A prototype was fabricated out of BK7 glass (Planar Optics, Inc., Webster, New York, USA), as shown in , and tested. The diffusing film (Model 3635-70, 3M, St. Paul, Minnesota, USA), having a transmission that was verified experimentally to be 60%, was applied at the input face of the lightpipe. The light source used was a fiber-coupled laser (Model BWF-670-300-E/55370, B & W TEK, Inc., Newark, Delaware, USA) emitting a power of 100 mW, with a central wavelength of 668 nm to match the 665 nm absorption peak of HPPH, a photosensitizer currently being investigated in clinical trials for PDT of the oral cavity [8
]. The internal transmittance of Schott N-BK7 glass for a thickness of 25 mm is 0.994 for a wavelength of 660 nm and 0.996 for a wavelength of 700 nm; this results in a glass absorption of less than 2% as light traverses the lightpipe.
Fig. 3 (Color online) BK7 lightpipe prototype, tested in two configurations: (a) Mylar reflective coating applied only to the 45° mirror while the rest of the lightpipe was left uncoated; (b) lightpipe device entirely coated with the exception of the (more ...)
A first experimental validation was done leaving the lightpipe device uncoated and only coating the 45° mirror with the reflective coating (metalized Mylar tape, CS Hyde, Inc., Lake Villa, Illinois, USA) as shown in . The reflectivity of the coating was verified experimentally to be greater than 90% at the wavelength of interest. The experimental results, obtained by sampling the irradiance with a scanning pinhole and power meter (Model 818-ST, Newport Corp., Irvine, California, USA) in steps of 0.5 mm, and the corresponding numerical simulation are shown in . It can be noted how the falloff in the irradiance was greater on the right side than the left both in the simulation and in the experimental measurement. The experimental average irradiance on the 25 mm2 target area was 87 mW/cm2 with an average deviation of 5% for an input power of 100 mW, in excellent agreement with the average irradiance of 92 mW/cm2 with 6% average deviation predicted from the numerical simulation.
(Color online) (a) Numerical simulation and (b) experimental data of the irradiance obtained at a distance of 1 mm from the output window for the uncoated lightpipe of . The square outline represents the 25 mm2 target area.
Light was confined in the uncoated lightpipe by total internal reflection (TIR): the rays that hit the glass–air interface with an angle greater than the critical angle were totally internally reflected inside the lightpipe with 100% efficiency. Since the device is to be used in the oral cavity and has as its primary purpose to limit irradiation of healthy tissue, it is necessary to avoid leakage of light from the lightpipe by adding an appropriate coating; the efficiency of the resulting device is expected to be lower than the TIR device because of reflection losses. The final device was entirely coated with the reflective coating, with the exception of the output window of 6.8 mm × 6.8 mm, as shown in .
The comparison between the numerical simulation and the experimental measurement for an input power of 100 mW is shown in . The experimental average irradiance of 76 mW/cm2 with an average deviation of 5% on the 25 mm2 target matched the average irradiance of 73 mW/cm2 with an average deviation of 6% calculated with the numerical model and amply satisfied the PDT illumination requirements of an average irradiance of at least 50 mW/cm2 with an average deviation well below 10%.
(Color online) (a) Numerical simulation and (b) experimental data of the irradiance obtained at a distance of 1 mm from the output window for the final lightpipe device of . The square outline represents the 25 mm2 target area.