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The main topic of this thesis is 3-D measurement of absorbed dose in radiotherapy using advanced optical imaging methods. The main ideas behind the implementation presented in this thesis come from X ray computed tomography, 3-D microscopy and schlieren techniques. Every tumour has different shape and radiotherapy aims to conform the absorbed dose to tumour shape thus sparing healthy tissue as much as possible. If a complex treatment plan is tested on tissue-equivalent 3-D dosimeter prior to treating a patient then the quality of treatment can be improved. 3-D dosimeters (often called gel dosimeters) have been developed extensively over the last 20 years. They consist of radiosensistive chemicals evenly distributed across the volume of the supporting matrix (e. g. gelatin). They are designed with the readout technique in mind, which can be Magnetic Resonance Imaging, X ray computed tomography or optical computed tomography (optical-CT). This PhD project focuses on optical-CT readout. Optical-CT can be performed with either a laser light source coupled to a photodiode or broadbeam light source coupled to camera - most often a charged coupled detector (CCD). Both options have been either improved or designed from scratch. The main results are as follows: 1) detailed analysis of improved focusing optics of CCD based optical-CT shows telecentric focusing enables low noise measurements with a simple design; 2) characterization of CCD based optical tomography instrument (optical-CT) shows signal-to-noise ratio to be greater than 80:1 for 1mm voxel; 3) a fast and novel laser scanning architecture is demonstrated and characterized. Both of the instruments developed are a significant improvement to the field of optical-CT in 3-D dosimetry paving the way, hopefully, for a clinical application.