Spinning Half-Wave Plate for Corneal Birefringence Compensation
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Solution Overview
Problem
Retinal birefringence scanning (RBS) is affected by corneal birefringence, which varies widely and can mask the desired signal from retinal birefringence, making it difficult to achieve consistent and reliable detection of eye fixation and focus.
Innovation Solution
The use of a thin-film, dichroic beam splitter/retarder and a spinning half wave plate, along with a non-rotating retarder, to modulate and analyze the polarization of light as it passes through retinal birefringent structures, generating frequency signatures that are independent of fixation and corneal birefringence, allowing for accurate assessment of eye fixation and focus.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If retinal birefringence scanning is used to detect eye fixation, then foveal fixation detection is achieved, but corneal birefringence masks the desired signal and creates variability
Solution Approach 1:
The patent converts the harmful effect of corneal birefringence into a beneficial signal by using a spinning half-wave plate to generate a characteristic 'spinning artifact' frequency signature. This artifact, originally considered noise, becomes a useful reference signal for normalization and artifact rejection, improving measurement precision while accounting for corneal birefringence variations
Solution Approach 2:
The patent changes the temporal parameter of the optical system by introducing a spinning half-wave plate that rotates at a specific fraction of the scan frequency. This generates frequency multiplication effects that transform the static corneal birefringence into a dynamic signal with characteristic frequencies, allowing differentiation between retinal and corneal contributions
2Measurement precision
If wave plates are used to enhance foveal fixation detection, then detection sensitivity is improved, but spinning artifacts are generated that vary with corneal birefringence
Solution Approach 1:
The patent implements feedback by using the detected spinning artifact signal to normalize the RBS signal strength. The system continuously monitors the artifact frequency and uses it to compensate for variations in corneal birefringence, thereby maintaining consistent detection sensitivity across different eyes and conditions
Solution Approach 2:
The spinning half-wave plate acts as an intermediary element that mediates between the incident light and the retinal birefringence. By introducing this controlled polarization modulation, the system creates a known reference signal that allows separation of corneal and retinal effects, reducing artifact variability
3Quantity of substance
If a fixed wave plate is optimized for one frequency component, then signal strength at that frequency is maximized, but other frequency components and spinning artifacts are not uniformly controlled
Solution Approach 1:
The patent makes the optical system multi-functional by using the spinning half-wave plate to simultaneously generate multiple frequency components (2.5f, 3.5f, 4.5f, etc.) that serve different purposes. The same optical configuration that maximizes the desired RBS signal also generates characteristic artifact frequencies that can be used for normalization and quality control
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach provides a reliable and consistent method for detecting eye fixation and focus, reducing noise and variability caused by corneal birefringence, and enabling normalization of signal strengths for improved detection accuracy.
Implementation Method 1
The fovea is surrounded by a uniquely arranged radial pattern of birefringent Henle fibers, fibers that change the state of polarization of transmitted light
Implementation Method 2
a spot of polarized near-infrared light is scanned on the retina
Implementation Method 3
a double-pass half wave plate (HWP) spinning at a specific fractional frequency of the scanning frequency (f), more precisely at an odd multiple of 1/16th as fast as the scanning frequency [(9/16)f] is used, generating so-called 'multiple-of-half' frequency components from birefringent patterns on the retina
Implementation Method 4
the changes in the polarization of light returning from the eye are analyzed to detect the projection into space of the Henle fibers surrounding the fovea
Data Source
AI summary
The present invention provides apparatus and methods for detecting fixation of an eye of a subject on a target. The methods provide for optimization of parameters of the spinning half wave plate and the fixed wave plate in the retinal birefringence scanning (RBS) design to enable uses of the “spinning artifact” frequency component. Frequency of the “spinning artifact” component is determined by half wave plate rotation speed and direction. Amplitude is determined by interaction of the spinning half wave plate with any retardance encountered in the double-pass optics such as the fixed wave plate, corneal birefringence, and small amount of retinal birefringence. With optimum selection of fractional spinning frequency of the half wave plate, and the orientation/retardance of the fixed wave plate, the “spinning artifact” frequency component is essentially independent of fixation direction and is also essentially independent of the normal range of corneal birefringence.