RF-excited plasma lamps for use as sources in OGSE integrating spheres

Abstract

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.