Sintered polytetrafluoroethylene (PTFE) is an extremely stable, near-perfect Lambertian reflecting diffuser and calibration standard material that has been used by national labs, space, aerospace and commercial sectors for over two decades. New uncertainty targets of 2 % on-orbit absolute validation in the Earth Observing Systems community have challenged the industry to improve is characterization and knowledge of almost every aspect of radiometric performance (space and ground). Assuming “near perfect” reflectance for angular dependent measurements is no longer going to suffice for many program needs. The total hemispherical spectral reflectance provides a good mark of general performance; but, without the angular characterization of bidirectional reflectance distribution function (BRDF) measurements, critical data is missing from many applications and uncertainty budgets. Therefore, traceable BRDF measurement capability is needed to characterize sintered PTFE’s angular response and provide a full uncertainty profile to users. This paper presents preliminary comparison measurements of the BRDF of sintered PTFE from several laboratories to better quantify the BRDF of sintered PTFE, assess the BRDF measurement comparability between laboratories, and improve estimates of measurement uncertainties under laboratory conditions.
Integrating spheres for optical calibration of remote sensing cameras have traditionally been made with Quartz Tungsten Halogen (QTH) lamps because of their stability. However, QTH lamps have the spectrum of a blackbody at approximately 3000K, while remote sensing cameras are designed to view a sun-illuminated scene. This presents a severe significant mismatch in the blue end of the spectrum. Attempts to compensate for this spectral mismatch have primarily used Xenon lamps to augment the QTH lamps. However, Xenon lamps suffer from temporal instability that is not desirable in many applications. This paper investigates the possibility of using RF-excited plasma lamps to augment QTH lamps. These plasma lamps have a somewhat smoother spectrum than Xenon. Like Xenon, they have more fluctuation than QTH lamps, but the fluctuations are slower and may be able to be tracked in an actual OGSE light source. The paper presents measurements of spectra and stability. The spectrum is measured from 320 nm to 2500 nm and the temporal stability from DC to 10 MHz. The RF-excited plasma lamps are quite small, less than 10mm in diameter and about 15 mm in length. This makes them suitable for designing reasonably sized reflective optics for directing their light into a small port on an integrating sphere. The concludes with a roadmap for further testing.
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