Figure 1 schematically depicts an improved multispectral imaging system of the type that utilizes a filter wheel that contains multiple discrete narrow-band-pass filters and that is rotated at a constant high speed to acquire images in rapid succession in the corresponding spectral bands. The improvement, relative to prior systems of this type, consists of the measures taken to prevent the exposure of a focal-plane array (FPA) of photodetectors to light in more than one spectral band at any given time and to prevent exposure of the array to any light during readout. In prior systems, these measures have included, variously the use of mechanical shutters or the incorporation of wide opaque sectors (equivalent to mechanical shutters) into filter wheels. These measures introduce substantial "dead" times into each operating cycle — intervals during which image information cannot be collected and thus incoming light is wasted. In contrast, the present improved design does not involve shutters or wide opaque sectors, and it reduces dead times substantially.

Figure 1. The Dichroic Beam Splitter and associated optics and electronics make it possible to utilize light from the telescope, even when the light beam straddles two adjacent narrow-band-pass filters. Light passed by one of these filters goes to one FPA; light passed by the other filter goes to the other FPA.

The improved multispectral imaging system is preceded by an afocal telescope and includes a filter wheel positioned so that its rotation brings each filter, in its turn, into the exit pupil of the telescope. The filter wheel contains an even number of narrow-band-pass filters separated by narrow, spokelike opaque sectors. The geometric width of each filter exceeds the cross-sectional width of the light beam coming out of the telescope. The light transmitted by the sequence of narrow-band filters is incident on a dichroic beam splitter that reflects in a broad shorter-wavelength spectral band that contains half of the narrow bands and transmits in a broad longer-wavelength spectral band that contains the other half of the narrow spectral bands. The filters are arranged on the wheel so that if the pass band of a given filter is in the reflection band of the dichroic beam splitter, then the pass band of the adjacent filter is in the longer-wavelength transmission band of the dichroic beam splitter (see Figure 2).

Each of the two optical paths downstream of the dichroic beam splitter contains an additional broad-bandpass filter: The filter in the path of the light transmitted by the dichroic beam splitter transmits and attenuates in the same bands that are transmitted and reflected, respectively, by the beam splitter; the filter in the path of the light reflected by the dichroic beam splitter transmits and attenuates in the same bands that are reflected and transmitted, respectively, by the dichroic beam splitter. In each of these paths, the filtered light is focused onto an FPA.

As the filter wheel rotates at a constant angular speed, its shaft angle is monitored, and the shaft-angle signal is used to synchronize the exposure times of the two FPAs. When a single narrow-band-pass filter on the wheel occupies the entire cross section of the beam of light coming out of the telescope, the spectrum of light that reaches the dichroic beam splitter lies entirely within the pass band of that filter. Therefore, the beam in its entirety is either transmitted by the dichroic beam splitter and imaged on the longer-wavelength FPA or reflected by the beam splitter and imaged onto the shorter-wavelength FPA.

Figure 2. The Filter Wheel contains narrow-band-pass filters arranged so that the pass bands of adjacent filters lie, alternately, in the transmission (T) and reflection (R) spectral band of the dichroic filter.

When the beam straddles two narrow-band-pass filters on the wheel, the spectrum of the light incident on the dichroic beam splitter includes one component in the transmission band and one component in the reflection band. The fraction of beam power in each component at a given instant of time is approximately equal to the fraction of the cross-sectional area of the beam occupied by the corresponding narrow-band-pass filter. The out-of-band signal on each path downstream of the dichroic beam splitter is further attenuated by the broad-band-pass filter on that path. Each FPA integrates incident light during frame times synchronized with the rotation of the filter wheel. Because the dichroic filter and the broad-band-pass filter on each path block out-of-band light, each FPA can integrate a spectrally pure image, not only when the light beam is passing through a single filter, but also when it is straddling two adjacent filters.

The dichroic beam splitter and the narrow-band filters, in combination, act like a shutter for each FPA at the end of its integration period, making it possible to read out each FPA without incurring degradation of the image. The focusing lens and the FPA for each optical path downstream of the dichroic beam splitter can be optimized over a range of wavelengths spanning half the spectral bands of the system.

This work was done by James C. Bremer of Swales Aerospace for Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www. techbriefs.com/tsp  under the Physical Sciences category. GSC-14783-1



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This article first appeared in the January, 2007 issue of NASA Tech Briefs Magazine.

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