Green lasers are in general preferred to red diode lasers, primarily because even
the best red diodes operate at a far longer wavelength than green lasers. The
best red diodes operate near 635 nm, close to the main laser wavelength of
632.8 nm emitted by legacy Helium-Neon gas lasers, which in many applications
were replaced by red diode lasers.
The Figure above shows the photopic relative human eye response curve, that
applies for daylight and indoor lighting conditions. At 635 nm, the response is
about 20% of that at the peak of about 545 nm. A green diode-pumped solid-
state (DPSS) laser typically operates at about 532 nm, or about 90 % of the peak
value. Thus under daylight and indoor lighting conditions, a green DPSS laser
appears to be 4.5 times brighter than a red diode laser operating at 635 nm. This
is the primary why DPSS green lasers have largely supplanted red diodes in
many applications. In many cases, the red diode laser operates at even longer
wavelengths to reduce cost. For a wavelength of 670 nm for example, the green
laser visibility advantage is about a factor of 18.