Patent ID: 8339615

Claim:
An edge detection method comprising: preparing a transparent substrate which includes a first main face that has a first main region and a first peripheral region outside the first main region and a second main face that has a second main region in parallel with the first main region and a second peripheral region outside the second main region, the first peripheral region having an inclination angle of θa 1 and the second peripheral region having an inclination angle of θa 2 ; causing measuring light to enter the first peripheral region of the first main face from a direction perpendicular to the first main region of the first main face; detecting a non-emitting region where the measuring light is not emitted from the second peripheral region of the second main face; and detecting an edge of the transparent substrate on the basis of the non-emitting region, wherein preparing the transparent substrate includes adjusting the inclination angles θa 1 and θa 2 so as to satisfy the following expression, where n is a refractive index of the transparent substrate: n ×sin(θ a 1+θ a 2−arcsin(sin θ a 1 /n ))≧1.