This paper presents the description and the characterization of the gonio-spectrophotometer GEFE (the acronym for ‘Gonio-EspectroFotómetro Español’). This device has been designed and built for the low-uncertainty absolute measurement of the bidirectional reflectance distribution function (BRDF). It comprises a fixed, collimated and uniform light source, a six-axis robot-arm to rotate the sample and a spectroradiometer that may revolve around the sample to be able to vary the source-to-detector angular separation. This gonio-spectrophotometer makes it possible to perform spectral measurements in the visible range, both inside and outside the incidence plane, as well as measurements in retroreflection conditions. This fully automated system is able to measure autonomously a sample's complete spectral BRDF (comprising around 1000 different angular configurations) in less than 4 h.

A new expression for the internal quantum efficiency of a photodiode is presented. It is obtained from the analysis of the photocurrent generated within the diode, considering the power and the cross-sectional diameter of the incident beam. The model explains variations of the internal quantum efficiency with irradiance that are not explained by other existing models, although this experimental fact was already known. The well-known phenomenon of supraresponsivity is also explained with this model. Finally, we show the dependence of the internal quantum efficiency on the variables involved in the model.

This paper describes an experimental setup, based on an optical-fibre system, to calibrate infrared photodiodes at 1300 nm and 1550 nm by comparison with a cryogenic radiometer. An integrating-sphere radiometer used as a transfer standard for optical-fibre power measurements has been characterized and calibrated, obtaining an uncertainty of less than 0.1% at these wavelengths.

An experimental set-up designed for the calibration of near-infrared detectors against a cryogenic radiometer, using a tuneable laser diode as source, is described. A complete characterization of the cryogenic radiometer has been performed at the Instituto de Fisica Aplicada (IFA, Madrid), at wavelengths of 1300 nm and 1550 nm. Two different secondary standards, an electrically calibrated pyroelectric radiometer and an InGaAs-based integrating-sphere radiometer, have been calibrated with relative uncertainties of less than 10−3 at these wavelengths.

Abstract:The Bidirectional Reflectance Distribution Function (BRDF) is essential to characterize an object's reflectance properties. This function depends both on the various illumination-observation geometries as well as on the wavelength. As a result, the comprehensive interpretation of the data becomes rather complex. In this work we assess the use of the multivariable analysis technique of Principal Components Analysis (PCA) applied to the experimental BRDF data of a ceramic colour standard. It will be shown that the result may be linked to the various reflection processes occurring on the surface, assuming that the incoming spectral distribution is affected by each one of these processes in a specific manner. Moreover, this procedure facilitates the task of interpolating a series of BRDF measurements obtained for a particular sample. 192-198 (1977). 9. E. P.