Like the CalcuLens™ Vertex LAS option, the DMI (Distance Measuring Interferometer) option enables the LAS to make relevant distance measurements in optical assemblies, but with higher accuracy and through deeper lens assemblies. LAS precision centering capability is combined with the Fogale nanotech LENSCAN low-coherence time-domain-scanning interferometry for measuring CT (center thickness) and air gaps of all optical elements (lenses, cubes, flats, etc.) of an assembly along the optical axis.
Whereas the Vertex option enables the LAS to measure air gaps and lens thicknesses accurate to within ±2.5 µm, LAS-DMI with LENSCAN system improves the LAS accuracy of these details to within ± 1 µm (for standard version) and ±0.2 µm (for high-res version). And while Vertex can measure air gaps and lens thicknesses to assembly depths up to 50 mm, or 100 mm if the assembly can be flipped bottom to top, LAS-DMI has a measurement range up to 600 mm. Measurements are performed non-contact, preventing damage to lens elements or assemblies. Both coated and uncoated lenses are measured equally well.
The figure below comes from a DMI inspection of a multi-lens assembly comprised of, from left to right with noted surface (#):
a) Air gap
b) Doublet surfaces comprised of bi-Convex (1) / doublet interface (2) / Concave-Flat (3)
c) air gap
d) bi-Convex surfaces (4 and 5)
e) air gap
f) Meniscus surfaces (6 and 7)
g) air gap
h) Flat-Concave surfaces (8 and 9).
Highlighted in the figure are the lens prescribed Thicknesses (mm) and the corresponding five measured results of the corresponding lenses in green columns to the right, as well as air gap results. In this example, all measurements fall well within the 0.1 mm Detection Tolerance required for surfaces to be captured. If surfaces cannot be found, adjustments can be made to centration, better focus on collimator, or adjusting gain.
Highest peak signals will be found at lens surfaces. Lower peaks will be noted at the interface of doublets. Gain must sometimes be lowered to reduce or eliminate signals from secondary reflections. Lens assemblies can be flipped physically, as can the lens prescriptions be flipped with the LENSCAN software for best results.
If little is known about a lens prescription, it is will be very difficult to inspect. However, the dependent variables in the lens prescription are 1) the material thickness and 2) the material index. If one of these is known, then the other along with air gaps can be determined using an iterative process.
Click HERE to download the "DMI Operation Tutorial" presentation.
Tremendous time and cost benefits can be realized by incorporating LAS-DMI option. Single lens incoming inspection and/or manufacturing quality control can be implemented for minimizing scrap and maximizing yield. cost further down the line. Multi-lens metrology improves the lens assembly operation with fast feedback on optimal pairing and spacing of lenses. Furthermore, scripts can be saved to automate inspection on batches of identical assemblies.
If you do not already own LAS, as the exclusive distributor for Fogale nanotech in North America, OATi offers the stand-alone LENSCAN system, pictured to the right. The stand-alone system performs the same measurements with equal precision and accuracy as the LAS-DMI.
See also our DMI Technology page for more details.