Complete hemispherical infrared laser-based reflectometer (CHILR)

A custom instrument, the Complete Hemispherical infrared Laser-based Reflectometer (CHILR), employs a gold integrating sphere and is used for the measurement of the normal directional-hemispherical reflectance of samples and complex structures such as cavities used in blackbodies and radiometers. Measurements are made using light input from a selection of lasers covering the infrared spectral range from 1 micron to 11 microns, a diffuse-gold-coated 200 mm diameter integrating sphere for collecting the scattered light, and pyroelectric and MCT detectors.

The reflectometer is designed to collect and measure nearly all the light that is reflected from a sample or cavity into the 2π solid angle in front of it. Because of the high power and small diameter of the collimated laser light (< 4 mm diameter), a small sphere entrance aperture can be used (6 mm diameter) This corresponds to a minimal 1.7° full angle reflected light lost from the sample or cavity. This is in contrast to the 12.5° full angle of the infrared integrating sphere used for spectral diffuse reflectance of samples measured at the Fourier transform infrared spectrophotometry (FTIS) facility, where a large solid angle is needed to collect sufficient flux.

A new, larger diameter reflectometer (CHILR II) has recently been added to the facility with a 4 times larger collection port to allow measurement of large diameter blackbody cavities that are commonly used as spectral radiance standards for infrared earth remote sensing systems. The use of both systems enables measurement of the sample reflectance over a large dynamic range as well as over a set of collection apertures.

The spatial uniformity and angular dependence of the hemispherical reflectance can also be measured for samples and cavities that have minimal amounts of reflected light into large angles. This is accomplished through a mount that is held separate from the sphere and incorporates motorized linear and rotation stages. The sample port size can be varied up to 50 mm in diameter, and can be used to measure cavities with aperture sizes up to 50 mm in diameter (25 mm for complete mapping).

Reflectance levels down to ~ 1 x 10^-5 can be measured using the CHILR system.