The possibility of humidity determination based on the use of spectral distributions in the wavelength range of up to 1 µm is analyzed. We discuss the use of an imaging hyperspectrometer for precision farming. A field experiment to determine the soil moisture index under the vegetation cover is described. A procedure of precise calibration of the hyperspectrometer based on the use of a tunable laser is described. We show the possibility of the practical determination of humidity based on the use of spectra in the wavelength range of up to 1 µm.

The paper presents results of the experimental investigation of “spectral” diffractive lenses the same focus position for several given wavelengths. Two spectral diffractive lenses designed to focus radiation of three and five specified wavelengths in the visible spectrum were investigated. Using a method of direct laser writing in photoresist with iterative correction of writing parameters, we fabricated a diffractive microrelief of the spectral lenses with the height deviation from the designed relief of less than 30 nm. Using a pinhole located at the focus of the fabricated lenses, we estimated the operation wavelengths. The point spread functions of the spectral lenses at the designed wavelengths were measured with the use of a tunable laser. The imaging properties of the spectral lenses were illustrated by the images of a reference color table.

We considered the feasibility of using a dual-range diffraction grating on the convex mirror in an imaging hyperspectrometer based on the Offner scheme. We conducted modeling in Zemax software. It is shown that the use of a dual-range grating allows one to use the hyperspectrometer based on the Offner scheme in two spectral ranges: the visible and mid-infrared.

The operation of the imaging spectrometer based on the axicon is modelled in the Matlab. The imaging spectrometer consists of an annular scanning slit, lenses and axicon. Simulation conducted for two schemes: non-imaging and imaging. We simulated and showed that it is possible to obtain a spectral resolution of 10 nm and a point spread function of 30-50 μm. We obtained the spectral distribution from a model object with parts in which the wavelength of monochromatic radiation took one of three values: 0.46 μm (blue), 0.54 μm (green), 0.70 μm (red).

We propose a novel type of spectral diffractive lenses that operate in the ±1-st diffraction orders. Such spectral lenses generate a sharp image of the wavelengths of interest in the +1-st and –1-st diffraction orders. The spectral lenses are convenient to use for obtaining remotely sensed vegetation index images instead of full-fledged hyperspectral images. We discuss the design and fabrication of spectral diffractive lenses for measuring vegetation indices, which include a Modified Red Edge Simple Ratio Index and a Water Band Index. We report synthesizing diffractive lenses with a microrelief thickness of 4 µm using the direct laser writing in a photoresist. The use of the fabricated spectral lenses in a prototype scheme of an imaging sensor for index measurements is discussed. Distributions of the aforesaid spectral indices are obtained by the linear scanning of vegetation specimens. Using a linear scanning of vegetation samples, distributions of the above-said water band index were experimentally measured.