White Light Endoscopy for Minimal Invasive Surgery
Endoscopy has become a very advantageous medical procedure particularly for investigative and operational surgery for cancer. The preferred method over open surgery, it offers the advantages of less invasion with shorter operation time, less trauma to the patient and quicker recovery time. The method proves beneficial to the patient as well as providing a cost saving to the NHS. This study made possible through a collaboration between The University of South Wales and the company partner, Cymtec Ltd. investigates the possibility of producing a solid-state LED or laser pumped phosphor white light system. The overall aim was to replace current xenon lamps, which have many unwanted characteristics with lower cost, higher efficiency and more environmentally friendly solid state devices to improve the benefits of endoscopic minimal invasive surgery even further. Instruments used in the procedures are small for use in restrictive spaces, the investigation therefore is focused on not just producing high power white light, but to provide high power white light that can be coupled into a small diameter liquid light guide (LLG), which can then be connected to endoscopic instruments. The etendue of the light therefore becomes a critical factor of the investigation. A patented light guide multiplexer technology designed by the company partner, Cymtec Ltd. which multiplexes light from red, green and blue high-power LEDs with relatively narrowband wavelengths is firstly reviewed. Following which development of the technology is achieved through careful consideration of optical bonding to increase efficiency whilst providing structural strength. From this investigation possible improvements to the system are identified and reviewed with a view to increasing the overall luminous output. As an alternative to multiplexing of blue, green and red LEDs to, phosphor technology is also investigated. YAG:Ce powdered phosphor coated on float glass in varying thicknesses is evaluated for suitability for pumping with high power LEDs and potentially lasers. The review highlights the importance of material properties and thermal dissipation in staying within the limits of the material. As a continuation, phosphor with ceramic or single crystal structures are identified and evaluated. High power blue lasers are used as the pumping source to overcome efficiency droop experienced by LEDs to increase output. Experimental analysis found the single crystal YAG:Ce option was deemed favourable due to the overall performance and light generated, proving suitable to be multiplexed with additional blue and red LEDs to produce white light. The combined light was then coupled into a Ø5mm liquid light guide, typically used for endoscopic equipment, providing over 1700 lm of D65 white light, a suitable alternative to xenon lamps for endoscopic procedures. To overcome the etendue limitation to using multiple LEDs as with lighting applications, a solution is designed to combine the output of multiple high power blue LEDs to pump phosphor whilst keeping the overall emitting area and etendue equivalent to that of the LED. The high reflective hollow light guide with internally coated phosphor design is firstly modelled in Zemax optical studio to prove the concept, using a suitable phosphor and binder combination. Bringing the concept to reality is achieved with custom components and specifically designed jigs. Testing the prototype proved the concept, with multi-LED phosphor converted light being generated and exiting the light guide with the same aperture size as the original LEDs. The output, found to be less than that modelled in the Zemax optical model is analysed with improvements to the design highlighted and discussed. Each method investigated proved to have advantages and disadvantages in comparison to each other. The study showed however the methods have merit in producing a soild-state based light engine to replace lamp based systems to provide optical performance whilst adding the advantage of colour tuning and overcoming the disadvantages and drawbacks of cost, lifetime and associated issues found with lamp based systems.
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