Non-disputable quality assurance of optical components

Sibylle Delaney

Delivering an objective and repeatable scoring solution of surface imperfections

The surface quality of an optical component is an evaluation of the surface imperfections that may be caused in the manufacturing or handling process. In most cases, these defects have little to no adverse effect on the overall system performance and cause only small increases in scattered light and minor reductions in throughput. However, certain surfaces are more sensitive to these defects –surfaces at image planes, for example, or surfaces used in high-power applications, where any imperfection can cause increased absorption of energy and damage the optic. The most common method of rating surface quality is the US Military Performance Specification MIL- 13830B, which uses ‘scratch and dig’ numbers based on calibrated standards prescribed therein. It determines the calibre of scratches and digs and sets guidelines limiting the allowable amount of each imperfection based on the component’s size and its scratch and dig number.

The Challenge

Although the optics industry demands the greatest accuracy, the standard approach for inspecting optics for scratches and digs has, up until now, relied upon human judgement, making it difficult to obtain repeatable results. Typically, the method involves holding the optic under inspection at a specific angle under specific light conditions – as defined by the standard – and the user will compare the optic to a set of criteria. ‘For the last 65-70 years, the optics industry has relied on looking at their optics to determine whether or not they can see a scratch or a mark on the surface such that they can pass or fail that optic during quality control,’ explained Mike Hobby, sales director at RedLux. This subjective and often undocumented Scratch-Dig inspection process all too often leads to discrepancies in the supply chain, which can increase production costs, added Hobby.

The Solution: Aligning Manufacturer and Customer

RedLux had developed the next generation scratch-dig inspection system OptiLux SD. It allows optics manufacturers and their customers to control the scratch-dig inspection process combining a high-resolution sensor head with a precision X-Y motion system to provide a fully-automated operator-independent surface measurement and analysis routine, compliant to the MILS-13830B standard in a highly repeatable and traceable way.

Customer Benefit: Objective and Traceable Quality Assurance

Crystran, a manufacturer of optical components, has been using RedLux’s device to measure its high-value 190 x 130 x 4 mm Zinc Selenide (ZnSe) windows since 2015. Zinc Selenide (ZnSe) is widely used for infrared components such as thermal imaging windows and lenses. These components are anti-reflection (AR) coated after polishing; this process makes up 40 per cent of the total production cost. So, the manufacturer must be confident that the surface quality is within specification both before and after coating takes place. By adopting an OptiLux SD machine, Crystran has been able to ship high-value components, such as its ZnSe windows, with a traceable pass/fail MIL- 13830B compliant report, therefore avoiding the costs associated with Scratch-Dig subjectivity within the supply chain. ‘Crystran can now provide direct assurance to their valued customers that their optical products have been fully assessed in exact accordance with MIL-13830B, by way of a detailed surface quality report. In addition to reducing the amount of manufacturing re-work and polishing effort, saving time and money, Crystran’s customers can be confident that even the most stringent S-D specification has been unambiguously met.’ commented Hobby. ‘We’ve been able to align supplier, customer and contractor’s expectations of optic quality so that everyone can sing off the same song sheet. It will make the optics industry far more efficient in how it can quality control these flat optics.’ In addition to automating the Sratch-Dig inspection process, the OptiLux SD limits the amount of component handling, minimising the potential of accidental damage on soft material such as ZnSe. Furthermore, the OptiLux SD allows technicians to understand how close an imperfection is to the grading boundary, allowing them to refine their production process by knowing exactly when to stop and when to continue polishing in order to meet the surface specification.

RedLux will be exhibiting the revolutionary OptiLux SD at Optifab in Rochester, New York, USA, 16 - 19 October - We are looking forward to your visit! / +44 (0)23 8026 3095 /

OptiLux SD - Inspection of Surface Imperfections

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