1. A method of obtaining radiation transmission data from an object comprising the steps of: providing a radiation source and a radiation detector system spaced therefrom to define a scanning zone therebetween, the detector system being capable of detecting and collecting spectroscopically resolvable information about incident radiation in that the detector system comprises at least one detector that exhibits a spectroscopically variable response allowing spectroscopic information to be retrieved; collecting one or more datasets of intensity information about radiation incident at the detector system and hence transmissivity of the object in the scanning zone at least one scanning position, from radiation transmitted through the object and received at the detector system; resolving each said intensity dataset across at least three frequencies or frequency bands within a spectrum of the source to produce an intensity data item for each frequency or frequency band; comparing the intensity data items numerically to a mass attenuation data library storing mass attenuation data for a plurality of expected constituent component materials of the object in the scanning zone to fit each intensity data item to a relationship given by the exponential attenuation law: 
 I/I o =exp[−(μ/ρ)ρ t] in respect of the constituent component materials and derive therefrom an indication of relative proportions of each component material in a transmission path producing the said intensity data items.