Helmholtz was the son of a gymnasium headmaster, Ferdinand Helmholtz, who had studied classical philology and philosophy, and who was a close friend of the publisher and philosoper Immanuel Hermann Fichte. Helmholtz's work is influenced by the philosophy of Johann Gottlieb Fichte and Kant. He tried to trace their theories in empirical matters like physiology.
As a young man, Helmholtz was interested in natural science, but his father wanted him to study medicine because there was financial support for medical students. His first important scientific achievement, an 1847 physics treatise on the conservation of energy was written in the context of his medical studies and philosophical background. He had discovered the principle of conservation of energy while studying muscle metabolism. He tried to prove that no energy is lost in muscle movement, because this also meant that there were no vital forces necessary to move a muscle. That was a rejection of the speculative tradition of Naturphilosophie which was at that time a dominant philosophical paradigm in German physiology.
In 1851 he invented the ophthalmoscope, an instrument which can be used to look into the human eye. Helmholtz's interests at that time were increasingly focused on the physiology of the senses. His main publication was the Handbuch der Physiologischen Optik (Handbook of physiological optics). During the second half of the nineteenth century it was the fundamental reference work in this field. The handbook provided empirical theories on spatial and color vision.
Helmholtz continued to work for several decades on several editions of the handbook. The work was frequently updated because of his dispute with Ewald Hering who held opposite views on spatial and color vision. This dispute divided the discipline of physiology during the second half of the nineteenth century.
In 1863 Helmholtz published a book called On the sensations of Tone as a Physiological Basis for the theory of music once again demonstrating his interest in the physics of perception. This book influenced musicologists into the twentieth century.
The sensory physiology of Helmholtz was the basis of the work of Wilhelm Wundt, a student of Helmholtz, who is considered one of the founders of experimental psychology. He, more explicitly than Helmholtz, described his research as a form of empirical philosophy and as a study of the mind as something separate. Helmholtz had in his early refutal of the speculative early nineteenth century tradition of Naturphilosophie stressed the importance of materialism, and was focusing more on the unity of "mind" and body.
In 1871 Helmholtz moved from Bonn to Berlin to become a professor in physics. He became interested in electromagnetism. Although he did not make any major contributions, his student Heinrich Rudolf Hertz became famous as the first to demonstrate electromagnetic radiation.
Helmholtz wrote about many topics ranging from the age of the earth to the origin of the planetary system.
Reference: Hermann von Helmholtz and the foundations of Nineteenth century Science Los Angeles: University of California Press, 1994.