A Multiple Direction Radiation Sensor, DIRAM

Abstract

The Directional Radiance Distribution Measurement (DIRAM) device was designed and built to determine the angular distribution of shortwave radiance as a function of height in cloudy and clear-sky conditions at various surface albedos. The construction contains 42 sensors, consisting of a collimation system and a detector, which are mounted in two domes (21 in each). The collimators are made of solid PERSPEX cylinders, 8 mm in diameter and ?24 mm long, which are fully transparent in the visible range. Three diaphragms are carved in each cylinder to reduce the field of view to ?7° and to reduce the stray light. The detector is a commerially available silicon photodiode for the visible spectral range. The domes can be placed at ground level, or on top and below a research aircraft. The collimators collect broadband visible radiation in 42 regularly distributed viewing angles. The aperture of each sensor is about 7°. The 42 signals are continuously stored by a datalogger with variable frequency up to 10 Hz. The angular dependence of solar radiation scattered from clouds (or the earth's surface) can be determined. The purpose is to collect data on the scattering of solar radiation by clouds (and surface) in order to gain more insight in the shortwave atmospheric radiation budget. Optical properties such as angular discrimination of the collimators and sensitivity of the device were investigated and summarized. During a test flight the operational aspects were investigated. It appeared that the device was able to collect data without interruption in severe conditions (vibrations, strong temperature, and humidity changes).