Darkvision and objects of varying transparency.

[Viridilucens]

So I've recently come across a problem in one of my games. A player of mine, whose character possesses darkvision, desperately wants to wear a pair of darkened because they're his only head item with an enchantment. That's not necessarily pertinent to the question I'll be posing, but I figured I'd let you know what spurred it.

My player says he should be able to see through the goggles with his darkvision, as they are transparent to some degree (they allow only very bright light through, like welding goggles). However, wearing these goggles, for a player without darkvision, would obstruct almost all visual input, unless everything were to suddenly glow very brightly. Would these goggles, or any darkened or clouded glass or otherwise barely transparent material, block darkvision? Along with this, would something like sunglasses or equivalent interfere with darkvision, making their black-and-white sight "dimmer"?

[Kayerloth]

Bestiary, Universal Monster Rules wrote:

Darkvision (Ex or Su) A creature with darkvision can see in total darkness, usually to a range of 60 feet. Within this range the creature can see as clearly as a sighted creature could see in an area of bright light. Darkvision is black and white only but otherwise like normal sight.

Based in part on the above I'd say I would treat the goggles as if worn by a creature with 'normal' vision and the Darkvision would function albeit sight would be in grayscale instead.

Consider that Darkvision functions even where essentially no light exists also tends to make me think the amount of light (bright vs normal vs dimmed) isn't related to how it would function owing to the 'light shielding' involved. It would just be in grayscale vs any color scale.

But ultimately I don't think there are any rules, certainly none I'm aware of concerning this question specifically and therefore it's really up to you/the DM. Might base it off whether you consider his Darkvision Ex or Su in nature with Su Darkvision less likely to be effected at all by whatever the amount of "normal" light present. The field of vision allowed might be more of an inhibition to vision than the light coming through the lenses depending on the design of the goggles.