In Light Detectors, Photoreceptors, and Imaging Systems in Nature, I explore some of the many ways light is intimately linked with life. Light is necessary for photosynthesis and vision, as well as the photobehavior of plants and animals. The reception of light is important to our health and well-being. Various wavelengths of light are being used as a tool for diagnosing diseases and to restore health. Therefore, I begin with the physical properties of light and the electromagnetic spectrum of energies, including the solar spectrum that reaches the Earth, which spans the ultraviolet, the visible, and the infrared—wavelengths from near 200 nm to about 950 nm. The effective energies for biological photoprocesses are generally limited to the visible wavelengths from around 340 nm to 780 nm. Many of the photobiological phenomena that are discussed in this book occur within these wavelengths and center around 500 nm, the solar energy peak in the blue-green.