High accuracy, absolute, cryogenic refractive index measurements of infrared lens materials for JWST NIRCam using CHARMS

TL;DR

This paper presents highly accurate cryogenic refractive index measurements for LiF, BaF2, and ZnSe used in JWST NIRCam lenses, covering 15-320 K and 0.4-5.6 microns, providing essential data for optical design.

Contribution

The study provides the first comprehensive cryogenic refractive index data for key infrared lens materials used in JWST NIRCam, including dispersion and thermo-optic coefficients.

Findings

Refractive index data for LiF, BaF2, and ZnSe from 15 to 320 K.

Dispersion and thermo-optic coefficients for the materials.

Validated Sellmeier fit coefficients for index prediction.

Abstract

The refractive optical design of the James Webb Space Telescope (JWST) Near Infrared Camera (NIRCam) uses three infrared materials in its lenses: LiF, BaF2, and ZnSe. In order to provide the instrument's optical designers with accurate, heretofore unavailable data for absolute refractive index based on actual cryogenic measurements, two prismatic samples of each material were measured using the cryogenic, high accuracy, refraction measuring system (CHARMS) at NASA's Goddard Space Flight Center (GSFC), densely covering the temperature range from 15 to 320 K and wavelength range from 0.4 to 5.6 microns. Data reduction methods are discussed and graphical and tabulated data for absolute refractive index, dispersion, and thermo-optic coefficient for these three materials are presented for selected wavelengths and temperatures along with estimates of index uncertainty. Coefficients for temperature-dependent Sellmeier fits of measured index are also presented with an example of their usage to predict absolute index at any wavelength or temperature within the applicable range of those parameters.