Coming back to: Light exposure: how much protection is proportionate?

Asking directly, because I could not find a straight answer: Light exposure: how much protection is proportionate?

Reading back through what has been written here about Light exposure, three questions come up every time and only one has ever been answered properly.

Listing all three, with what I think the state of the answer is for each, so the thread can start further along than the last one did.

Clear enough that I do not think I have a follow-up, which is unusual.

The opening post is the version of this I will quote in future. One addition.

Light protection is easy and cheap. The original carton exists for a reason and a vial stored in it is protected without anybody having to think about it again.

Protecting from light: amber vials or opaque containers reduce light exposure. If you are storing for long periods and light matters, this is worth doing. For short-term prepared solutions, ordinary storage away from direct sunlight is usually adequate.

Offering a way to settle Light exposure rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.

Post #4 describes the usual case. This is about the unusual one.

Door storage in a refrigerator is the warmest and most variable position in the appliance. A shelf at the back is a materially different environment.

What is reasonably well supported: refrigeration slows degradation, repeated warming and cooling is worse than either state alone, light exposure is worth avoiding, and adsorptive losses matter more in dilute solutions.

A qualification I should have led with rather than closed on.

I want to gently push back on the reply above about storage extrapolation. The advice is sound and the confidence is not.

We are extrapolating from stability data on a licensed formulation to a preparation that differs in concentration, diluent and container. That extrapolation is reasonable and it is still an extrapolation. Saying so does not weaken the practical recommendation; it just stops the recommendation from hardening into a fact that someone cites as certain.

Reporting the observation and leaving the explanation open deliberately.

Picking up post #6: that is the part I would want checked first.

Reconstituted material carried in a bag for a day is a temperature excursion even in a cool climate. Where that matters is cumulative exposure over weeks rather than a single afternoon.

On post #8 — agreed on the reasoning, with one qualification.

Clarity is a weak test. A solution can lose a significant fraction of its active content to degradation or adsorption while remaining perfectly clear.

For anyone finding this later: the short answer on Light exposure is that it depends on one thing, and the rest of the thread is people identifying which thing.