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Going beyond pass/fail optics characterization

03 Oct 2008

The demand for higher quality images using small-diameter optics is fostering competition amongst manufacturers. Xavier Levecq of Imagine Optic tells OLE why its latest product allows industrial R&D teams to push new lens designs through to market more efficiently.

The digital camera technology incorpor-ated into today's hottest selling products has become an important criterion of choice for customers. Do a simple search on Google for the "five most important features in a digital camera" and almost every article will put lens quality at the top of the list. For mobile phones and other devices, image quality comes in just after signal strength, size/weight and battery life. Consumers today understand that image quality is just as much about lens quality as it is megapixels.

"Smaller. Better. More powerful", these are keywords for almost any product these days but, in optoelectronics, they are synonymous with survival. From design and prototyping to online quality control, manufacturers are hard pressed to develop new lens designs in record time and to make sure that production lines function at peak efficiency. The combination of new optical designs, novel lens materials, complex assemblies that require precision alignment leaves no shortage of opportun-ities for problems to appear.

Characterizing history
The first lens characterization systems to appear on the market functioned using the line spread function (LSF) to calculate the modulation transfer function (MTF) in one direction and at one point in the field of view. The LSF is obtained by projecting a slit image onto a CCD detector after it has passed through the optical system being tested.

Next came cross-slit systems that used the information from tangential and sagittal slits to measure the optical system's MTF on two axes but still at only one point in the field. Finally, MTF systems attained their maximum with the advent of multi-source platforms that measured the MTF at a set number of points in the field. The major shortfall of all of these systems is that they are only capable of giving a pass/fail response.

The next generation of characterization systems was based on wavefront metrology. A very small handful of systems rely on interferometry (standard or shearing interferometry) but the majority use Shack-Hartmann wavefront sensors. Wavefront measurements provide information on all of the aberrations in the optical system and use that information to calculate the MTF. This was a major advance because characterization moved beyond simple pass/fail responses and users gained access to the cause of any problems in their element.

Although they offer significantly higher functionality than simple MTF systems, the vast majority of commercially available wavefront characterization systems have some important limitations. For these devices, measuring components with a high numerical aperture (NA) – microscope lenses for example – can be difficult without additional optics. In addition, they can generally only measure at a single wavelength whereas most optical systems are used over a wide spectral range.

Perhaps the most important limitation of commercially available wavefront-based systems is the fact that they can only measure on-axis whereas often users want to equally understand the off-axis optical qualities of the element being characterized. This is especially useful for optical assemblies that are used in high-performance imaging systems.

It is also worth noting that, while measuring the MTF and aberrations are important, other factors including effective focal length (EFL), chromatism, field curvature, distortion, relative illumination, and vignetting play equally important roles in defining overall quality of the individual optical elements in the assembly. Until recently, the state of the art required that researchers or production managers rely on several different machines to fully characterize optical elements.

Enter the SL-Sys neo
To respond to the growing optics industry need for the characterization of ever smaller optics and objectives, Imagine Optic launched the first of its SL-Sys products in 2006 with the SL-Sys liquid for testing miniature liquid optics. After several months of intense market research and nearly two years of technological development, the company followed up on the success of the SL-Sys liquid by launching the SL-Sys neo earlier this year.

"We didn't want to just put another characterization system on the market," explained Imagine Optic co-founder and vice-president of marketing Xavier Levecq. "We wanted to provide a truly novel solution that would have a measurable impact on the way that industrial R&D engineers and production line managers work."

The SL-Sys neo offers the features you would expect from a wavefront lens characterization system and equally proposes some surprisingly innovative pluses that make it a product that's worth a closer look. For starters, the product boasts a wavefront measurement sensitivity of λ/100, resolutions up to 7600 measurement points and a maximum MTF acquisition frequency higher than 1000 lp/mm. What's more, the neo not only measures on-axis but equally off-axis over a field with a range of ±45°.

SL-Sys neo: how it works
To understand how the SL-Sys neo works, it is best to start with how the device is built (see figure 1). It consists of two collimated sources functioning at 532 and 635 nm; a rotating diffuser, mounted on a translation stage that is adjusted along the z-axis with micrometre precision; two rotation stages of which one rotates the diffuser and objective on their common y-axis and the other that enables the objective to rotate on its z-axis; a specially designed, extended-wavelength HASO wavefront analyser; a beam expander; and a complex assembly of high-quality optical elements that make up the light path. The tabletop instrument is housed in a compact assembly with a footprint of just 32 × 35 cm2.

The beam expander plays two important roles because it adapts the size and precisely conjugates the pupil of the objective being characterized to that of the wavefront sensor. This conjugation is key to the SL-Sys neo's functionality because it enables the system to measure any objective between 1 and 12 mm in diameter (depending on configuration options), regardless of its numerical aperture, and equally enables users to observe the evolution of vignetting over the entire field.

Once lenses or objectives are loaded into the SL-Sys neo, characterization begins. The sources are activated one at a time and the objective focuses the incoming beam onto the diffuser whose position is automatically adjusted. The light reflected by the diffuser creates a secondary source that is retro-diffused through the objective and directed towards the wavefront analyser.

Simply knowing the diffuser's z-axis position provides the information necessary to calculate the objective's back focal length (BFL). Measuring the variations in curvature observed during the translation of the diffuser through the objective's focal plane enables the SL-Sys neo to provide a precise evaluation of the element's effective focal length (EFL). Using the EFL measurement, the device also provides measurements on the objective's distortion for all of the measured points in the field.

During the characterization process, the rotation stage on which both the diffuser and objective are mounted rotates on the y-axis, whereas the objective itself simultaneously rotates on its z-axis. This double rotation allows the SL-Sys neo to measure the objective's aberrations at any point in the field of view and, even more, to measure its field curvature. Excessive field curvature is one of the major causes of lost resolution in the field of view. Given the importance of field curvature and distortion on overall image quality, the SL-Sys neo draws a curve (see figure 2) to represent their evolution as a function of the field angle.

By using the aberration measurements obtained both on-axis and in the field (see top image), the SL-Sys neo calculates the three-dimensional MTF as well as the through-focus MTF for each analysed point in the field (see figure 3). By performing complete individual characterizations with each of the device's two sources that function at two different wavelengths, users are provided with information on the objective's chromatism as well as the aberrations at different points in the spectrum. Quantifying chromatism allows engineers to ensure that the component's value is within acceptable limits but also to evaluate the adequacy of lens materials.

In an R&D department, these core functionalities enable users to rapidly verify the conformity of a new lens assembly to its theoretical potential as it might be observed in optical design software. More importantly, these measurements provide meaningful insight into the root cause of any abnormalities such as irregularities in polishing, alignment (tilt) or centering of individual lenses as well as basic mechanical issues. When a deviation from established quality standards is detected on the production line, understanding the reason behind the anomaly can help save time and money.

By packing EFL, BFL, chromatism, wavefront aberrations on-axis and in the field, field curvature, distortion, 3D MTF, through-focus MTF and vignetting measurements into one device, Imagine Optic has developed a truly innovative product.

• Xavier Levecq is VP of marketing at Imagine Optic, a leading supplier of Shack-Hartmann wavefront sensing hardware and software, as well as adaptive optics technologies. For more information see www.imagine-optic.com.

• This article originally appeared in the October 2008 issue of Optics & Laser Europe magazine.

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