Radiometric Measurement System (RAMS)


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Optics: We employed very simple optical arrangement for the radiometer. The diffuser-light trap arrangement provides a hemispherical field of view with incident radiation being collimated by the high reflectance walls of the exponential-logarithmic cavity. Enough collimation of the radiation is achieved with this design that narrow spectral bandpass interference filters can be used to select desired wavelength regions.

Electronics: The instrument electronics includes five major functional blocks. They are the detectors signal conditioning block, the data processing block, the system controller block, the shadow ring drive and control block, and the data storage block.

The signal detectors are silicon photodiodes operating in the photovoltaic mode and covering the spectral range from about 0.3 to 1.1µm. Their signals are converted into electrical voltages by low noise FET input operational amplifiers. Programmable gain amplifiers allow adjustments for dynamic range, and filter circuits condition the signals for analog to digital processing. Data processing units consist of an analog multiplexer circuit, a sample-and-hold circuit, and an analog to digital converter providing a 12-bit resolution output. The shadow ring is driven by a DC motor rotating at a constant speed. A motor controller is used to maintain motor speed. The system controller provides the timing necessary to perform all the system's tasks. It sets the shadow ring in motion and steps through the detector's outputs, maintaining the proper dynamic range for the analog to digital converter by selecting the proper amplifier gain. It also controls the analog to digital conversion and selectively stores data.

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