638nm vs 637nm (single mode vs multimode)

[Soulstorm]

Another thing to consider is not just final beam specifications at full power, which are a little better for SM, particularly at higher power levels, long distance projections outdoors, large events where the beam has a long path etc, but the matter of the constitution of beam profile and colour mixing as a result. Most projectors don't just use 100% modulation at all times, especially for those fades + blends, this is where SM pays for itself in professional use, MM is not bad but side by side you can definitely see a difference in most applications.

A good SM build will deliver breathtaking mixes, with an extremely sharp red, to the point I was amazed it was so bright compared to green, on multiple balanced set ups, across the power spectrum.

There is a reason the top manufacturers go to the hassle of building SM reds. And the really good ones, if treated with respect, will stay perfectly aligned for an extremely long time.

[buffo]

Keep in mind that red scatters less in haze and that combined with the lower visual sensitivity really hits red hard compared with green.

This ^^^^ is the key. The human eye is much, much less sensitive to red than it is to green (or blue). A good rule of thumb is to call it roughly 5 times more sensitive to green verses red. But of course, there is a lot of variation based on the wavelength of red you use, and if you go below 640 nm, the sensitivity falls off like a cliff! (Example: 650 nm red is 8 times dimmer than 532 nm green at the same power, while 671 nm red appears over 26 times dimmer than 532 nm green at the same power!)

If you want to light the room up with red, you need a lot of it.

Adam

Edit: Handy chart to have if you want to do the calculations yourself:

RELATIVE SPECTRAL SENSITIVITY OF THE EYE
λ (nm)	Photopic	Scotopic	λ (nm)	Photopic	Scotopic	λ (nm)	Photopic	Scotopic	λ (nm)	Photopic	Scotopic
380	0.000039*	0.000589	480	0.139020	0.793000	580	0.870000	0.121200	680	0.017000	0.000072
390	0.000120*	0.002209	490	0.208020	0.904000	590	0.757000	0.065500	690	0.008210	0.000035
400	0.000396*	0.009290	500	0.323000	0.982000	600	0.631000	0.033150	700	0.004102	0.000018
410	0.001210*	0.034840	510	0.503000	0.997000	610	0.503000	0.015930	710	0.002091	0.000009
420	0.004000*	0.096600	520	0.710000	0.935000	620	0.381000	0.007370	720	0.001047	0.000005
430	0.011600*	0.199800	530	0.862000	0.811000	630	0.265000	0.003335	730	0.000520	0.000003
440	0.023000*	0.328100	540	0.954000	0.655000	640	0.175000	0.001497	740	0.000249	0.000001
450	0.038000*	0.455000	550	0.994950	0.481000	650	0.107000	0.000677	750	0.000120	0.000001
460	0.060000	0.567000	560	0.995000	0.328800	660	0.061000	0.000313	760	0.000060	0.000000
470	0.090980	0.676000	570	0.952000	0.207600	670	0.032000	0.000148	770	0.000030	0.000000

*according to CIE (Commission Internationale de L'Eclairage - International Commission on Illumination) 1931 V(λ) function. In the modified function from 1978 (Judd and Vos), sensitivity assigned to wavelengths shorter than 460nm is somewhat higher:
450nm-0.0468, 440nm-0.0379, 430nm-0.0273, 420nm-0.0175, 410nm-0.0074, 400nm-0.0028, 390nm-0.0008, 389nm-0.0002