Graduated neutral-density filter - Wikipedia
ND Filter: A Beginner's Guide to Neutral-Density Lens Filters
Optics - Northwestern Mechatronics Wiki
Herschel wedge - Wikipedia
Motion blur - Wikipedia
Extinction (astronomy) - Wikipedia
Landscape photography - Wikipedia
Hologon - Wikipedia
Neutraldichtefilter – Wikipedia
B+W 702 77mm 25 Percent MRC Graduated Neutral Density Filter : Camera Lens Neutral Density Filters : Electronics
ND Filter. Don't go Variable! – jonasrask
Pulfrich effect - Wikipedia