Abstract: The spectral absorption of a body, placed between a source for emitting electromagnetic radiation and a detector for detecting the radiation, is measured. The technique includes emitting the electromagnetic radiation in a determined spectral range towards the body, filtering the electromagnetic radiation, and detecting the electromagnetic radiation attenuated by the absorption due to the body and deducing therefrom a measure of the spectral absorption of the body. The filtering and detection steps are implemented by a detector of the quantum type. The technique also applies to determining the calorific value of a gas, for measuring the energy of electromagnetic radiation, and an apparatus for implementing the technique.

Abstract: The invention concerns a method for wavelength calibration of an electromagnetic radiation filtering device (16) included in an apparatus (10; 32) measuring spectral transmission of a propagating medium external to said apparatus and wherein said radiation flows through, said filtering device having spectral transmission tuneable on a wavelength range based on the value of a physical parameter. The invention is characterized in that it consists in: selecting at least one absorbing gaseous line which is always present in natural form in the propagation medium and whereof the corresponding wavelength is included in said filtering device tunability wavelength range; and in calibrating the filtering device with respect to said at least one absorbing gaseous line which is used as natural reference.

Abstract: The invention provides a method consisting successively in producing two electromagnetic radiations of the same wavelength that is modulated as a function of time over a range of wavelengths, a "measurement" one of said radiations passing through a gas that presents selective absorption for said radiation in the range, while the other radiation constitutes a "reference" radiation. Each of said radiations is transformed into a respective "measurement" or "reference" electrical signal. The measurement signal is divided by the reference signal to obtain a first quotient electrical signal which is in turn split into two portions: a time-varying AC portion and a DC portion that does not vary with time. The AC portion is divided by said DC portion in order to obtain a second quotient electrical signal representative solely of the electromagnetic radiation absorbed by the gas.

Abstract: An optical absolute pressure sensor constituted by a body made of a substance having photoelastic properties and provided with at least one internal duct extending parallel to the longitudinal direction of said body, the pressure inside said duct being substantially zero and the ends thereof being closed in sealed manner. The invention also applies to a method of making the sensor, to apparatus for implementing the method, and to a sensor constituting the product of the method.

Abstract: In a device for optical-cable transport and combination of light images, the ends of an optical cable forming part of the device are coupled with lenses which are in turn associated with an optical system for dispersing radiation as a function of the wavelength in order to suppress any raster effect. This optical system is constituted at one end of the optical cable by a reflecting holographic mirror, the chromatic aberrations of which form the dispersion. At the other end of the cable, the optical system consists of a partially transparent holographic mirror for reflecting a first image to be transported and re-forming this image by suppressing the aberrations and combining said first image with a second image. The first image can be a synthetic image displayed on a CRT indicator and the second image can be the surrounding landscape in an application such as a helmet sight visor.