An x-ray source includes a carbon nanotube FE (field emission) cathode that is heated with laser light propagated down a fiber optic cable. The resulting two-terminal x-ray tube allows for independent control of tube voltage and tube current. Control of the tube voltage is typically achieved using a control electrode, whereas the field emission tube current is controlled by thermal modulation. A technique is provided for accurately measuring beam current in the x-ray source. The method includes activating the cathode and adjusting the laser intensity, until a desired dose of output x-ray radiation is attained, then measuring the total x-ray tube current. The cathode is then turned off, so that the electron beam is turned off. The total x-ray tube current is measured again. The cathode is turned on again, immediately thereafter. The difference between the two values of the total x-ray tube current gives the correct x-ray tube beam current.
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