What is the difference between light red (635 nm) and red (650 nm)?

Both wavelengths appear red to the eye, but light red (635 nm) sits closer to the peak sensitivity of the human retina than red (650 nm). Specifically: 635 nm light appears roughly 2× brighter than 650 nm light at the same optical power. Practical effect: a 20 mW light-red module looks visually comparable to a 40 mW red module – with a lower laser class and longer diode service life. Light-red modules are slightly more expensive to manufacture but pay off in brighter environments or where a lower laser class is required.