What is the category name to describe "day", "night", or "dusk"?

Solution 1:

These periods like day, night, evening, morning, dusk, twilight, afternoon, etc. are simply called "times of day".

Even the National Institute of Standards and Technology calls them that and I suppose that's as official as one can get, at least in the US.

Solution 2:

How about LightingCondition

which I would understand given a choice of day/daylight, night/artificial light no light...

It seems my other suggestion LightSource (Flash/daylight/artificial light) is less relevant for you

Solution 3:

If you would like something a little more poetical and less stuffy, you could always use hour (M-W)

4a : a customary or particular time lunch hour, in our hour of need

The word hour existed long before we started dividing the day into 24 equal periods; it could refer to a particular time of day (one of the canonical hours lauds, prime, terce, sext, nones, vespers, compline, and vigils) when prayers were read.

(Etymonline):

Church sense is oldest in English. Meaning "one of the 24 equal parts of a natural solar day (time from one sunrise to the next), equal hour; definite time of day or night reckoned in equal hours," and that of "one of the 12 equal parts of an artificial day (sunrise to sunset) or night, varying in duration according to the season; definite time of day or night reckoned in unequal hours" are from late 14c.

Solution 4:

Since diurnal-nocturnal course refers to the whole astronomical day, you can use the phrase DiurnalNocturnalSpan to describe the time of the day, even though not universally applicable. The camera users will automatically use the context and work out to realize that DiurnalNocturnalSpan refers to the time of the day they're working in.