWT optimized model and measurements was 0.05 microns RMS for test case 1 and 0.2 microns RMS for test case 2. Our implementation of OWT is a valid, feasible, and robust method for constructing an optical model that is anatomically and functionally similar to the eye over a wide field of view.

Applicability of Infrared Photorefraction for Measurement of Accommodation in Awake-Behaving Normal and Strabismic Monkeys

This study was designed to use infrared photorefraction to measure accommodation in awake-behaving normal and strabismic monkeys and describe properties of photorefraction calibrations in these monkeys. Ophthalmic trial lenses were used to calibrate the slope of pupil vertical pixel intensity profile measurements that were made with a custom-built infrared photorefractor. Day to day variability in photorefraction calibration curves, variability in calibration coefficients due to misalignment of the photorefractor Purkinje image and the center of the pupil, and variability in refractive error due to off-axis measurements were evaluated. The linear range of calibration of the photorefractor was found for ophthalmic lenses ranging from -1 D to +4 D. Calibration coefficients were different across monkeys tested (two strabismic, one normal) but were similar for each monkey over different experimental days. In both normal and strabismic monkeys, small misalignment of the photorefractor Purkinje image with the center of pupil resulted in only small changes in calibration coefficients, that were not statistically significant (P>0.05). Off-axis measurement of refractive error was also small in the normal and strabismic monkeys (approximately 1 D to 2 D) as long as the magnitude of misalignment was <10 degrees. Remote infrared photorefraction is suitable for measuring accommodation in awake, behaving normal, and strabismic monkeys. Specific challenges posed by the strabismic monkeys, such as possible misalignment of the photorefractor Purkinje image and the center of the pupil during either calibration or measurement of accommodation, that may arise due to unsteady fixation or small eye movements including nystagmus, results in small changes in measured refractive error.

Reconstruction of the optical system of the human eye with reverse ray-tracing

We present a practical method for reconstructing the optical system of the human eye from off-axis wavefront measurements. A retinal beacon formed at different locations on the retina allows probing the optical structure of the eye by the outgoing beams that exit the eye through the dilated pupil. A Shack-Hartmann aberrometer measures the amount of wave aberrations in each beam at the exit pupil plane. Wavefront data obtained at different oblique directions is used for tomographic reconstruction by optimizing a generic eye model with reverse ray-tracing. The multi-configuration system is constructed by tracing pre-aberrated beams backwards from each direction through the exit pupil into the optical system of the aberrometer followed by the generic eye model. Matching all wave aberrations measured at each field point is equivalent to minimizing the size of the beacon spots on the retina. The main benefit of having a personalized eye model is the ability to identify the origin of the ocular aberrations and to find the optimal way for their correction.

Off-axis cavity ringdown measurements of trace iodine

Off-axis cavity ringdown spectroscopy has been employed to measure trace levels of molecular iodine using continuum absorptions near 404 nm and 469 nm. Iodine concentrations ranging from 0 to 2 × 1012 molecules/cm3 were accurately measured to within ±4% and a measurement precision of 7.8 × 1011 molecules/(cm3 Hz1/2) in a 1 cm pathlength was empirically determined. The system, which does not require calibration and works in a variety of background gases, should find utility in quantifying residual iodine in oxygen–iodine chemical lasers and flow tube experiments.