Photoacoustic Depth Resolution via Angular Incidence
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Solution Overview
Problem
Conventional photoacoustic methods are limited in achieving micrometre-scale depth resolution due to the short duration of pressure signals from deep sample depths, making it difficult to analyze inhomogeneous samples with absorption occurring several tens of micrometres beneath the surface.
Innovation Solution
A photoacoustic method using measuring light with a predetermined wavelength range and varying angles of incidence to calculate energy density absorbed by the sample, modeling the sample as a stack of layers with fit parameters for absorption coefficients, and performing a fitting procedure to determine depth-resolved absorption coefficients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional photoacoustic methods use short measuring light pulses to achieve high temporal resolution, then pressure signals from deep sample depths become too short to resolve, but using long measuring light pulses extends pressure signal duration and enables micrometre-scale depth resolution
Solution Approach 1:
The patent applies periodic pulsed illumination with controlled duty cycles, using repeating measurement cycles at different angles of incidence. Each cycle includes illumination phases and measurement phases, allowing accumulation of depth-resolved information over multiple periodic cycles while maintaining temporal resolution through controlled pulse durations.
Solution Approach 2:
The patent introduces angular dimension by varying the angle of incidence of measuring light across multiple measurements. This transforms the problem from purely temporal resolution to a combined angular-spatial domain, where depth information is extracted through angular variations rather than relying solely on time-resolved pressure signal duration.
2Measurement precision
If measuring light is absorbed predominantly by localized substances at shallow depths, then high temporal resolution can be achieved, but analysis of inhomogeneous samples with absorption at deeper depths (tens of micrometres) becomes impossible
Solution Approach 1:
The patent transitions from time-domain analysis to angular-domain analysis by varying the angle of incidence. This allows extraction of depth information from spatial angular variations rather than temporal pressure signal variations, preserving information from deep absorbing structures that would otherwise be lost in conventional time-resolved methods.
Solution Approach 2:
The patent systematically varies the angle of incidence parameter across multiple measurements. This parameter change enables differentiation of absorption contributions from various depths by exploiting the angular dependence of light penetration and absorption paths, thereby recovering depth-resolved information without relying on short pressure signal durations.
3Reliability
If the measuring light wavelength is chosen for characteristic absorption by specific substances, then substance-specific detection is achieved, but the sample absorbs light completely within tens of micrometres, limiting depth resolution
Solution Approach 1:
The patent resolves the contradiction by moving from temporal to angular domain analysis. Substance-specific detection is maintained through wavelength selection, while depth resolution is achieved through angular variations of incidence, allowing both substance identification and depth localization without the fundamental limitation of complete light absorption within tens of micrometres.
Solution Approach 2:
The patent segments the depth information extraction by angle of incidence, where each angular measurement provides information about a specific depth range. This segmentation allows reconstruction of depth-resolved absorption profiles while maintaining substance-specific detection through wavelength-selective absorption measurements.
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
Enables micrometre-scale depth resolution for analyzing inhomogeneous samples by effectively apportioning absorbed energy density to individual layers, improving the accuracy of properties determination, particularly suitable for non-invasive analysis of living tissue like skin.
Implementation Method 1
electromagnetic radiation is emitted into a surface of a sample and is absorbed by the sample while generating a pressure wave
Implementation Method 2
The energy transmitted with the light absorption causes a heating and a thermomechanical expansion of the sample
Implementation Method 3
the pressure wave is propagated to the measurement area and is detected as a pressure transient
Implementation Method 4
the pressure waves are detected by piezoelectric sonic transducers as pressure transients at the surface of the sample
Data Source
AI summary
A method comprises a) radiating at least one measurement light pulse having a pulse duration and intensity onto a measurement area of the inhomogeneous sample; b) detecting at least one pressure transient at the measurement area; and c) calculating a value for the energy density absorbed by the sample during the pulse duration from a curve of the at least one pressure transient at the start and at the end of the at least one measurement light pulse. The method further includes: repeating steps a) to c) for different angles of incidence of the measurement light; modelling the inhomogeneous sample as a stack of layers, each layer being assigned at least a layer thickness and an absorption coefficient, at least one absorption coefficient of a layer being a fitting parameter; and performing a fitting procedure for the fitting parameters; outputting the fitted fitting parameters.