If someone with no night vision were in the cockpit of a nocturnal flight, that person's situation would resemble Instrument Flight Rules. Am I correct? So:
In theory, can someone lacking night vision operate an aircraft with no more danger than a pilot on IFR?
Can someone lacking night vision get a pilot's licence?
An answer that is valid for India, China, EU or the US is appreciated
The name of the condition is nyctalopia. I found it mentioned by the UK's CAA (sorry it doesn't match your array of jurisdictions, but maybe this information will lead to more):
If the pilot is found to have a demonstrable nyctalopia, a medical flight or simulator test may be required, depending on the degree of severity. For pilots with demonstrated nyctalopia enough to cause concerns for night flying, a VCL limitation will be required.
"VCL" being a daytime-only limitation, which would typically not allow a Class 1.
This answer is valid for EASA jurisdiction.
If someone with no night vision were in the cockpit of a nocturnal flight, that person's situation would resemble Instrument Flight Rules. Am I correct?
Only to some minor extent, because at night VMC, you still see a lot around you. Surface lights, quite often horizon line, sometimes backlit layer of clouds above you etc. You have some kind of perspective around you, limited, comparing to day VMC, but much less disorienting than IMC. This is why night rating requires 5 hours of practice and some theory (8 hrs, IIRC) whilst instrument rating (IR) minimum is 45 hours, plus about 4 months of theory (talking about PPL ratings now).
In EASA member states at least, night rating and instrument rating are not dependent on each other. You can acquire night rating only or IR only. Or both.
In theory, can someone lacking night vision operate an aircraft with no more danger than a pilot on IFR?
There is no «night vision» at least in EASA terminology. If you are referring to VCL mark (limitation to fly during daytime hours only + civil twilight) in your medical, then you are probably not color safe (like I am). Color blindness, mild or severe, as far as I know, is the most frequent reason for getting VCL mark.
EASA officials state that in dark environments, color vision is essential, especially recognition of pale colors. In US, for example, pilots that fail Ishihara, Anomaloscopy and CAD tests can still be granted a SODA (Statement of Demonstrated Ability) if they show their color recognition in practice. In Australia, color blind people can fly almost without any restrictions.
So the question is somewhat irrelevant, because a person with VCL mark («lacking night vision», as you say) can be IR rated and can fly IFR during daytime, officially and safely.
Can someone lacking night vision get a pilot's licence?
Absolutely, I am not color safe and have PPL pilot license, acquiring IR now to become fully instrumental.
However, with VCL mark, you will not be able to fly as a professional pilot (CPL or ATPL) at least in EASA jurisdiction, because VCL mark is only applicable to Class 2 medical. Class 1 medical has no tolerance towards conditions that result in your VCL limitation in Class 2 medical. Not being «color safe», you won't get Class 1 medical, which is required for CPL/ATPL.
And to complete the story, some countries require only regular driver's license medical certificate to issue ultralight class B (airplanes lighter that 472.5kg, like beautiful Evektor Eurostar, for example) license. But ultralights fly in Day VMC only, so the whole night vision discussion is irrelevant in context of ULAs.
US-based answer, and I'm not a doctor.
I don't know what "no night vision" actually means. But let's assume you mean someone has some kind of difficulty seeing clearly in low lighting conditions (again, whatever that means). You seem to be asking two questions here: is night vision important for pilots; and can someone with limited night vision get a medical?
At least theoretically, night vision only matters to pilots who fly at night. I know several pilots who have daytime-only restrictions on their medicals because of limited color vision. I assume (but don't know) that if someone had a hypothetical condition that meant they could see perfectly during the day but not at night, the medical examiner could issue the same limitation:
Not valid for night flying or color signal control.
You suggested that a pilot with poor night vision could just fly on instruments at night, but they have to get the aircraft to the runway first. And takeoff and landing both require visual cues, including seeing airport lighting at night. If the pilot really has difficulties seeing at night, it isn't obvious how they could safely operate an aircraft on the ground, never mind in the air.
As for a medical, I couldn't find anything in the FAA's medical standards about low light vision specifically, but there is some general wording that seems like it could apply:
Congenital or acquired conditions (whether acute or chronic), of either eye or adnexa that may interfere with visual functions
I have no idea how a medical examiner would address any issues. They might have to defer the case to the FAA for review.
