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Abstract We present a wave‐optics‐based BSDF for simulating the corona effect observed when viewing strong light sources through materials such as certain fabrics or glass surfaces with condensation. These visual phenomena arise from the interference of diffraction patterns caused by correlated, disordered arrangements of droplets or pores. Our method leverages the pair correlation function (PCF) to decouple the spatial relationships between scatterers from the diffraction behavior of individual scatterers. This two‐level decomposition allows us to derive a physically based BSDF that provides explicit control over both scatterer shape and spatial correlation. We also introduce a practical importance sampling strategy for integrating our BSDF within a Monte Carlo renderer. Our simulation results and real‐world comparisons demonstrate that the method can reliably reproduce the characteristics of the corona effects in various real‐world diffractive materials. 

Optical heterodyne detection (OHD) employs coherent light and optical interference techniques (Fig. 1-(A)) to extract physical parameters, such as velocity or distance, which are encoded in the frequency modulation of the light. With its superior signal-to-noise ratio compared to incoherent detection methods, such as time-of-flight lidar, OHD has become integral to applications requiring high sensitivity, including autonomous navigation, atmospheric sensing, and biomedical velocimetry. However, current simulation tools for OHD focus narrowly on specific applications, relying on domain-specific settings like restricted reflection functions, scene configurations, or single-bounce assumptions, which limit their applicability. In this work, we introduce a flexible and general framework for spectral-domain simulation of OHD. We demonstrate that classical radiometry-based path integral formulation can be adapted and extended to simulate the OHD measurements in the spectral domain. This enables us to leverage the rich modeling and sampling capabilities of existing Monte Carlo path tracing techniques. Our formulation shares structural similarities with transient rendering but operates in the spectral domain and accounts for the Doppler effect (Fig. 1-(B)). While simulators for the Doppler effect in incoherent (intensity) detection methods exist, they are largely not suitable to simulate OHD. We use a microsurface interpretation to show that these two Doppler imaging techniques capture different physical quantities and thus need different simulation frameworks. We validate the correctness and predictive power of our simulation framework by qualitatively comparing the simulations with real-world captured data for three different OHD applications—FMCW lidar, blood flow velocimetry, and wind Doppler lidar (Fig. 1-(C)). 

Transient absorption spectroscopy (TAS) is a field of study that investigates the dynamic process of chemical compounds. Thanks to the recent emergence of ultrafast pulsed lasers, TAS now extends its reach to studying photochemical reactions occurring within few femtosecond to nanosecond timescales. With ultrafast TAS, changes in sample absorbance or transmittance over time following excitation by pulsed light can be measured at a high temporal resolution -tens of femtoseconds. An application of ultrafast TAS is lifetime measurement for fluorescence decay. However, due to various noise sources (sensor noise, shot noise, unintended photochemical reactions, etc.) during measurement, obtaining a reliable lifetime value often necessitates extensive repetition resulting in experiments lasting several hours. In this paper, we introduce an effective time sampling strategy tailored for lifetime measurement from noisy transient signals. We start with a well-established non-linear curve fitting algorithm and demonstrate that sampling time shifts that maximize the signal derivative (t=τ) will minimize the variance in lifetime estimation. Additionally, we reduce the number of parameters by normalization to ensures the correctness of our algorithm. We demonstrate using simulation that our proposed method outperforms conventional time sampling or normalization methods across various conditions. Especially, we found that proposed method gives same error with 5.5 x less samples compared to the common TAS measurement strategy that uses exponential time sampling with full parameter curve-fitting. Moreover, through real-world TAS measurements, we show that our technique results in 2 - 8 x less standard deviation compared to baseline methods. We expect that our algorithm will be valuable not only for researchers who use TAS but also for researchers across various fields who use time-gated transient cameras for lifetime analysis. 

A chip-scale spectrometer is proposed to resolve linear polarization information in the visible-wavelength range. The design was supported with preliminary experimental data based on elliptical cross-section nanopillar-shaped GaN photodetectors. 

We introduce Doppler time-of-flight (D-ToF) rendering, an extension of ToF rendering for dynamic scenes, with applications in simulating D-ToF cameras. D-ToF cameras use high-frequency modulation of illumination and exposure, and measure the Doppler frequency shift to compute the radial velocity of dynamic objects. The time-varying scene geometry and high-frequency modulation functions used in such cameras make it challenging to accurately and efficiently simulate their measurements with existing ToF rendering algorithms. We overcome these challenges in a twofold manner: To achieve accuracy, we derive path integral expressions for D-ToF measurements under global illumination and form unbiased Monte Carlo estimates of these integrals. To achieve efficiency, we develop a tailored time-path sampling technique that combines antithetic time sampling with correlated path sampling. We show experimentally that our sampling technique achieves up to two orders of magnitude lower variance compared to naive time-path sampling. We provide an open-source simulator that serves as a digital twin for D-ToF imaging systems, allowing imaging researchers, for the first time, to investigate the impact of modulation functions, material properties, and global illumination on D-ToF imaging performance.