Investigation of a spectral lens for the formation of a normalized difference vegetation index NDVI0.705

Abstract

Subject of study. A new type of diffractive optical element separating radiation at the wavelengths of 705 and 750 nm into the ±1 diffraction orders is investigated as a basis for a sensor for the detection of the normalized difference vegetation index NDVI0.705. Method. The study was performed as a natural experiment using a white light source with a continuous spectrum. Main results. A diffractive optical element separating radiation at the wavelengths of 705 and 750 nm into the ±1 diffraction orders was investigated. Using this optical element allows index images to be obtained directly instead of through the complex acquisition of a full hyperspectral image for the detection of NDVI0.705 in the region of a near-infrared slope. The diffractive optical element was fabricated using direct laser writing in a photoresist. The height of the formed microrelief was 4 µm, which implies that a combination of diffractive and refractive properties of this element contribute to the image formation process. The suggested term for these diffractive optical elements is a spectral diffractive lens. A laboratory setup was assembled based on the fabricated spectral diffractive lens to confirm the possibility of operating this lens in the imaging mode. In first experiments, a high-power halogen lamp behind an opaque screen with holes with diameters of 0.4–0.5 µm (individual and in linearly positioned groups) was used to simulate small-scale radiating objects. Two diffraction orders were recorded on a light-sensitive matrix, namely, −1 for radiation with a wavelength of 705 nm and +1 for radiation with a wavelength of 750 nm. The similarity of the geometrical parameters of the images of the group of sources in the +1 and −1 orders confirmed the possibility of using this spectral diffractive lens in the imaging mode. Therefore, a slit aperture was mounted instead of the linearly positioned sources in the next experiment. Scanning of a section of a color table with subsequent assembly of an index image was performed through this aperture. Practical significance. The spectral diffractive lens proposed in this work can be used to create simple compact sensors for the real-time monitoring of vegetation cover and for specialized agricultural equipment.