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.
CCD based spectrometers are commonly used to characterize the optical performance of LEDs. All CCD based
spectrometers exhibit varying amounts of stray light. This situation is exacerbated by using tungsten-halogen based
sources for calibration. NIST has come up with methods to characterize the stray light associated with CCD based
spectrometers using multiple lasers spanning the visible and near IR spectrum. At Sphereoptics we have developed
methods to map out and correct for stray light using a double monochromator and software. We have been able to
mathematically remove the effects of stray light on many common LED characterization measurements such as
spectral flux for instance. These effects were found to be quite substantial especially in the UV part of the spectrum.
In this paper we will present our findings on the nature of stray light found in a B&W BRC112E UV spectrometer as
well as our progress in correcting the errors it causes.
A vacuum compatible integrating sphere was built to operate inside a thermal vacuum chamber. This paper presents the
design and test results for a 1.65 meter diameter vacuum compatible integrating sphere with a 1.0 meter diameter exit
port and approximately 10kW of internal tungsten lamps. Liquid nitrogen is used as cooling medium to remove the heat
generated by these lamps. There are no moving parts inside the vacuum chamber.
The radiance is monitored with two filter-wheel detectors, one TE-cooled silicon and one TE-cooled germanium, as well
as a TE-cooled silicon array spectrometer. All three detectors are located outside the thermal vacuum chamber and view
the sphere radiance through fiber optic cables.
The system was tested inside a thermal vacuum chamber at NASA Goddard Space Flight Center before commissioning
in the 5.5 meter thermal vacuum chamber at Space Applications Centre in Ahmedabad, India. Results of tests of radiance
uniformity, radiance levels, and radiance stability are presented. Comparisons of the filter radiometers with the array
spectrometer are also presented.
The content in this Field Guide starts with traditional illumination in imaging systems, followed by the recent advances in computer-aided design of high-efficiency nonimaging illumination optics, along with the modern source models that support these techniques. Sections on the illumination of visual displays are included as well as some important topics on architectural illumination.
The complete and accurate colorimetric characterization of fluorescent materials requires the use of a 'bispectrometer;' a two-monochromator instrument designed to provide a two- dimensional matrix of bispectral radiance factor values for any given sample. This paper presents the general principles of operation, and a definition of the properties measured by such an instrument; expressing the actual physical quantities measured as an approximation to corresponding theoretical quantities. Approaches to calibration are outlined, with particular attention to the need for separation of reflected and luminescent components. Finally, this paper describes the calculations required to reduce the bispectral data produced by the instrument to more familiar colorimetric terms.
This paper is a report on the ongoing flight evaluation testing of Spectralon, a diffuse reflectance material which is slated for use as a calibration panel for several satellite-based earth-observing instruments. The present study focuses on tests of the stability of the material under exposure to levels of UV/VUV radiation which match those of the low-earth orbit environment. In earlier UV/VUV exposure tests, some degradation of the optical properties of the material were observed; this optical degradation has been linked to photochemical degradation of organic contaminants. A more stringent manufacturing protocol was designed to eliminate these contaminants. The second phase of UV/VUV exposure testing, reported here, was undertaken with the object of validating and optimizing this new manufacturing protocol. Results of this testing indicate that the new manufacturing protocol yields a significant improvement in the optical stability of Spectralon under UV/VUV exposure. These results also indicate that most of the observed degradation is caused by exposure to radiation in the 200 - 380 nm band. This finding suggests new avenues of investigation, as well as providing justification for a simplification of future test requirements.
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