N-BK7 Plano-Concave Lenses, Uncoated

Zemax Files
Click on the red Document icon next to the item numbers below to access the Zemax file download. Our entire Zemax Catalog is also available.

Features

• Material: N-BK7
• Wavelength Range: 350 nm - 2.0 μm (Uncoated)
• Choose from 6 Diameters: Ø6 mm, Ø9 mm, Ø1/2" (Ø12.7 mm), Ø25 mm,
  Ø1" (Ø25.4 mm), or Ø2" (Ø50.8 mm)
• Offers Excellent Transmission Throughout the Visible and Near Infrared

These Plano-Concave lenses are fabricated from RoHS-compliant N-BK7 glass. N-BK7 is one of the most common types of optical glass used for high quality optical components. It is typically chosen whenever the additional benefits of UV fused silica (i.e., good transmission further into the UV and a lower coefficient of thermal expansion) are not necessary. However, for particularly harsh environments where the optic will be exposed to alkalis and acids, consider our N-SF11 plano-concave lenses. Since the Abbe Number of N-SF11 (25.76) is lower than that for N-BK7 (64.17), lenses fabricated from N-SF11 will exhibit higher dispersion than those fabricated from N-BK7.

These lenses, like all plano-concave lenses, have negative focal lengths and can be used to diverge collimated beams. In this case, the curved surface of the lens should face the source to minimize spherical aberration. In addition, they can be employed to offset the effects of spherical aberration caused by other lenses in an optical system.

N-BK7 lens kits are also available. Please click here for information.

Below is the transmission curve for N-BK7, a RoHS-compliant form of BK7. Total Transmission is shown for a 10 mm thick, uncoated sample and includes surface reflections.

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Click Here for Raw Data

The lenses sold on this page are also available with broadband antireflective coatings, the reflectance traces of which are shown in the graph below.

These high-performance multilayer AR coatings have an average reflectance of less than 0.5% (per surface) across the specified wavelength ranges and provide good performance for angles of incidence (AOI) between 0° and 30° (0.5 NA). The plot shown below indicates the performance of the standard coatings in this family as a function of wavelength. Broadband coatings have a typical absorption of 0.25%, which is not shown in the reflectance plots.

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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-13830B (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-13830B. 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-13830B: Performance Specifications for Optical Components
MIL-PRF-13830B 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.

	-A Coating 350 nm to 700 nm	-B Coating 650 nm to 1050 nm	-C Coating 1050 nm to 1700 nm		Singlet Irregularity
Coating Variance: Transmission	Click to Enlarge	Click to Enlarge	Click to Enlarge		Click to Enlarge
Coating Variance: Reflectance	Click to Enlarge	Click to Enlarge	Click to Enlarge

Disclaimer: The coated transmission data presented here are of 2 mm thick witness samples of N-BK7 or equivalent glass, and all the data here are typical. Some variations in performance data will occur from lot to lot. Please contact Technical Support with any questions regarding the use or reliability of these data.