Simultaneous multispectral imaging of flame species using Frequency Recognition Algorithm for Multiple Exposures (FRAME)

Application of FRAME for Simultaneous LIF and LII Imaging in Sooting Flames Using a Single Camera

“…Keeping in mind the issues of stray light and the cost and complexity of the optical setups mentioned above, recently, the SLIPI-based approach known as FRAME (Frequency Recognition Algorithm for Multiple Exposures) was demonstrated for multispecies two-dimensional (2D) imaging in flames [ 33 ]. In FRAME, the sample is excited by multiple lasers and the corresponding optical signals are recorded on a single camera—using a single acquisition and utilizing the full sensor area.…”

“…However, in FRAME, it comes at the cost of (usually a very minute) loss in spatial resolution as mentioned above. Developed by Kristensson et al, the FRAME technique combines 1p-SLIPI (of high modulation frequency—from 10 to 30 line-pairs/mm Ronchi gratings) and multispectral imaging, where the LIF signals of two or more probed species are recorded simultaneously [ 33 , 34 ]. For instance, in non-combusting media, four modulated continuous lasers of wavelengths 405 nm, 447 nm, 450 nm, and 532 nm were combined to perform multispectral imaging of the dynamic samples of fluorophores on a single camera [ 34 ].…”

“…For instance, in non-combusting media, four modulated continuous lasers of wavelengths 405 nm, 447 nm, 450 nm, and 532 nm were combined to perform multispectral imaging of the dynamic samples of fluorophores on a single camera [ 34 ]. Li et al [ 33 ] employed the method on a turbulent dimethyl ether/air flame using two modulated pulsed laser sheets—one at 283 nm was exciting OH-LIF, while another at 355 nm was exciting CH 2 O-LIF, simultaneously. Therefore, unlike single laser sheets used in 1p-SLIPI, in FRAME, two or more laser sheets are utilized for probing the sample.…”

“…Therefore, unlike single laser sheets used in 1p-SLIPI, in FRAME, two or more laser sheets are utilized for probing the sample. Moreover, these multiple laser sheets are reasonably aligned (see Figure 1 ) or rearranged in the sample illumination plane so that they can be recognized later during the FRAME image post-processing using a frequency-sensitive algorithm [ 33 ].…”

“…In this investigation, we present a modified setup of FRAME to simplify the optical design in [ 33 ], which improves the setup’s practicality in terms of minimizing the loss of the laser fluence. The combination of a square Ronchi grating and a spatial filter used for generating a sinusoidal modulation in [ 33 ] is replaced here by a custom-made transmission grating of quartz (fabrication procedure is mentioned elsewhere [ 30 ]). Using the modified FRAME setup, we demonstrate, for the first time: (i) simultaneous PAH-LIF and soot-LII imaging and (ii) simultaneous OH-LIF and soot-LII imaging using a single camera, sequentially.…”