This paper presents the method, implementation and validation of a borescope probe design tool devised for the challenges of optical fluid measurement techniques. The design tool is capable of predicting the path and power distribution of the laser beam through the probe and into the region interest, ensuring a cost and time-efficient design process that removes the need for experimental trials. The associated code is available as supplementary material. Optical measurement techniques have become established methods within fluid dynamics research. In contrast, their application to turbomachinery rigs is usually limited due to the restricted optical access. A small number of studies have circumvented this problem by employing borescopes to introduce the laser beam into the measurement region but wider application is inhibited because these probes are difficult to design, expensive and usually require several iterations until a suitable design is achieved. The first part of the paper presents the structure of the software program and the mathematical modelling of the optics for predicting the beam path into the measurement region. The second part presents different design options and the manufacture of a typical probe with validation in a wind tunnel facility using volumetric velocimetry. The borescope results agree very well with measurements acquired using direct illumination through a window demonstrating the efficacy of the method.