Over the last decade, portable, active optical sensors (AOS) have become tools in production agriculture both for mapping crops and soils and also for applying agrochemicals. These sensors are often referred to as “proximal” active optical sensors, in recognition of their deployment at sensor-target distances in the order of meters. Unfortunately most users have little understanding of the underlying physics (optics) or construction of these sensors. This paper sets out to document the fundamental electro-optical principles by which these devices operate and to document the mathematical rules governing the use of data produced by these devices. In particular, emphasis is placed on the inverse-square law of optics and how it affects AOS measurements. The basis for utilizing sensor measurements in more complex mathematical functions is also presented, including manipulation of the individual wavelength response data to derive information regarding the distance variation between the sensor and the target.