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Dual-Confocal Laser Caliper Method for Refractive Index and Thickness Measurements of Intraocular Lens Implants and Thin Films

Catalog of Regulatory Science Tools to Help Assess New Medical Devices 

Technical Description

This RST is a dual-confocal laser caliper (DCLC) test method for noncontact measurement of refractive index and thickness of IOL implants and thin films. DCLC technology is based on a fiber-optic self-calibrating dual-confocal laser caliper approach for sensing the confocal response of the laser beam reflection from the object surfaces, enabling measurements without constraints on object shape, thickness, or optical transparency within the tested range [1–3]. The DCLC method has been applied for noncontact measurements of refractive-index and thickness of IOLs, thin films, and other related optical materials [1–4]. Refractive index and thickness are optical properties of IOL implants addressed in consensus standards such as ISO 11979-2:2024 [5]. In the quantitative evaluation of refractive-index and thickness of IOL implants and thin films, the DCLC tool provides the following features:

  • Noninvasive, noncontact, and minimally operator-dependent testing.
  • Self-calibrating operation, requiring no independent calibration testing.
  • Provides measurements across a wide range of object shapes, thicknesses, and optical transparencies.
  • Thickness measurements for 1-5 µm thickness optical materials, depending on the focusing objective numerical aperture (NA) and test conditions [1–3].

Intended Purpose 

The DCLC method is intended for noncontact, noninvasive, self-calibrating measurement of refractive index and thickness of IOL implants and thin films, with refractive-index accuracy of 10⁻³ and thickness accuracy of <2 µm [1–3].

Testing

To evaluate the key properties of the DCLC tool, multiple tests reported in peer-reviewed studies were performed, including:

  • Self-calibration: The DCLC tool demonstrated intrinsic self-calibration using non-transparent etalon plates, such that separate independent calibration was not required under the tested conditions [1–3]. 
  • Testing a transparent glass plate: Under the conditions described in [3], thickness accuracy of <1.8 µm and refractive-index accuracy of 10⁻³ were achieved for a glass etalon plate. The IOL samples showed thickness accuracy <2 µm and refractive-index accuracy of 10⁻³ [1–3].
  • Testing IOL samples: The IOL thickness measurement accuracy is <2 µm, and the refractive index accuracy is 10⁻³. For applications where accuracy below 2 µm is desired, focusing objectives with higher NA and magnification may be used [1–3].
  • Testing thin film samples: The DCLC tool was applied for noninvasive, in-situ measurement of refractive index and thickness of silk fibroin thin film samples [4].

Limitations

The DCLC tool operates within specific optical and environmental conditions depending on targeted accuracy and test setup. For thickness accuracy below 2 µm, focusing objectives with higher NA and magnification (but shorter working distance) may be used. For IOL refractive index measurements, the primary laser wavelength of 546 nm ± 10 nm corresponds to the photopic reference wavelength specified in the applicable recognized standard ISO 11979-2:2024 [5]. Generally, refractive index measurements use laser sources with various wavelengths in the 400-780 nm visible spectral range.

Supporting Documentation

Patents

  1. I. Ilev, D. Kim, R. James, and D. Calogero, “Systems and methods for conducting contact-free thickness and refractive-index measurements of intraocular lenses using a self-calibrating dual confocal microscopy system”, U.S. Patent No. 11,454,794 (September 27, 2022). https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/11454794 
  2. I. Ilev, D. Kim, R. James, and D. Calogero, “Systems and methods for conducting contact-free thickness and refractive-index measurements of intraocular lenses using a self-calibrating dual confocal microscopy system”, U.S. Patent No. 10,712,545 (July 14, 2020). https://image-ppubs.uspto.gov/dirsearch-public/print/downloadPdf/10712545 

Peer-Reviewed Publications and Papers

  1. D. Kim, R. James, D. Calogero, and I. Ilev, “Noncontact method for sensing thickness and refractive index of intraocular lens implants using a self-calibrating dual-confocal laser caliper”, Journal of Biomedical Optics, v. 23, pp. 067004, 2018. https://doi.org/10.1117/1.JBO.23.6.067004 
  2. X. Tan, D. Hammer, and I. Ilev, “A novel fiber-optic confocal laser caliper approach for non-contact optical characterization of silk fibroin thin films created by riboflavin photo-crosslinking”, International Conference on Lasers and Electro-Optics (CLEO-2021), Paper AF2Q.3, May 9­14, 2021, San Jose, CA. https://www.nihms.nih.gov/pmc_docs/1956292/
  3. International Organization for Standardization. Ophthalmic implants – Intraocular lenses – Part 2: Optical Properties and Test Methods. Geneva, Switzerland, ISO, 2024 (ISO 11979-2:2024).

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