An optical detector device (105, 230, 700), comprising: a fixed angle multispectral sensor; one or more memories; and one or more processors, communicatively coupled to the one or more memories, configured to: receive spectral data from a spectroscopic measurement from the fixed angle multispectral sensor, wherein the spectroscopic measurement is performed across a range of wavelengths; determine, based on the spectral data from the spectroscopic measurement, a particulate size of a particulate; determine, using a spectroscopic classification model, a classification of a spectrum of the particulate based on the spectroscopic measurement and determine the identification of the type of the particulate based on the classification of the spectrum of the particulate; determine, based on the particulate size and the identification of the type of the particulate, that an alert condition relating to a fire is satisfied; and trigger an alert based on determining that the alert condition is satisfied characterised in that the one or more processors, when determining that the alert condition is satisfied, are to: determine, based on the particulate size and the identification of the particulate, that the particulate is smoke-based particulate; and determine that the smoke-based particulate is a first type of smoke-based particulate that satisfies the alert condition rather than a second type of smoke-based particulate that does not satisfy the alert condition. a fixed angle multispectral sensor; one or more memories; and one or more processors, communicatively coupled to the one or more memories, configured to: receive spectral data from a spectroscopic measurement from the fixed angle multispectral sensor, wherein the spectroscopic measurement is performed across a range of wavelengths; determine, based on the spectral data from the spectroscopic measurement, a particulate size of a particulate; determine, using a spectroscopic classification model, a classification of a spectrum of the particulate based on the spectroscopic measurement and determine the identification of the type of the particulate based on the classification of the spectrum of the particulate; determine, based on the particulate size and the identification of the type of the particulate, that an alert condition relating to a fire is satisfied; and trigger an alert based on determining that the alert condition is satisfied receive spectral data from a spectroscopic measurement from the fixed angle multispectral sensor, wherein the spectroscopic measurement is performed across a range of wavelengths; determine, based on the spectral data from the spectroscopic measurement, a particulate size of a particulate; determine, using a spectroscopic classification model, a classification of a spectrum of the particulate based on the spectroscopic measurement and determine the identification of the type of the particulate based on the classification of the spectrum of the particulate; determine, based on the particulate size and the identification of the type of the particulate, that an alert condition relating to a fire is satisfied; and trigger an alert based on determining that the alert condition is satisfied characterised in that the one or more processors, when determining that the alert condition is satisfied, are to: determine, based on the particulate size and the identification of the particulate, that the particulate is smoke-based particulate; and determine that the smoke-based particulate is a first type of smoke-based particulate that satisfies the alert condition rather than a second type of smoke-based particulate that does not satisfy the alert condition. determine, based on the particulate size and the identification of the particulate, that the particulate is smoke-based particulate; and determine that the smoke-based particulate is a first type of smoke-based particulate that satisfies the alert condition rather than a second type of smoke-based particulate that does not satisfy the alert condition.