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Refocused Optical Profilometer With Telecentric Scanning for Reliable Height Measurements

IP.com Disclosure Number: IPCOM000060998D
Original Publication Date: 1986-Jun-01
Included in the Prior Art Database: 2005-Mar-09
Document File: 2 page(s) / 45K

Publishing Venue

IBM

Related People

Ayers, GL: AUTHOR [+6]

Abstract

The Refocused Optical Profilometer (ROP) is a device developed for noncontact measurement of magnetic tape pancake profiles. (A pancake is defined here as a large reel of tape.) It utilizes a focused HeNe laser beam which is telecentrically scanned across the pancake surface by a galvo mirror. The specularly reflected and divergent laser beam is refocused by two separated cylindrical lenses which provide imaging in two perpendicular planes. In the vertical plane (i.e., plane of incidence), the first cylindrical lens images the pancake laser spot onto the position sensor. This ROP arrangement is shown to be able to detect height variations at the pancake surface, independent of local gradient, curvature, or reflectivity. The schematic layout of ROP is indicated in Fig. 1, and the operation principle outlined in Fig. 2.

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Refocused Optical Profilometer With Telecentric Scanning for Reliable Height Measurements

The Refocused Optical Profilometer (ROP) is a device developed for noncontact measurement of magnetic tape pancake profiles. (A pancake is defined here as a large reel of tape.) It utilizes a focused HeNe laser beam which is telecentrically scanned across the pancake surface by a galvo mirror. The specularly reflected and divergent laser beam is refocused by two separated cylindrical lenses which provide imaging in two perpendicular planes. In the vertical plane (i.e., plane of incidence), the first cylindrical lens images the pancake laser spot onto the position sensor. This ROP arrangement is shown to be able to detect height variations at the pancake surface, independent of local gradient, curvature, or reflectivity. The schematic layout of ROP is indicated in Fig. 1, and the operation principle outlined in Fig. 2. A laser beam that is scanned by a galvo mirror is focused onto the tape surface by a spherical lens L1
. The laser beam is incident on a location on the rotation axis of the mirror so that the laser spot on the mirror does not move appreciably as the mirror rotates. The reflected and diverging laser beam is collected by two cylindrical lenses: L2 images (in the "vertical plane") the laser spot at the tape onto the position sensor, while L3 images (in the "horizontal plane") the laser spot at the galvo mirror onto the position sensor. By locating the laser spot on the galvo mirror at a focus of L1 and the pancake apex at the other focus of L1, we achieve telecentric scanning, which means that as the laser beam is scanned across the pancake, the central ray of the focused beam is always perpendicular to the scan direction at the pancake. This is important, since the long distance from the pancake apex to the detector means that deviations from a telecentric scan result in excessive la...