Hi, people. I have a not so scientific, but hopefully a simple question. I want to get a replacement lens for my machine. All I know is that I currently have an 18mm (diameter) 38.1mm plano-convex lens. Without talking about any specific needs, like material, etc. what should I get? Specifically what is the difference between 38.1mm and a 50.8mm focal lenses? And also between plano-convex and meniscus lenses? I read that 38.1mm is better in terms of energy loss (i.e. less loss) and meniscus one had a more precise focal point. So does it mean 38.1mm meniscus is the best combination?
Thank you for your time.
A Thorlabs technician measuring the irregularity of one of our singlets using a Zygo GPI-XP/D Interferometer.
Introduction
Thorlabs has a series of quality control procedures in order to ensure our singlets meet our standards and specifications. This starts with in-process inspections of the lens&#; imaging capabilities and ends with a final inspection of surface quality and dimensions. Specifications for particular products can be found in their linked documentation by clicking the symbol. This tab will take you through the general process used to check for quality.
Singlet Quality Practices
In-process inspection begins once the singlet has been shaped to specifications. Focal length, surface irregularity, and surface power are checked, following sampling plan Level VI given in MIL-PRF-B (see below). These three specifications are imperative for proper imaging. Surface irregularity of parts is kept to below either a quarter wavelength or a half wavelength at 633 nm, depending on the material of the singlet. Below is a graph of over 200 batches of singlets with irregularity data of both their front and back sides.
At this point, some uncoated singlets will proceed to final inspection, while others will receive an antireflective (AR) coating. The application of optical coatings has its own in-process inspections. To ensure the AR coating is applied properly, we verify both reflectance and transmission performance by scanning witness pieces using spectrophotometry; the material of these 2 mm thick witness samples matches the other parts in the run. For reflectance verification, we use at least one witness sample for each coating run. Transmitting optics receive two AR coatings, one on each surface, so for the verification of transmission, we use one witness sample that is also coated on both of its sides. Large runs use multiple witness samples to ensure the uniformity across the deposition chamber. By testing coating performance during every run, variance over time is kept low. To see how coatings vary, see the table below.
Final inspection of both uncoated and AR-coated singlets includes a batch check of diameter and thickness and a 100% visual check to ensure that the surface quality, chamfer, and clear aperture meet our published specifications. While surface quality is cosmetic to a degree, scratches, digs, and other inclusions in the surface of a part can increase the chances of damage to the singlet when used with high-power sources. These inspections are done in a clean, dark room under lighting that meets the requirements of MIL-PRF-B. Inspection under a single light source in a dark room allows for inconsistencies in the glass to be located without being obscured by glare or reflections.
MIL-PRF-B: Performance Specifications for Optical Components
MIL-PRF-B is a document created by the U.S. Army Armament Research, Development and Engineering Center's Defense Quality and Standardization Office for the specifications covering how finished optical components should be manufactured, assembled, and inspected. While primarily for use in letting the military dictate how products they use can be incorporated into their equipment, these standards have been adopted by many optics manufacturers. To download a copy of the full document, click here.
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