Spectral reflectivity and interferometry for optical lens analysis

David Rideout, Olympus America Inc.

There are more than 500 companies in the US alone that make optical equipment. The number of products with built-in lenses is increasing. Few cell phones or personal computers are sold today without a still or video camera included. We see the predominance of microlenses in CCD arrays and the move toward the use of LED lighting as a green alternative. One day, every Christmas light might even include a lens.

Today’s optical lenses are being manufactured according to ever-increasing requirements for precision, rendering easy, accurate and reportable inspection critical. In addition, industrial use of optical components is increasing exponentially, so efficiency and cost-effectiveness in manufacturing and quality control are more important than ever.

Although inspection of lenses and coatings is vital to the success of many products, quality and measurement issues must be balanced with the need to keep costs down and throughput up in a production environment. The cost/capability balance is a key driver for what instrument is used. In addition, experience shows that various technologies have particular strengths in handling specific types of lens inspections.

Today’s spherical and aspherical optical lenses are inspected and analyzed for a number of features, including flatness, curvature, coating thickness, color measurement and reflectivity. Any deviation in curvature or flatness of a lens from its original design will change the focal point of the light passing through the lens and also may increase the level of spherical aberration within the optical system.

This will alter the specifications and effectiveness of the lens, changing the performance of the primary product as well, be it an LED or a camera. In addition, lens coatings continue to evolve, and the coatings themselves often require additional levels of inspection. Changes in the thickness, reflectivity or color level of a coating affect the way light is transmitted through the lens. For example, antireflective coatings absorb light – thus reducing reflection – and other coatings may change the level of static electricity being transmitted through the system. If a coating does not match design specifications regarding reflectivity, evenness or thickness, it can affect the usability of the final product.

Two technologies most often used in the inspection of optical lenses are interferometry and a type of spectroscopy called spectral reflectivity. Interferometers analyze the actual shape of the lens by examining how light entering it is refracted or reflected.

Spectral reflectivity systems allow measurement of the thickness, optical properties and reflectivity of coatings, defining the way the lens absorbs or reflects various wavelengths of light. As such, they allow manufacturers to ensure that the correct types of coating are used and that the proscribed thickness and evenness of the coating on each lens element is correct in a manufactured optical system. Both technologies address part of the overall application, but neither alone is sufficient to do a complete analysis.

Olympus has been building lenses for microscopy, endoscopy, photography and other advanced applications since 1919 and uses both technologies extensively to measure and analyze its optical products. The company builds its own spectral reflectivity and interferometry systems to meet its exacting standards for both optical performance and cost efficiency. These systems currently are not sold in the US, but there are numerous commercially manufactured inspection systems that are available to lens manufacturers.

Spectral reflectivity

Spectral reflectivity systems usually consist of a specialized type of microscope and custom software. The sample is struck with light at specific wavelengths, and all reflected light is analyzed. The returning light has been altered by the sample, and the alteration can be analyzed to determine the reflected or incident direction and wavelength. Analyzing the reflected spectral composition from a coated sample and comparing the data received to reference optical properties can help engineers determine the precise wavelength characteristics of a coating (Figure 1). Coating thicknesses also can be analyzed using a laser confocal microscope if the user knows the refractive index of the coating. Current software platforms can do this automatically for quick inspections.

Figure 1. Spectral reflectivity is commonly used to analyze lens coatings. An Olympus USPM-RU captured this image.

Using the same spectral data, users can determine the thickness of the coating because spectral reflectance also is affected by the difference between the light reflected by the coating and the light reflected by the lens surfaces. Comparing thickness levels with corresponding spectral reflections provides direct correlation that can be used for measurement.

The greatest advantage of spectral reflectivity systems is that they offer nanometer- or even angstrom-level repeatability. In addition, analysis can be performed using either transmitted or reflected light, depending upon the system and the inspection requirements. One drawback in the past has been the need to avoid internal reflection from the rear surface of the lenses being examined. However, a key advantage of some more recent systems is that they allow rapid and highly accurate spectroscopic measurement of thin samples without interference from rear surface-reflected light, which was not possible with traditional spectroscopes. Thus, companies now can avoid back-side interference without incurring the additional time and cost of special sample preparation.

Interferometry

Interferometers typically are tabletop devices that use light to compare a fabricated product to a reference sample whose dimensions it is intended to match (Figure 2). Light is passed through a reference lens to the fixtured sample (lens) and then onto a reflecting mirror, where it is sent back to the receiver. When the sample image is combined with a reference image, the fringe pattern generated is used to analyze the shape of the product’s optical surface.

Figure 2. Lens curvature is typically inspected using interferometry. An Olympus KIF-20 captured this image.

Depending upon the specific technique being used, an interferometry system may use white light or laser illumination. The instrument most commonly used for lens inspection is a laser-based technology called a Fizeau system, which typically uses a reference sample similar to the surface being inspected. Some higher-end models can accommodate both flat and spherical surfaces, but most systems do just one or the other.

The results reveal any variations in height from the theoretical smooth surface of the lens, with the user receiving both an image of the fringe pattern itself and data describing the deviation from the reference surface. Software then can analyze these differences to provide highly precise and repeatable measurement data on how much the manufactured product varies from the reference sample (Figure 2).

Importantly, because interferometers can be used to examine surfaces larger than 100 mm, entire lenses can be analyzed at one time. And interferometers can provide reproducible data below 100 nm at very low magnifications. Fizeau interferometers also have the advantage of flexibility, accommodating not only lenses but also almost any surface needing precision flatness inspection, including wafers, mirrors, solar collectors and magnetic heads.

Other surface analysis technologies are available to provide similar measurements such as profilometers and laser confocal microscopes, but each has trade-offs. Profilometers, although sometimes easy to use, can be slow and offer little beyond what the interferometer can achieve. In addition, they often feature touch probes, which can damage delicate lenses.

Figure 3. As industrial use of optical components continues to expand, efficiency, cost-effectiveness and quality control in manufacturing become more important than ever. Most digital cameras today include still and video, and their lenses must be inspected carefully. The PEN E-P2 digital camera was inspected with the Olympus USPM-RU.

Laser scanning confocal microscopes provide enhanced lateral measurement and slope inspection. They are, however, usually not able to provide Z-axis measurements as precise as those offered by interferometers. And though they have enormous capabilities for a wide variety of purposes, confocal microscopes are neither the least expensive nor the easiest systems to use in a manufacturing environment. They are most often devoted to off-line engineering and research and development work, while interferometers are optimized for volume production.

Today’s lens manufacturers most often use both spectral reflectivity systems and interferometers to analyze their products. For instance, a phone manufacturer might demand that the curvature of the tiny lenses in its mobile phone cameras be inspected via interferometry, while the evenness of the coatings on those lenses is verified via spectral reflectivity.

In the future, product advances may lead to further reduction of unwanted light reflecting from rear surfaces in lenses, better reflectivity for minute areas, quicker and more accurate results, enhanced measurement wavelengths and improvements in software that one day could allow a manufacturer to combine data from various lens and coating inspections for enhanced process control.

Meet the author

David Rideout is group marketing manager of industrial microscopes at Olympus America Inc., Scientific Equipment Group; e-mail: david.rideout@olympus.com.