title: Applications of ray tracing to a pseudophakic eye model

authors: Turuwhenua, Jason

abstract: The calculation of IOL power using keratometry is adversely affected by recent corneal reshaping surgeries. This thesis investigates the application of ray tracing and general anterior corneal surface modeling, for the purpose of improving ophthalmic measurements and in particular, the estimation of IOL power.

A new algorithm (based on a multi-step approach) for the recovery of the corneal height using videokeratography is presented. The method ensures a cubic recovery with continuous curvature; skew rays are treated in post-processing. The RMS height error is measured for three simulated (with two skewed) cornea. The total errors are 6.2 x 10⁻⁴ mm ignoring the skew ray error, and 1.7 x 10⁻⁴ mm accounting for it. The individual height errors are submicron in the latter case. The algorithm gives average errors of 2.5 x 10⁻⁴ mm for a set of calibration balls. The completion time is 2.3 s over all cases, using a standard desktop PC.

A new method for the recovery of the internal ocular radii of curvature is investigated. The method is used to recover the posterior corneal radii (PII) and the anterior lens radii (PIII) given several anterior cornea models (PI) in simulation. The recovered surface powers are no more than 0.1 D(PII) and 0.006 D(PIII) in error of the true surface powers. 

A framework is then presented for modeling the effect of lens decenter and tilt on perceived image quality. The SQRI image quality metric is determined for a range of lens tilt and lens decenter values. These are compared with the statistical moments of the spot diagrams. The SQRI shows asymmetric degradation (with tilt for a particular decenter value) of imaging for a plane displaced -0.1 mm from best focus. For a plane displaced +0.1 mm from best focus, the SQRI is symmetric and improves regardless of the sign of tilt. The statistical moments suggest that skew does not necessarily imply poor imaging.

Finally, the modeling methods developed are tested on two clinically measured eyes. Minimizing the spot size, predicts the spectacle prescription to 0.0 D(OS) and 0.1 D(OD) of the mean spherical equivalent. Adding prescribed lenses to the model eye, estimates best focus to 0.03 mm and 0.02 mm of the retinal plane; consistent with better than 6/6 VA measured for OS/OD. A VisTech VCTS 6500 contrast sensitivity chart is used to verify the eye model. A 75% match with theory is found for OS, a 50% match is found for OD.

text: Applications of ray tracing to a pseudophakic eye model The calculation of IOL power using keratometry is adversely affected by recent corneal reshaping surgeries. This thesis investigates the application of ray tracing and general anterior corneal surface modeling, for the purpose of improving ophthalmic measurements and in particular, the estimation of IOL power.

A new algorithm (based on a multi-step approach) for the recovery of the corneal height using videokeratography is presented. The method ensures a cubic recovery with continuous curvature; skew rays are treated in post-processing. The RMS height error is measured for three simulated (with two skewed) cornea. The total errors are 6.2 x 10⁻⁴ mm ignoring the skew ray error, and 1.7 x 10⁻⁴ mm accounting for it. The individual height errors are submicron in the latter case. The algorithm gives average errors of 2.5 x 10⁻⁴ mm for a set of calibration balls. The completion time is 2.3 s over all cases, using a standard desktop PC.

A new method for the recovery of the internal ocular radii of curvature is investigated. The method is used to recover the posterior corneal radii (PII) and the anterior lens radii (PIII) given several anterior cornea models (PI) in simulation. The recovered surface powers are no more than 0.1 D(PII) and 0.006 D(PIII) in error of the true surface powers. 

A framework is then presented for modeling the effect of lens decenter and tilt on perceived image quality. The SQRI image quality metric is determined for a range of lens tilt and lens decenter values. These are compared with the statistical moments of the spot diagrams. The SQRI shows asymmetric degradation (with tilt for a particular decenter value) of imaging for a plane displaced -0.1 mm from best focus. For a plane displaced +0.1 mm from best focus, the SQRI is symmetric and improves regardless of the sign of tilt. The statistical moments suggest that skew does not necessarily imply poor imaging.

Finally, the modeling methods developed are tested on two clinically measured eyes. Minimizing the spot size, predicts the spectacle prescription to 0.0 D(OS) and 0.1 D(OD) of the mean spherical equivalent. Adding prescribed lenses to the model eye, estimates best focus to 0.03 mm and 0.02 mm of the retinal plane; consistent with better than 6/6 VA measured for OS/OD. A VisTech VCTS 6500 contrast sensitivity chart is used to verify the eye model. A 75% match with theory is found for OS, a 50% match is found for OD.

year: 2001

title: Preparation and characterisation of decoloured Novatein and modified PLA blends

authors: Izuchukwu, Sandra C.P.

abstract: Modified Poly (lactic acid) (PLA) was blended with Decoloured Novatein® (DNTP), a thermoplastic protein material using reactive extrusion to produce a degradable material with improved properties compared to neat Decoloured Novatein®. PLA was modified through free radical grafting of itaconic anhydride to create reactive side-chain groups. Varying ratios of DNTP/ PLA-g-IA or PLA were prepared. Blending DNTP with PLA was found to increase tensile strength  between 22% to 538% and modulus between 201 GPa to 3193 GPa, whereas the strain at break decreased between 80% to 94% depending on the blend ratio. The glass transition temperature of the blends which was measured as the tan δ peak, also revealed an increase when compared to neat DNTP.  Scanning electron microscope revealed an enhanced interfacial adhesion between the two phases in the blends with PLA-g-IA suggesting a more homogenous microstructure. WAXS result revealed an insignificant decrease in the crystallinity of the blends compared to neat DNTP, indicating that blending with PLA had no structural effect on DNTP. The results show the possibility and feasibility of blending DNTP with PLA for use in agricultural and packaging applications.

text: Preparation and characterisation of decoloured Novatein and modified PLA blends Modified Poly (lactic acid) (PLA) was blended with Decoloured Novatein® (DNTP), a thermoplastic protein material using reactive extrusion to produce a degradable material with improved properties compared to neat Decoloured Novatein®. PLA was modified through free radical grafting of itaconic anhydride to create reactive side-chain groups. Varying ratios of DNTP/ PLA-g-IA or PLA were prepared. Blending DNTP with PLA was found to increase tensile strength  between 22% to 538% and modulus between 201 GPa to 3193 GPa, whereas the strain at break decreased between 80% to 94% depending on the blend ratio. The glass transition temperature of the blends which was measured as the tan δ peak, also revealed an increase when compared to neat DNTP.  Scanning electron microscope revealed an enhanced interfacial adhesion between the two phases in the blends with PLA-g-IA suggesting a more homogenous microstructure. WAXS result revealed an insignificant decrease in the crystallinity of the blends compared to neat DNTP, indicating that blending with PLA had no structural effect on DNTP. The results show the possibility and feasibility of blending DNTP with PLA for use in agricultural and packaging applications.

year: 2017

title: Teaching Māori in English-medium schools in New Zealand: Teacher responses to aspects of the curriculum guidelines for te reo Māori in English-medium school settings

authors: Takurua, Nātana

subjects: New Zealand

abstract: Opinions are divided about how the content of language courses should be specified and whether educational authorities should include specifications/ suggestions about language content in national curriculum documents. Issues associated with language content are, however, not ones that can be avoided, particularly as teachers are expected to prepare students for national examinations at the upper levels of schooling and these examinations are necessarily predicated on general expectations about proficiency achievements and more specific expectations about the types of language with which students will be familiar. In this article, we explore the complex nature of some of the issues involved and the ways in which the New Zealand Ministry of Education has sought/ is seeking to resolve them in the case of the curriculum for Māori in English-medium schools. We also report on the responses of a sample of teachers of Māori in secondary schools in New Zealand to the omission of language examples from the final version of the curriculum guidelines document.

text: Teaching Māori in English-medium schools in New Zealand: Teacher responses to aspects of the curriculum guidelines for te reo Māori in English-medium school settings Opinions are divided about how the content of language courses should be specified and whether educational authorities should include specifications/ suggestions about language content in national curriculum documents. Issues associated with language content are, however, not ones that can be avoided, particularly as teachers are expected to prepare students for national examinations at the upper levels of schooling and these examinations are necessarily predicated on general expectations about proficiency achievements and more specific expectations about the types of language with which students will be familiar. In this article, we explore the complex nature of some of the issues involved and the ways in which the New Zealand Ministry of Education has sought/ is seeking to resolve them in the case of the curriculum for Māori in English-medium schools. We also report on the responses of a sample of teachers of Māori in secondary schools in New Zealand to the omission of language examples from the final version of the curriculum guidelines document.

year: 2009-09