1. A welding controller for a spot welder having a welding tip for contacting a workpiece and injecting an electric welding current into the workpiece to produce a spot weld and having a control signal input for receiving a signal for controlling the magnitude of the welding current comprising a well located in the interior of said welding tip, a thermographic phosphor disposed in said well in thermal contact with said welding tip so that the intensity of phosphorus radiation emitted by the phosphor when it is illuminated is a measure of the temperature of the phosphor, a multi-filament fiber optic cable having a trunk portion extending into said well in close proximity to said phosphor and a plurality of branches extending outwardly therefrom, a phosphor illumination source disposed adjacent the end of a first one of said branches for supplying radiation for illuminating said phosphor, first radiant-energy sensor means disposed adjacent a second one of said branches responsive to radiant-energy in a first wavelength range for producing an output signal, second radiant energy sensor means disposed adjacent the end of a third one of said branches responsive to radiant energy in a second wavelength range for producing an output signal, memory means adapted to contain data indicative of the desired thermal pattern for a weld and electronic circuit means operatively associated with said first and second sensor means and said memory means for comparing the output signals of said first and second sensor means with the data in said memory means to provide a control signal for controlling the magnitude of the welding current.