Achromatic diffractive features on lenses are widely used in industry for color correction, however there is not a welldefined standard to quantify the performance of the lenses. One metric used to qualify a lens is the sag deviation from the nominal lens profile. Imperfections in the manufacturing of the diffractive feature may cause scattering and performance loss. This is not reflected in sag deviation measurements, therefore performance measurements are required.
There are different quantitative approaches to measuring the performance of an achromatic diffractive lens. Diffraction efficiency, a measure of optical power throughput, is a common design metric used to define the percent drop from the modulation transfer function (MTF) metric. The line spread function (LSF) shows a layout of the intensity with linear distance and an ensquared energy specification can be implemented. The MTF is a common analysis tool for assemblies and can be applied to a single element. These functional tests will be performed and compared with diffractive lenses manufactured by different tool designs.
This paper displays the results found with various instruments. Contact profilometry was used to inspect the profile of the diffractive elements, and a MTF bench was used to characterize lens performance. Included will be a discussion comparing the results of profile traces and beam profiles to expected diffraction efficiency values and the effects of manufacturing imperfections.
Finished lens molding, and the similar process of precision lens molding, have long been practiced for high volume, accurate replication of optical surfaces on oxide glass. The physics surrounding these processes are well understood, and the processes are capable of producing high quality optics with great fidelity. However, several limitations exist due to properties inherent with oxide glasses. Tooling materials that can withstand the severe environmental conditions of oxide glass molding cannot easily be machined to produce complex geometries such as diffractive surfaces, lens arrays, and off axis features. Current machining technologies coupled with a limited selection of tool materials greatly limits the type of structures that can be molded into the finished optic.
Tooling for chalcogenide glasses are not bound by these restrictions since the molding temperatures required are much lower than for oxide glasses. Innovations in tooling materials and manufacturing techniques have enabled the production of complex geometries to optical quality specifications and have demonstrated the viability of creating tools for molding diffractive surfaces, off axis features, datums, and arrays. Applications for optics having these features are found in automotive, defense, security, medical, and industrial domains.
This paper will discuss results achieved in the study of various molding techniques for the formation of positive diffractive features on a concave spherical surface molded from As2Se3 chalcogenide glass. Examples and results of molding with tools having CTE match with the glass and non CTE match will be reviewed. The formation of stress within the glass during molding will be discussed, and methods of stress management will also be demonstrated and discussed. Results of process development methods and production of good diffractive surfaces will be shown.
This paper will demonstrate a graphical method for selecting a pair of optical components to simultaneously achromatize and passively athermalize an imaging lens for use in a housing with a particular coefficient of thermal expansion. The effort will be presented for a generic spectrum, so readers may apply the method to whatever waveband and set of materials that are of interest. The term “component” is used in place of “material” since this paper will explore combinations of refractive and diffractive optics. The method for creating an achromat with two refractive materials will be reviewed. To create an athermal doublet, or lens that does not change focus under temperature changes, the same color equations are used with a slight modification to include housing thermal effects. The paper will culminate by demonstrating how a set of two materials can be used to both color correct and passively athermalize a single lens; these materials can be quickly chosen to match a particular housing material via nomograph. A sample chart for a common waveband will be demonstrated.
Long wave infrared (LWIR) optical systems are prone to defocus with changes in temperature. IR refractive materials
are more thermally sensitive compared to conventional visible glass due to their larger therm-optic coefficients. LWIR
systems can be designed to be passively athermal (little or no change to focus with varying temperatures). Chalcogenide
glasses provide additional material choices for IR lens designers. In particular, AMTIR5 has been engineered so its
therm-optic coefficient matches the coefficient of thermal expansion (CTE) of aluminum, allowing for an athermal
singlet. This paper explores the benefits of using engineered chalcogenide glass for color corrected, passively athermal
Initially, we present color corrected and passively athermal doublets that are designed with different materials and / or
diffractive surfaces. Their thermal and color performance are cataloged for axial beams only. These are intended to be
starting components, which readers may then insert into common design forms, such as Petzval, Double Gauss,
Telephoto, and Inverse Telephoto.
A F/1.3, 20° full field of view, aspheric Petzval lens design form is explored and the MTF is evaluated for -50°C to 85°C
in an aluminum housing. From this design, we explore the tradeoffs between using chalcogenide versus crystalline
materials, diffractive versus pure refractive surfaces, and engineered chalcogenide (AMTIR5) versus "catalog" materials.
The following paper provides the practicing engineer with guidelines on the relationships between cost and various
performance factors for different types of linear stages. When multiple precise motions need to be made in a system,
stages are typically the solution. A number of factors should be considered before choosing a stage: cost, load capacity,
travel range, repeatability, resolution, encoding accuracy, errors in motion, stiffness, stability, velocity of motion,
environmental sensitivity, and additional features like over-travel protection and locking mechanisms. There are a
variety of different bearing types for linear stages, each with their own advantages and disadvantages. This paper
presents charts that provide relationships between the cost, travel range, angular deviation, and load capacity of various
types of manual one-axis linear stages. The stages considered were those that had less than a 2.5" travel range and sold
by major optomechanical vendors. The bearing types investigated were dovetail, flexure, ball bearing, double row ball
bearing, crossed roller bearing, and gothic arch ball bearing. Using the charts and general guidelines provided in this
paper, a more informed decision may be made when selecting a linear stage.
In this paper, simple relationships are presented to determine the amount of focal shift that will result from the axial
motion of a single element or group of elements in a system. These equations can simplify first-order optomechanical
analysis of a system. Examples of how these equations are applied are shown for lenses, mirrors, and groups of optical
elements. Limitations of these relationships are discussed and the accuracy is shown in relation to modeled systems.