Filters of the highest quality. Durable glassQuartzLineB270PlasticRefractive index1.4561.5231.58Surface reflectance0.0340.0420.056Stress-relief temperature1120C510C463CRobust quartz glassLow reflective surface resistant to extreme temperatures and a refractive index of only 1.456.Different types, many possibilitiesHandmade 4-part frame, allows smoother filter adjustment than a frame made of aluminum