Brightness is a complicated question for several reasons:
- Eye sensitivity as a function of wavelength depends on whether the eye is adapted for light conditions (photopic vision), dark conditions (scotopic vision) or somewhere in the middle (mesopic vision)
- It depends on the spectral interaction of the laser with the surface on which you're projecting
- It depends on contrast with the environment, both in terms of brightness and color
- The mixing of monochromatic light sources (very sharp peaks in the spectrum) does not necessarily work the way that the mixing of broader band sources does
- The brightness response of the human eye is nonlinear
- The divergence of the laser will affect the irradiance/illuminance