Sensitivity analysis of a multibranched light guide for real time hyperspectral imaging systems

Abstract

Hyperspectral imaging (HSI) is a spectroscopic technique which captures images at a high contrast over a wide range of wavelengths to show pixel specific composition. Traditional uses of HSI include: satellite imagery, food distribution quality control and digital archaeological reconstruction. Our lab has focused on developing applications of HSI fluorescence imaging systems to study molecule-specific detection for rapid cell signaling events or real-time endoscopic screening. Previously, we have developed a prototype spectral light source, using our modified imaging technique, excitation-scanning hyperspectral imaging (HIFEX), coupled to a commercial colonoscope for feasibility testing. The 16 wavelength LED array was combined, using a multi-branched solid light guide, to couple to the scope's optical input. The prototype acquired a spectral scan at near video-rate speeds (∼8 fps). The prototype could operate at very rapid wavelength switch speeds, limited to the on/off rates of the LEDs (∼10 μs), but imaging speed was limited due to optical transmission losses (∼98%) through the solid light guide. Here we present a continuation of our previous work in performing an in-depth analysis of the solid light guide to optimize the optical intensity throughput. The parameters evaluated include: LED intensity input, geometry (branch curvature and combination) and light propagation using outer claddings. Simulations were conducted using a Monte Carlo ray tracing software (TracePro). Results show that transmission within the branched light guide may be optimized through LED focusing lenses, bend radii and smooth tangential branch merges. Future work will test a new fabricated light guide from the optimized model framework.

Keywords: Colonoscopy; Colorectal Cancer; Endoscopy; Light Emitting Diode; Spectroscopy.

Figure 1:

The alpha prototype multi-furcated optical grade acrylic solid light guide

Figure 2:

“Pipe Curve Angle_45” Pipe curve model of 45° over the radius of revolution range. The arc length is used in the figures for comparison. The radius of the pipe remains 5 mm for all iterations the size changes due to the relative size of the frame to fit a longer arc length pipe. http://dx.doi.org/10.1117/12.2510506

Figure 3:

“Pipe Bend Angle_45” Pipe bend model of 45° over the range of lengths which associate to the radii of the pipe curve for equivalent arc lengths. The radius of the pipe remains 5 nun for all iterations the size changes due to the relative size of the frame to fit a longer arc length pipe. http://dx.doi.org/10.1117/12.2510506

Figure 4:

Comparison of the optical ligthpipe curve and bend models showing optical transmission as a function of % transmission vs total pipe length. (A) Results from the light pipe curve model over the entire range of arc radii. (B) an expanded view of the first ten iterations of panel A to visualize the overlapping data. (C) results from the light pipe bend model over the entire range of equivalent arc radii and (D) an expanded view of the first ten iterations to visualize the overlapping data.

Figure 5:

A comparison of the 525 nm LED (left) physical irradiance characteristics measured with a spectrometer and (right) TracePro simulation of the LED model. Experimental data were acquired by projecting the LED beam profile onto grid paper a fixed distance from the LED. Theoretical data were generated by projecting the simulated LED source onto a flat interrogation plane at the same fixed distance from the LED.

Figure 6:

Verification of the number of rays to determine the accuracy of the measurements for (top) the irradiance map of the curved light pipe model — angle of revolution 30°, radius of revolution 100 mm and (bottom) polar iso candela plot of the curved light pipe model — angle of revolution 30°, radius of revolution 100 mm