A security device for a means of transport comprising a radiation source to direct radiation into the eye of a person intending to take control of the means of transport, so that the radiation is internally reflected within the eye and emerges from the eye with a substantial spectral content corresponding to the spectral content of food flowing through the retina of the eye, and a receiver arrangement to receive the radiation emanating from the eye of the person and to perform a spectral analysis of the reflected radiation to determine the concentration of at least one chemical within the blood of the person, the device further comprising an alarm unit to generate an alarm and/or an immobilizer unit to immobilize the means of transport, the or each unit being responsive to the said determination of concentration when the concentration of the said chemical is outside a permitted threshold range. Preferably, radiation is transmitted to the retina, and the spectral analysis is carried out for radiation reflected from the retina.