My personal take (that is, I'm making this up, not reading it somewhere) on this issue is this:
Wherever possible, the technology works by overlaying the new information over a real, optical image in a semi-transparent fashion. When this is not possible, it is displayed as an AR image screen. Based on that, I have broken down the vision enhancements into three categories:
No modifiers (works just like normal vision): - Smartlink
- Flare Compensation
- Image Link (Assuming it doesn't cover the target in a window)
- Optical Magnification (Binoculars, scopes and some goggles only!)
Modifiers exactly cancel bonus - Low Light
- Thermographic
- Ultrasound
Can not cast spells while active - Vision Magnification (electronic, this is the standard)
Undecided My rational for these categories is this:
For no modifiers, this technology does not obscure in any way natural vision (except maybe to put a targeting retecicle on it that is easily identified.)
For the category where modifiers exactly cancel the bonuses, I figure this works from both a logic standpoint (the more obscured the target is, the more the digital image enhancement covers it with its "updated" information) and a game balance standpoint (the technology can be used by spell caster, but does not affect spell casting.) The basic idea is that if the caster would normally suffer, for example, a -2 low-light penalty, and the low-light goggles remove that penalty, that spell casting is penalized by a -2 "vision clouded by technical enhancement" penalty. One more thing to point out about this category is if the modifiers reach -6, the character no longer has line of sight -- that is, if the only reason they can see the target at all is because of the thermographic enhancement, then they can't really see the target.
For the category where spell casting is impossible, I figure that vision magnification would pretty much have to overlay an AR window on your normal vision, thus blocking out normal vision. The exception to this is binoculars, scopes, and goggles, which do use real lenses. I figure such real optical equipment is probably a bit more rare than the cheaper, smaller, lighter electronic versions. Glasses and contact lenses are unlikely to have the necessary amount of room for optical magnification, and would therefore have to use electronic.
As for the undecided category, well frankly I have no idea how "image enhancement" would work. I lean one of two ways -- either applying no penalty (the overlay is not significant, an expert system simply uses icons and other tools to draw your attention to relevant looking details) or a negative modifier equal to the rating (the system overlays a sharpened image on top of the real image.)
In any case, assuming you have 'trodes or something, and either skinlink or wireless enabled, you could change the modes (or turn the device off) as a free action using DNI. This would be nearly identical to the "everything that gives a bonus has a penalty equal to that bonus" category, except it requires a free action.