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Ken Tran, Thomas Mendel, Kristina Holbrook, Paul Yates; Cross polarization kit to enhance slit lamp fundoscopy. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1478.
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We incorporated a low cost cross-polarization kit for ophthalmic slit lamps to enhance fundus examination. Light reflections off of handheld lenses used for viewing the retina at the slit lamp reduce image contrast and obscure retinal details. These reflections are partially overcome by narrowing the slit beam and changing the incident angle of the illumination, reducing field of view. Linearly polarized light reflected from smooth surfaces, like a handheld lens or cornea, maintains the incident polarization to a greater degree than light reflected from molecularly complex surfaces, like the retina. By illumination with polarized light, we aim to filter reflected polarized light from the lens, allowing for a wider field of view, more rapid examination, and more detailed funduscopic view.
We designed and manufactured a low cost prototype polarization kit that can be easily retrofitted into commonly used slit lamps. One linear polarizer was installed near the slit lamp light source, resulting in polarized incident light to the patient eye. A resolving polarizer was installed on the slit lamp eye piece housing rotated 90 degrees from the first to achieve a cross polarizing filter. Healthy and pathologic fundi were examined at variable polarization settings under an approved IRB protocol.
Our prototype kit was easily installed on existing Topcon and Haag-Streit slit lamps. Photos acquired using a Volk Transequator or 78D lens and Nikon D7000 camera demonstrated a wide view of a healthy fundus was enhanced with the utilization of cross-polarization, substantially filtering bright lens reflections of the slit lamp tungsten element. Additionally, retinal pathology was more easily observed, including diabetic retinopathy with macular edema, macular degeneration, and peripheral retina during pan-retinal photocoagulation. With the resolving polarizer set to maximally filter polarized light, about two camera f-stops of overall illumination were lost.
With proper use of cross-polarization, we demonstrate that a wider field of view is accomplished while diminishing image-obscuring reflections, potentially increasing fundus examination efficiency, accuracy, and safety of slit lamp based laser treatment.
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