[Ahmad Abdel]

I've seen a lot of mamluk,ottoman,etc... helmets most of them have a mail aventail, what are the disadvantages of a visor ,and when is it better to use a helmet without a visor?

[Dan Howard]

All of the myths about armour being hot and stifling should actually be attributed to enclosed helmets.

[Ahmad Abdel]

[T. Kew]

Visors are very restrictive for vision, airflow and temperature control. Open faced helmets are way more comfortable to wear and give much better peripheral vision and general awareness in a combat situation. The main reason to actually use a visor is the threat of missiles - when that's a major factor, keeping your visor down becomes pretty important just to deal with random unlucky hits. When you're in a melee, by contrast, having more vision and breath can be pretty useful.

[Ahmad Abdel]

[Henry O.]

also you can protect your face from missiles by tilting your head forward if you need to

[Paul Hansen]

Another thing that's not yet mentioned: closed helmets restrict proper hearing a lot. Which can be a real hinderance, e.g. in a melee.

[Bartek Strojek]

Question is a bit tricky one, because it's not Islamic helmets that didn't have visor, it's vast majority of helmets, ever.

Ancient helmets didn't have a visor, heavy cavalry sometimes, quite rarely, had masks, mail hanged all the way on the forehead, covering most of the face, and so on, but not visors.

Japanese samurai used iron or leather masks.

Visors got invented in late medieval period in Western Europe, peaked somewhere in the early 16th century, and later fell out of favor. As far as I am aware, most combatants didn't use them anyway.

They theoretically combine the advantages of open helmet and closed one, due to ability to lift the visor, but at the cost of increased weight, adding of additional, cumbersome elements, and so on.

At least as far as I know, maybe other cultures somewhere came up with the visor too?

I know that they didn't catch on in the Muscovy, despite the widespread contacts with Poles/Lithuanians, so Polish Ruthenian lands were mostly eastern border of visor's usage.