Beyond MHz image recordings using LEDs and the FRAME concept

Resolution enhancement via guided filtering for spatial-frequency multiplexing single-shot high-speed imaging

The frequency recognition algorithm for multiple exposures (FRAME) is a progressive single-shot high-speed videography technique that employs the spatial-frequency multiplexing concept to provide high temporal and spatial resolution. However, the inherent crosstalk from the zero-frequency component to the carrier-frequency component leads to resolution degradation and artifacts. To improve recovered frames' quality, we propose a FRAME reconstruction method using guided filters for a removal of the zero-frequency component, which can minimize the artifacts while enhance spatial resolution. A total variation (TV) denoising operation is involved to remove artifacts further to achieve optimized performances. Simulations and experiments were conducted to demonstrate the robust and efficient post-processing capability of the proposed method. With a two-frame experimental system, the results of a USAF 1951 resolution target reveal a 1.8-fold improvement in spatial resolution from 16 lp/mm to 28.5 lp/mm. For complex dynamic scenarios, the wide field of high-speed fuel spray was shot and the proposed method can resolve two droplets with a 30 μm distance which outperforms the traditional method.

Single-shot videography with multiplex structured illumination using an interferometer

Frequency recognition algorithm for multiple exposures (FRAME) is a high-speed videography technique that exposes a dynamic object to time-varying structured illumination (SI) and captures two-dimensional transients in a single shot. Conventional FRAME requires light splitting to increase the number of frames per shot, thereby resulting in optical loss and a limited number of frames per shot. Here, we propose and demonstrate a novel FRAME method which overcomes these problems by utilizing an interferometer to generate a time-varying SI without light splitting. Combining this method with a pulsed laser enables low-cost, high-speed videography on a variety of timescales from microseconds.

Investigation of single-shot high-speed photography based on spatial frequency multiplexing

The frequency recognition algorithm for multiple exposures (FRAME) is a spatial frequency multiplexing method that enables high-speed videography with high spatial resolution across a wide field of view and high temporal resolution up to femtoseconds. The criterion to design encoded illumination pulses is an essential factor that affects the sequence depth and reconstruction accuracy of FRAME but was not previously discussed. When the spatial frequency is exceeded, the fringes on digital imaging sensors can become distorted. To exploit the Fourier domain for FRAME with deep sequences and avoid fringe distortion, the maximum Fourier map for sequence arrangement was determined to be a diamond shape. The maximum axial frequency should be a quarter of the sampling frequency of digital imaging sensors. Based on this criterion, the performances of reconstructed frames were theoretically investigated by considering arrangement and filtering methods. To ensure optimal and uniform interframe quality, the frames near the zero frequency should be removed and optimized super-Gaussian filters should be employed. Experiments were conducted flexibly with a digital mirror device to generate illumination fringes. Following these suggestions, the movement of a water drip dropping on a water surface was captured with 20 and 38 frames with uniform interframe quality. The results prove the effectiveness of the proposed methods to improve the reconstruction accuracy and promote the development of FRAME with deep sequences.

Fluorescence lifetime imaging through scattering media

Fluorescence lifetime determination has proven to be useful, e.g. identification of molecules, quantitative estimation of species concentration and determination of temperatures. Lifetime determination of exponentially decaying signals is challenging if signals of different decay rates are being mixed, resulting in erroneous results. Such issues occur when the contrast of the measurement object is low, which can be limiting in applied measurements due to spurious light scattering. A solution is presented here where structured illumination is used to enhance image contrast in fluorescence lifetime wide-field imaging. Lifetime imaging determination was carried out using Dual Imaging Modeling Evaluation (DIME), and spatial lock-in analysis was used for removing spurious scattered signal to enable fluorescence lifetime imaging through scattering media.

Snapshot multicolor fluorescence imaging using double multiplexing of excitation and emission on a single detector

Fluorescence-based multispectral imaging of rapidly moving or dynamic samples requires both fast two-dimensional data acquisition as well as sufficient spectral sensitivity for species separation. As the number of fluorophores in the experiment increases, meeting both these requirements becomes technically challenging. Although several solutions for fast imaging of multiple fluorophores exist, they all have one main restriction; they rely solely on spectrally resolving either the excitation- or the emission characteristics of the fluorophores. This inability directly limits how many fluorophores existing methods can simultaneously distinguish. Here we present a snapshot multispectral imaging approach that not only senses the excitation and emission characteristics of the probed fluorophores but also all cross term combinations of excitation and emission. To the best of the authors’ knowledge, this is the only snapshot multispectral imaging method that has this ability, allowing us to even sense and differentiate between light of equal wavelengths emitted from the same fluorescing species but where the signal components stem from different excitation sources. The current implementation of the technique allows us to simultaneously gather 24 different spectral images on a single detector, from which we demonstrate the ability to visualize and distinguish up to nine fluorophores within the visible wavelength range.