 After bacteriorhodopsin, halorhodopsin may be the best type I (microbial) opsin studied.Peak absorbance of the halorhodopsin retinal complex is about 570 nm.Just as the blue-light activated ion channel channelrhodopsin-2 opens up the ability to activate excitable cells (such as neurons, muscle cells, pancreatic cells, and immune cells) with brief pulses of blue light, halorhodopsin opens up the ability to silence excitable cells with brief pulses of yellow light.