The High Throughput Virtual Slit enables compact, inexpensive Raman spectral imagers

摘要

Raman spectral imaging is increasingly becoming the tool of choice for field-based applications such as threat, narcotics and hazmat detection; air, soil and water quality monitoring; and material ID. Conventional fiber-coupled point source Raman spectrometers effectively interrogate a small sample area and identify bulk samples via spectral library matching. However, these devices are very slow at mapping over macroscopic areas. In addition, the spatial averaging performed by instruments that collect binned spectra, particularly when used in combination with orbital raster scanning, tends to dilute the spectra of trace particles in a mixture. Our design, employing free space line illumination combined with area imaging, reveals both the spectral and spatial content of heterogeneous mixtures. This approach is well suited to applications such as detecting explosives and narcotics trace particle detection in fingerprints. The patented High Throughput Virtual Slit1 is an innovative optical design that enables compact, inexpensive handheld Raman spectral imagers. HTVS-based instruments achieve significantly higher spectral resolution than can be obtained with conventional designs of the same size. Alternatively, they can be used to build instruments with comparable resolution to large spectrometers, but substantially smaller size, weight and unit cost, all while maintaining high sensitivity. When used in combination with laser line imaging, this design eliminates sample photobleaching and unwanted photochemistry while greatly enhancing mapping speed, all with high selectivity and sensitivity. We will present spectral image data and discuss applications that are made possible by low cost HTVS-enabled instruments. Raman spectral imaging is increasingly becoming the tool of choice for field-based applications like threat, narcotics and hazmat detection; air, soil and water quality monitoring; material ID and the like. Conventional fiber-coupled point source Raman spectrometers effectively interrogate a small sample area and identify bulk samples via spectral library matching routines. However, these devices are very slow at mapping over macroscopic areas. In addition, the spatial averaging performed by instruments that collect binned spectra, particularly when used in combination with orbital raster scanning, tends to dilute the spectra of trace particles in a mixture. Free space line illumination combined with area scanning imaging, by contrast, reveals both the spectral and spatial content of heterogeneous mixtures. This approach is well suited to applications such as explosives or narcotics trace particle detection in fingerprints, for example. Spectral imaging is also preferred over conventional spectroscopy for forensics applications, where it preserves a unique mapping relationship between a familiar RGB image and an information-rich but less intuitive spectral image. This renders the technology far more approachable for nontechnical users. Spectrometer design requires a tradeoff between light-gathering power, spectral resolution, and instrument size. A wide entrance slit and large instrument aperture maximize the number of photons that can be collected from the sample, but decrease the spectral resolution. A long focal length increases the resolution, but requires a larger instrument, with larger optics to accommodate the larger pupil. The High Throughput Virtual Slit is an innovative optical design that mitigates these tradeoffs. By manipulating the pupil in collimated space, we obtain spectral resolution approximately 3X greater than in conventional designs at an equivalent slit width and with no sacrifice of entrance aperture. Alternatively, we can reduce the instrument dimensions by an equivalent amount to produce extremely compact instruments with the resolution of much larger Raman spectrometers. In principle, a 3X reduction in focal length translates to a 9X reduction in optical surface area and a 27X reduction in overall instrument volume, so we can use much smaller components inside a much smaller housing to build these instruments, achieving very low unit costs while retaining the analytical power of larger devices. We also make more efficient use of sensor real estate and available laser power with a line illumination/collection design that increases total photon flux while reducing the average intensity. Our approach takes full advantage of the high powers available from modern diode laser arrays. When used in combination with laser line imaging, this design eliminates sample photobleaching and unwanted photochemistry while greatly enhancing mapping speed, all with high selectivity and sensitivity. We will present spectral image data and discuss applications that are made possible by low cost HTVS-enabled instruments.