In Prometheus, why did all the astronauts use helmets completely made of glass? I mean, is the idea that a person will look out the back of his head? Is it not safer to have helmets like astronauts have nowadays?
- The more glass, the easier it is to see around. While a person can't see directly behind, it is still a benefit to have a full range of motion, even when turning their head.
- A statement about it being safer to have non-glass helmets make the assumption that the non-transparent material is not strong enough. Prometheus takes place over 70 years in the future. Looking back at the last 70 years and what changes can happen, there's no reason to assume that whatever transparent material is being used in space helmets in that time is not re-enforced. We have materials like bullet-proof glass now. In that time, even stronger and safer glass is not only possible, but likely. Heck, by then they might have transparent aluminum or even stronger metals in transparent forms.
In addition to the wearer's field of view and the technological advances in durable (and presumably radiation-shielding [variable light intensity filtering already exists]) transparent materials mentioned by Tango, a entirely transparent helmet allows others to see the head of the wearer providing a limited improvement in identification and observation of status (this applies both in-universe and out-of-universe).
Using a single material surface would also reduce the number of material interfaces which can be more likely to fail/leak (different thermal expansion and elasticity properties and requiring a matrix/glue) and be more difficult to see failures--and a transparent helmet might make damage more obvious. Allowing the turning of the head without turning of the helmet would also reduces stresses on and design complexity for the connection with the rest of the spacesuit.
Such an "open" design might also reduce feelings of claustrophobia and perhaps anxiety (minimal limitations on peripheral vision and unencumbered head movement might reduce a sense of blind spots).
A mostly out-of-universe explanation could be that a plastic bubble is much cheaper both to design and to manufacture than a composite helmet. (DJClayworth also mentioned the rule of cool as an out-of-universe reason.)