This book describes the design and development of an instrument called the Solar Vector Magnetograph or SVM. This instrument maps the magnetic field of the solar active regions. The magnetic field of solar active regions is responsible for the eruptive phenomena on the sun which expel large amounts of high-energy particles and radiation in the interplanetary space posing danger for human activities in space. The careful measurement of solar magnetic field is very important to track its evolution which may be useful for forecasting the solar eruptive events. The instrument SVM was developed by Dr. Sanjay Gosain during his Ph.D. at Udaipur Solar Observatory, India for which he received best thesis award by Astronomical Society of India. This book describes how the high-cadence magnetic field measurements can be done by using a tunable Fabry-Perot etalon and a polarimeter. This book is useful for any solar astronomer who wishes to develop an instrument for measuring solar magnetic fields or wishes to understand how solar magnetic field is measured using Zeeman-effect diagnostics. The techniques of optical polarimetry is also discussed in depth.