To measure the concn. of a body contained in a gas, such as water vapour in the atmos., the gas is first irradiated with a radiation having a sufficient wave length to photodissociate the body to be measured. At least one of the prods. of photodissociation is then photometered to determine its concn. which is proportional to the concn. of the body to be measured. Pref. the wave length used is sufficient to produce one of the products of photodissociation in an excited state; the photometry consisting in the measurement of the intensity of the light emitted when the product relaxes to the ground state. When the product is not produced in the excited state, the measurement may be by absorption photometry. System makes it possible to measure accurately the concn. in a gas of a body which does not have an absorption spectrum easily distinguished from other absorption spectra.
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