Calibration panels are used as reference materials with known reflectance characteristics to convert remote sensing image pixel values to scientific units such as reflectance. At the time of purchase, the panel vendor typically provides the customer with reflectance data for the panel. The vendor-provided spectral data as a reference for each panel’s reflectance values at the time of purchase. Over time, the panel’s reflectance can change due to exposure of its surface to elements such as the sun, dirt, dust, sand, and more during the panel’s use in the field. The purpose of this dataset is to enable the assessment of calibration panel reflectance changes over time. In addition to the vendor-provided spectral data for a series of panels at each panel's respective time of purchase, we measured the reflectance characteristics of each panel using a handheld spectroradiometer instrument and share those spectra in this data release. These Calibrated Reflectance Panels were manufactured by MicaSense (MicaSense Inc., Seattle, WA, USA; http://www.micasense.com/), which was also the source of the vendor-provided reflectance data. The vendor-provided data is provided as absolute reflectance (a value between 0.0 and 1.0) in the range of 400 nm to 850 nm (in increments of 1 nm). These vendor-provided spectra are provided in Comma Separated Value file format (files with a .csv extension) with two columns (wavelength, reflectance). We combined the vendor-provided spectral data into a single .csv file for this data release. On February 10, 2020, USGS researchers collected a series of reflectance spectra using a handheld field spectroradiometer for five calibration panels, each delivered to the customer on different dates between 2015-2019 and subjected to different field work conditions over time. This enables an assessment of calibration panel reflectance changes over time. The instrument used to measure the panel reflectance spectra was the Analytical Spectral Devices FieldSpec® 4 Hi-Res (ASD FS4; ASD Inc., a Malvern PANalytical Company, Longmont, Colorado) covering the range from 0.35 to 2.5 µm. These handheld collected reflectance spectra were exported in the .txt format from the ASD ViewSpecPro software, and we combined the collected spectral data into a single Comma Separated Value file format (with a .csv extension).