The Lens Alignment Station (LAS™) employs a focused laser beam in reflection to measure the surface decenter and tilt. Following reflection from the lens surface, the laser beam is focused to a bright spot on the LAS™ camera at two distinct vertical locations - Normal (or Cat’s Eye) and Confocal. The Normal signal occurs when the focused laser reflects directly from the lens surface. The Confocal signal occurs when the laser focus is coincident with the Radius of Curvature (ROC) of the surface. This will be above the surface for a concave surface and below the surface for a convex surface.
In operation, the laser focus is scanned vertically by moving the LAS™ Optical Module up and down the vertical stage to bring both Normal and Confocal signals into focus at known vertical spacings. These spacings are calculated by the CalcuLens™ software using ray trace algorithms and the optical prescription on the lens (radius of curvature, diameter, refractive index and air gap).
How does it work?
The LAS™ measures the tilt & centration of a lens relative to the spin axis of the rotary air bearing. This is done in three steps:
Locate the Normal or Confocal focus from the top surface of the lens. Measure the diameter and angle of the focused laser spot on the LAS™ camera as it prescribes an orbit while spinning the rotary air bearing. The CalcuLens™ software uses this vector information to determine the precise 3D location (x, y, z) of the Center of Curvature (CC) of the top surface.
Repeat step #1 for the bottom surface of the lens. The result is a precise determination of the CC of the bottom surface.
Knowing the 3D locations of the CCs of the top and bottom surface allows the CalcuLens™ software to calculate the Optical Axis (OA) of the lens relative to the spin axis for the rotary air bearing. This is done by simply connecting a straight vector between the two CCs as shown below.
The sensitivity to decenter/tilt, image magnification, and depth of focus depends on the Working Distance (WD) of the LAS™ objective lens. The shorter the WD, the lower the sensitivity, the higher the magnification, and the shorter the depth of focus. A WD of 90mm is a good middle-of-the-road for most size lenses and is the default objective delivered with all LAS™. A WD of ≤ 40mm is useful when measuring lenses smaller than 10mm in diameter or for accurate air gap, lens thickness and radius of curvature measurements. A WD ≥ 150mm is useful for higher alignment sensitivity on larger optics or when inspecting through deep lens assemblies. The maximum usable WD is 300mm.