A short communication to the International Journal of Optoelectronics 7(3):449-452 (1992), by Spooncer et al, entitled “A humidity sensor using a wavelength-dependent holographic filter with fibre optic links”, describes the response of gelatin-based Bragg reflection holograms to ambient humidity. It concludes that optical response to an increasing and decreasing cycle of humidity shows a hysteresis which limits its industrial application as a sensor.
WO-A-9526499 discloses a holographic sensor, based on a volume hologram. This sensor comprises an analyte-sensitive matrix having an optical transducing structure disposed throughout its volume. Because of this physical arrangement of the transducer, the optical signal generated by the sensor is very sensitive to volume changes or structural rearrangements taking place in the analyte-sensitive matrix as a result of interaction or reaction with the analyte. For example, a sensor comprising a gelatin-based holographic medium may be used to detect trypsin. Trypsin acts on the gelatin medium, irreversibly destroying the integrity of the holographic support medium.