In this work different patented optical sensing solutions are reviewed, mostly for their application of measuring fluid level within tanks, containers or bio-mass boilers in condominiums, thus providing an intrinsically safe sensing scenario within harsh and hazardous environments, due to the passive nature of light. In general, the intensity modulation of the light is very attractive since it is simple in concept, reliable, and can produce optical sensors which offer a wide range of applications at lower costs, thus facilitating their final commercialization and market spread. The optical sensing solutions discussed in this work include a fiber-optic sensor for fluid detection/level with intrinsic self-referencing property thus solving one of the main drawbacks when using intensity-based sensors, namely the undesirable and unexpected optical power fluctuations when operating. Moreover, a low-cost intensity-based POF sensor for fuel level measurements in paramotoring and powered paragliding is presented. It features a very simple and a very low-cost solution for these sports, attending the increasing demand of new and more sophisticated embedded sensors in the flight instrumental. Going on from this, a fiber-optic fluid level sensor for a range greater than 2m is described. The system uses a single lens and a single fiber but being possible to increase the measuring safety area by optical multiplexing. Finally, a fluid level sensor based on a Light Dependent Resistor (LDR), which includes a temperature control scheme, is presented. Its final embodiment design is also shown and described.