A system for spectral calibration of a remote sensing sensor, comprising: a plurality of reflective mirrors (72A-72G) configured and arranged to reflect radiation from a source of radiation onto a remotely located radiation sensor, wherein a first mirror (72A, 72B, 72C) in the plurality of mirrors is configured as a first wavelength dependent reflectance target to reflect a first portion of the radiation in a first wavelength range toward the remotely located radiation sensor and a second mirror (72D, 72E, 72F, 72G) in the plurality of mirrors is configured as a second wavelength dependent reflectance target to reflect a second portion of the radiation in a second wavelength range different from the first wavelength range toward the remotely located radiation sensor, wherein the first portion of the radiation and the second portion of the radiation are selected to calibrate the remotely located radiation sensor so as to provide a quantitative spectral relationship between the radiation detected at the remotely located sensor and the radiation reflected by the plurality of mirrors; wherein the first mirror has a first radius of curvature and the second mirror has a second radius of curvature different from the first radius of curvature; and the first mirror has a first base diameter and the second mirror has a second base diameter different from the first base diameter; and the relationship between the first radius of curvature and the first base diameter and the relationship between the second radius of curvature and the second base diameter is such that the cone angle of the radiation reflected by the first and second mirrors is the same. a plurality of reflective mirrors (72A-72G) configured and arranged to reflect radiation from a source of radiation onto a remotely located radiation sensor, wherein a first mirror (72A, 72B, 72C) in the plurality of mirrors is configured as a first wavelength dependent reflectance target to reflect a first portion of the radiation in a first wavelength range toward the remotely located radiation sensor and a second mirror (72D, 72E, 72F, 72G) in the plurality of mirrors is configured as a second wavelength dependent reflectance target to reflect a second portion of the radiation in a second wavelength range different from the first wavelength range toward the remotely located radiation sensor, wherein the first portion of the radiation and the second portion of the radiation are selected to calibrate the remotely located radiation sensor so as to provide a quantitative spectral relationship between the radiation detected at the remotely located sensor and the radiation reflected by the plurality of mirrors; wherein the first mirror has a first radius of curvature and the second mirror has a second radius of curvature different from the first radius of curvature; and the first mirror has a first base diameter and the second mirror has a second base diameter different from the first base diameter; and the relationship between the first radius of curvature and the first base diameter and the relationship between the second radius of curvature and the second base diameter is such that the cone angle of the radiation reflected by the first and second mirrors is the same.