Low Coherence Interferometry Autofocus for Reflective Samples
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Solution Overview
Problem
Existing microscopy systems face challenges in achieving fast and precise autofocus, especially when imaging samples under reflective surfaces or covered by refractive materials, due to limitations in low coherence interferometry techniques, which struggle with high precision and reflection sensitivity, and fail to optimize signal when imaging objectives are changed.
Innovation Solution
The implementation of a low coherence interferometry system with a beam splitter that transmits light parallel to the optical axis, using a detection aperture offset from the illumination aperture to suppress surface reflections and compensate for refraction-induced offsets, allowing for automatic focus adjustment and objective interchangeability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If low coherence interferometry is used for autofocus, then focus adjustment speed is improved, but measurement precision deteriorates due to signal overlap from reflective surfaces
Solution Approach 1:
The patent applies asymmetry by offsetting the detection aperture from the optical axis while keeping the illumination aperture centered. This asymmetric detection aperture configuration creates a geometric situation where reflected light from reflective surfaces is excluded from the detection area, while still allowing sufficient light from the sample to be detected. This resolves the contradiction by maintaining fast interferometric autofocus while eliminating the precision-degrading signal overlap.
Solution Approach 2:
The patent extracts the harmful reflected light signals from the detection path by positioning the detection aperture to sample only the region where sample-scattered light exists, excluding the regions where surface reflections occur. This separation of signal sources enables precise measurement of sample position without contamination from reflective surface signals.
2Object-affected harmful factors
If detection aperture is offset from optical axis to suppress reflections, then signal-to-background ratio is improved, but device complexity increases
Solution Approach 1:
The solution introduces asymmetry only in the detection aperture positioning, keeping the illumination path simple and centered. This selective asymmetry approach suppresses reflection interference without requiring complex modifications to the entire optical system, thereby limiting the increase in device complexity to only the necessary aperture configuration.
3Speed
If low coherence interferometry is used for fast autofocus, then response time is improved, but manufacturing precision deteriorates due to refraction-induced offsets
Solution Approach 1:
The patent implements feedback by measuring the actual sample position using low coherence interferometry and using this information to adjust the focus position. The system continuously monitors and corrects for refraction-induced offsets by detecting the true sample location and compensating accordingly, thereby maintaining high focus accuracy despite the fast measurement process.
Solution Approach 2:
The patent changes the measurement parameter from direct geometric distance to optical path length, which accounts for refraction effects. By measuring the optical path length through the cover slip and adjusting accordingly, the system compensates for refraction-induced focus errors while maintaining fast measurement speeds.
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 enables precise autofocus with <1µm axial accuracy on thin samples near reflective surfaces and automatic calibration for different imaging objectives, improving signal-to-background ratio and maintaining focus despite refractive material thickness, particularly beneficial for TDI, confocal, and two-photon microscopy.
Implementation Method 1
using low coherence interferometry for determining the location of a sample, leading to a focus adjustment
Implementation Method 2
detect a portion of back-scattered light from the sample
Implementation Method 3
a beam splitter that transmits light to a microscope and reflects light from the low coherence interferometer into an optical path parallel to an optical axis
Implementation Method 4
the imaging objective that focuses light from the low coherence interferometer into a sample material
Implementation Method 5
the low coherence interferometer is configured to have a detection aperture that is not coincident with an aperture that is defined by an area with a point symmetry of an illumination aperture
Data Source
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AI summary
Disclosed are several technical approaches of using low coherence interferometry techniques to create an autofocus apparatus for optical microscopy. These approaches allow automatic focusing on thin structures that are positioned closely to reflective surfaces and behind refractive material like a cover slip, and automated adjustment of focus position into the sample region without disturbance from reflection off adjacent surfaces. The measurement offset induced by refraction of material that covers the sample is compensated for. Proposed are techniques of an instrument that allows the automatic interchange of imaging objectives in a low coherence interferometry autofocus system, which is of major interest in combination with TDI (time delay integration) imaging, confocal and two-photon fluorescence microscopy.