Terrarium lighting

You can use light meters (in fc or lux) or Quantum PAR meter (better, but more expensive). Or you can try Ray's method:
Measuring Light Intensity » First Rays LLC
You have to be able to access exif data of digital photo.

With seasonality, there are quite a bit of variation. The day length may not vary much near the equator, but precipitation pattern does.

I'm not sure what you mean by short day plants. Are you referring to the photoperiodism for flowering? In other words, photoperiodic plants are those whose flower induction is influenced by night length. If so, there are variations among different species. But some of them are photoperiodic. If you are interested, Joseph Arditti's book: Fundamentals of Orchid Biology has a table of how flower induction occurs for many orchid species (p.209-212).

I personally change the day length by 1 hour or so (some year I forget).

 

It is probably not too relevant how long the plant experiences day light in its natural habitat (unless their physiology is influenced by the photoperiods like long-day or short-day "flowering" or like the induction of dormancy from the photoperiods).

From the perspective of growth (photosynthesis), day light integrals (DLI) of photosynthetic photon flux density (PPFD) is probably more relevant. That is the total amount of light received in a day. Within a reasonable range, you can use weaker light for a longer time. This is a bit of simplification because if you use too strong light, the rate of break down in photosynthetic mechanism (something called Photosystem II is the common part which breaks down) accelerates. This phenomenon is called photo-inhibition (the process isn't completely understood yet). Also, in CAM-dominated plants daily growth might not be limited by amount of light (they are more limited by the CO2 storage during the night), so DLI might not be relevant.

 

I agree that the day length - by itself and not pushed to extremes - is relatively unimportant. Photoperiod-driven plants (some cattleyas, e.g.) respond to changes (shortening) in the day length.

 

I wonder that, too. Photoperiod is a good cue for the seasonality because it will tell them the absolute time of the year. Some plants are strictly photoperiodic, but more frequently others use it as one of many cues. For example, my friends transplanted poplars from southern and northern populations in Alaska. For some of them, they increased the temperature putting greenhouse like structure to each of them. Basically they were interested in how trees respond to the global warming. The northern and southern trees behaved quite differently, and southern trees (probably using photoperiod) started to grow early and kept growing later in the fall because that is what they do in south. But early spring and late fall is dangerous in Alaska, and they could get killed because of freezing. If they don't get frozen, they can grow bigger, but it is a risky strategy. But they do modify the timing of growth by the artificial warming experiment. So in this case, they were photoperiod dominated, but they can modify their behavior by temperature.

I use reflective grow tents, so the natural light is completely cut-off. But many of them seem to flower in the correct time. I have assumed that they are probably sensing small seasonal differences in the temperature. When plants are imported from the southern hemisphere to the northern hemisphere, it is interesting that some plants can acclimate quickly (in terms of blooming season), but others take a long time. For example, my Cattleya warneri took 2-3 years before it started to flower in the correct spring. It has been shown that some of Cattleya are photoperiodic. So I wonder that those phoeperiod-dominated plants take longer to acclimate when there isn't much variation in day length.