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: Ozone Depletion and Global Warming

authors: Fow, Alista John

subjects: Ozone Depletion

abstract: Abstract
This thesis examines global warming and the possible contribution that ozone
depletion provides to this warming. An examination is performed to determine
the extent of any warming/cooling events within the Earth-atmosphere
system. The change in energy corresponding to this warning of the Earth-
atmosphere system is estimated as being equivilent to an increase of mean
solar input of 0.22W/m2. This is compared to the predicted changes of solar
input for the two most common global warming scenarios: greenhouse gases
and solar irradiance variance; and for a less well explored scenario, snow-ice
albedo change. Examination of ozone depletion data shows that an absence of
ozone in the stratosphere produces an increase in UV-B radiation at the surface
of the Earth. This increase in UV-B light has not previously been thourougly
examined in any of the global warming scenarios. This is presented as a fourth
scenario for global warming.
An analytical three layer model of the Earth-atmosphere, based on an earlier
two layer model, is developed. Using this model it is determined that
greenhouse gases, solar irradiance, snow-ice albedo feedback and ozone depletion
can cause warming of the Earth's atmosphere. After comparison with
other models, a snow-ice albedo mechanism is incorporated into the three layer
model. This produces an amplification effect of any warming that occurs.
Compared to the observed increase of surface temperature between 1975-2000
of 0.55 K, the model using a snow-ice albedo feedback, produced an increase
of temperature of 1.4 K for greenhouse gases, 0.294 K for a solar irradiance increase
and 0.119 K caused by a decrease in the ozone layer. Of the greenhouse
gas, solar irradiance and ozone depletion scenarios, ozone depletion demonstrates
the most realistic relative changes with a cooling of the stratosphere
and a warming of the troposphere and Earth's surface as has been observed.
It is concluded that ozone depletion is likely for a reasonable part of observed
global warming.

text: Ozone Depletion and Global Warming Abstract
This thesis examines global warming and the possible contribution that ozone
depletion provides to this warming. An examination is performed to determine
the extent of any warming/cooling events within the Earth-atmosphere
system. The change in energy corresponding to this warning of the Earth-
atmosphere system is estimated as being equivilent to an increase of mean
solar input of 0.22W/m2. This is compared to the predicted changes of solar
input for the two most common global warming scenarios: greenhouse gases
and solar irradiance variance; and for a less well explored scenario, snow-ice
albedo change. Examination of ozone depletion data shows that an absence of
ozone in the stratosphere produces an increase in UV-B radiation at the surface
of the Earth. This increase in UV-B light has not previously been thourougly
examined in any of the global warming scenarios. This is presented as a fourth
scenario for global warming.
An analytical three layer model of the Earth-atmosphere, based on an earlier
two layer model, is developed. Using this model it is determined that
greenhouse gases, solar irradiance, snow-ice albedo feedback and ozone depletion
can cause warming of the Earth's atmosphere. After comparison with
other models, a snow-ice albedo mechanism is incorporated into the three layer
model. This produces an amplification effect of any warming that occurs.
Compared to the observed increase of surface temperature between 1975-2000
of 0.55 K, the model using a snow-ice albedo feedback, produced an increase
of temperature of 1.4 K for greenhouse gases, 0.294 K for a solar irradiance increase
and 0.119 K caused by a decrease in the ozone layer. Of the greenhouse
gas, solar irradiance and ozone depletion scenarios, ozone depletion demonstrates
the most realistic relative changes with a cooling of the stratosphere
and a warming of the troposphere and Earth's surface as has been observed.
It is concluded that ozone depletion is likely for a reasonable part of observed
global warming.

year: 2006

title: Inclusion of indigenous peoples in CONFINTEA VI and follow-up processes

authors: Morrison, Sandra L.

subjects: indigenous people and CONFINTEA VI

abstract: This paper discusses key issues raised by indigenous peoples during CONFINTEA VI and proposes strategies to enable them to participate in ongoing processes. Indigenous peoples are not involved in the design, implementation and monitoring of adult education programmes, and this often results in a “one-size-fits-all” model. This article takes the position that indigenous people must have full and effective participation in all matters which concern them and that well-meaning policy statements are only as effective as the display of real effort to make them work. One example of an indigenous community taking initiative in order to free itself of overwhelming deficit positioning by mainstream educational and other systems is the Māori community of Aotearoa/New Zealand. The paper argues that through CONFINTEA VI, there is still space for the voice of indigenous peoples to be heard.

text: Inclusion of indigenous peoples in CONFINTEA VI and follow-up processes This paper discusses key issues raised by indigenous peoples during CONFINTEA VI and proposes strategies to enable them to participate in ongoing processes. Indigenous peoples are not involved in the design, implementation and monitoring of adult education programmes, and this often results in a “one-size-fits-all” model. This article takes the position that indigenous people must have full and effective participation in all matters which concern them and that well-meaning policy statements are only as effective as the display of real effort to make them work. One example of an indigenous community taking initiative in order to free itself of overwhelming deficit positioning by mainstream educational and other systems is the Māori community of Aotearoa/New Zealand. The paper argues that through CONFINTEA VI, there is still space for the voice of indigenous peoples to be heard.

year: 2011