Portable PVS-02 spectrometer for transfer of the spectral radiance scale in the 0.4–2.5 μm range

Abstract

We describe selection of an optical layout and calculation of the spectral sensitivity and measurement uncertainty in a portable spectrometer. The spectrometer is used for transfer of the brightness scale in the 0.4-2.5 μm range from one brightness reference standard to another.Introduction. For remote measurements in the optical range, there is a need for uniform calibration of the energy scales for spectral and multispectral instrumentation [1]. Such calibration is done on metrological test benches, where wide-angle radiation sources (photometric spheres) are used as the working standard for the spectral radiance scale. The exit aperture of such a sphere should span the aperture of the entrance objective lens of the survey system. The diameter of the entrance aperture for survey systems used for remote sensing of the Earth with high spatial resolution reaches 1 m or more. For calibration of such systems, photometric spheres are required with a corresponding size of the exit aperture. Calibration in turn of spheres of diameter ~2 m directly on existing certified metrological test benches is not always possible. The need arises for transfer of the spectral radiance scale from the radiation source of a certified metrological test bench to a large photometric sphere.Comparators are used for calibrating working standards for the spectral radiance scale according to the GOST 8.195-89 scheme [2]. Generally stationary monochromators are used as comparators, such as the MDR-23 with a system for detecting radiation after the exit slit of the monochromator. However, for comparison of spatially separated radiation sources, we need a compact portable spectrometer. With this aim, a portable PVS-02 spectrometer was built at the A. N. Sevchenko Institute of Applied Physical Problems at Belorussian State University (NII PFP BGU) with working spectral range 0.4-2.5 μm. Using the PVS-02, the spectral radiance scale is transferred from a diffuse emitter (photometric sphere with exit aperture 240 mm in the Kameliya-M metrological system) to a photometric sphere with large exit aperture (500-1000 mm). Cooled radiation detectors are used in the PVS-02 to improve the stability and fidelity of reproduction of the spectral radiance scale.Optical Layout of the Spectrometer. A single detector is not capable of providing detection of radiation in the range 0.4-2.5 μm. Two radiation detectors make it possible to completely span the given range: detectors based on silicon and InGaAs. In this case, the need arises to use two polychromators, arbitrarily called the visible range polychromator (VisPoly) and the IR range polychromator (IRPoly).The optical layouts of the visible and IR polychromators are given in Fig. 1. Projection objective lenses focus the light flux from the radiation source onto the entrance slits of the polychromators. The entrance slits of the polychromators are "knife-edge" type. The rotating plane mirrors are designed to deviate the optic axes of the polychromators, with the aim of reducing the external dimensions of the i...