I've seen a lot of folks saying that closed emitter optics are better for preventing lens fogging than open ones, due to the sealed nitrogen-purged nature of the closed design. But, the point of the sealing/nitrogen-purging is to remove moisture from inside the optic and prevent it from entering, therefore preventing internal fogging of the optic. Internal fogging pretty much means the optic is trashed, because there's not enough airflow for the moisture to easily escape. On an open emitter design, there is no internal portion to the optic, so no internal lens fogging concerns.
However, to my understanding the sealed designs should do nothing to prevent external lens fogging. If your glass is cold and you go to a warmer, moister environment, (or breathe directly on it) you will get external lens condensation and thus fogging. I've had no problem inducing external fogging on binoculars or scopes in cold weather just by accidentally breathing on them. Aimpoint literally instructs you to fog up the lens before wiping it with a cloth to clean it, so I don't see sealed optics as being especially anti-fog.
I know there are other potential issues with open designs in re blocking of the emitter or water on the lens, but fogging should not be worse, right? Yet, I'm seeing that everywhere at the moment, and feel like I'm taking crazy pills. Maybe it's because the same amount of fogging is less of an issue for closed designs, because the emitter is not projecting onto a fogged lens?