Interferometric Eye Length Measurement Using Segmented Reference Arm
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Solution Overview
Problem
Conventional axial eye length measurement apparatuses using partially coherent light face challenges in achieving accurate measurements due to the need for highly linear movement of mirrors over substantial distances, which can be time-consuming and prone to errors from eye movement during measurement.
Innovation Solution
An axial eye length measurement apparatus that employs a light source producing partially coherent light, a beam splitter, and two mirrors in the reference arm, with a variable delay optical element and a processor to control the mirrors such that the corneal and retinal interference peaks have a predetermined non-zero separation, ensuring a substantially linear delay signal derivative of less than 30%, facilitating accurate and efficient eye length measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single movable mirror is used in the reference arm to achieve eye length measurement, then the measurement can be obtained, but the mirror must move in a highly linear manner over a substantial length (15-35 mm) which increases measurement time and is prone to errors from eye movement
Solution Approach 1:
The reference arm is segmented into two separate reference paths, each with its own mirror (first and second mirrors). This segmentation allows the measurement to be divided into two interference measurements (first and second interference signals) that can be acquired more rapidly, eliminating the need for a single mirror to traverse the entire 15-35 mm range in a highly linear manner over an extended period.
2Reliability
If a single movable mirror is used in the reference arm, then the apparatus structure is simpler, but the measurement is prone to errors from eye movement during the substantial measurement time
Solution Approach 1:
The reference arm is divided into two separate reference paths with distinct mirrors, allowing simultaneous or rapid sequential acquisition of interference signals. This segmentation reduces the measurement time window during which eye movement could introduce errors, thereby improving reliability. The additional mirror and path are manageable complexities that trade off against significant gains in measurement reliability.
Solution Approach 2:
The apparatus performs periodic interference measurements by alternating between the first and second reference paths. This periodic action allows for rapid acquisition of multiple measurements that can be averaged, reducing the impact of eye movement during any single measurement cycle and improving overall reliability.
3Measurement precision
If multiple measurements are taken and averaged to improve accuracy, then the precision improves, but the total measurement time increases and eye movement errors accumulate
Solution Approach 1:
By segmenting the measurement into two rapid interference signal acquisitions using separate reference paths, the apparatus can obtain multiple measurements more quickly. The first and second interference signals can be processed and averaged in a shorter total time, maintaining precision while improving productivity and reducing the window for eye movement errors to accumulate.
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 enhances measurement accuracy by maintaining a linear delay signal between interference peaks, reducing errors from eye movement and improving the precision of eye length measurements, even when multiple measurements are averaged.
Implementation Method 1
The amplitude of the signal will increase and decrease due to interference (i.e., interference spikes will arise) when the length of the reference arm is within a distance equal to the coherence length of the light
Implementation Method 2
at least one variable delay optical element extending across the first portion and the second portion, the delay element having a delay signal, wherein the delay signal corresponding to region between the corneal interference peak and second interference peak is substantially linear
Data Source
Figure 1~2
Figure 3~4D
Figure 5A~5B
AI summary
An interferometric axial eye length measurement apparatus having a light source adapted to produce a beam of partially coherent light, a first mirror and a second mirror disposed in the reference arm, and a processor adapted to control at least the second mirror such that a corneal interference peak and a second interference peak have a non- zero separation. There is at least one variable delay optical element having a delay signal, wherein the delay signal corresponding to a region between the corneal interference peak and second interference peak is substantially linear.