Optical device for detecting volatile compounds (A, B, C), characterized in that it comprises: - a sensitive reflecting element the reflected light intensity of which varies as a function of the volatile compounds (A, B, C) contained in an atmosphere to be tested and on their concentration, the sensitive reflecting element comprising: ∘ a substrate, ∘ at least one sensitive layer that is transparent between 400 nm and 1000 nm and porous, said sensitive layer being configured to allow volatile compounds (A, B, C) contained in the atmosphere to be tested to adsorb and desorb, and ∘ an electrically conductive layer sandwiched between the substrate layer and the sensitive layer and placed directly in contact with the substrate layer and with the sensitive layer, said electrically conductive layer being configured to allow the sensitive layer to be heated by Joule heating in order to allow the volatile compounds (A, B, C) adsorbed on the sensitive layer to desorb, ∘ at least one monochromatic or quasi-monochromatic light source placed to illuminate the sensitive layer at an incident angle (α), ∘ at least one light detector configured to measure the light intensity reflected by the sensitive reflecting element at a detection angle (β), the electrically conductive layer being transparent and non-scattering between 400 nm and 1000 nm so as not to adversely affect the light intensity reflected by the sensitive reflecting element at the detection angle, and ∘ a computing and processing unit configured to determine the volatile compounds (A, B, C) present in the atmosphere to be tested depending on their desorption temperatures and their concentration via variation in the light intensity reflected by the sensitive reflecting element. - a sensitive reflecting element the reflected light intensity of which varies as a function of the volatile compounds (A, B, C) contained in an atmosphere to be tested and on their concentration, the sensitive reflecting element comprising: ∘ a substrate, ∘ at least one sensitive layer that is transparent between 400 nm and 1000 nm and porous, said sensitive layer being configured to allow volatile compounds (A, B, C) contained in the atmosphere to be tested to adsorb and desorb, and ∘ an electrically conductive layer sandwiched between the substrate layer and the sensitive layer and placed directly in contact with the substrate layer and with the sensitive layer, said electrically conductive layer being configured to allow the sensitive layer to be heated by Joule heating in order to allow the volatile compounds (A, B, C) adsorbed on the sensitive layer to desorb, ∘ at least one monochromatic or quasi-monochromatic light source placed to illuminate the sensitive layer at an incident angle (α), ∘ at least one light detector configured to measure the light intensity reflected by the sensitive reflecting element at a detection angle (β), the electrically conductive layer being transparent and non-scattering between 400 nm and 1000 nm so as not to adversely affect the light intensity reflected by the sensitive reflecting element at the detection angle, and ∘ a computing and processing unit configured to determine the volatile compounds (A, B, C) present in the atmosphere to be tested depending on their desorption temperatures and their concentration via variation in the light intensity reflected by the sensitive reflecting element. ∘ a substrate, ∘ at least one sensitive layer that is transparent between 400 nm and 1000 nm and porous, said sensitive layer being configured to allow volatile compounds (A, B, C) contained in the atmosphere to be tested to adsorb and desorb, and ∘ an electrically conductive layer sandwiched between the substrate layer and the sensitive layer and placed directly in contact with the substrate layer and with the sensitive layer, said electrically conductive layer being configured to allow the sensitive layer to be heated by Joule heating in order to allow the volatile compounds (A, B, C) adsorbed on the sensitive layer to desorb, ∘ at least one monochromatic or quasi-monochromatic light source placed to illuminate the sensitive layer at an incident angle (α), ∘ at least one light detector configured to measure the light intensity reflected by the sensitive reflecting element at a detection angle (β), the electrically conductive layer being transparent and non-scattering between 400 nm and 1000 nm so as not to adversely affect the light intensity reflected by the sensitive reflecting element at the detection angle, and ∘ a computing and processing unit configured to determine the volatile compounds (A, B, C) present in the atmosphere to be tested depending on their desorption temperatures and their concentration via variation in the light intensity reflected by the sensitive reflecting element.