Sounds like a very interesting project!
So: can it be done? Absolutely. The physical measuring devices exist, and there are quite a few that are pretty inexpensive.
Has it been done? I’m not aware of anyone who has done this with SmartThings, because most people have just gone for “circadian light,” which is changing the color temperature based on time of day and maybe date. You’ll be able to find at least a couple of those. But—those projects don’t try to match the actual light outside. Just the most likely. Which means if it’s raining or overcast or a smog day the circadian light calculation will be wrong.
So most “circadian light” projects are simulations based on time of day, not weather conditions. Using one of those means your bathroom might look bright and sunny at noon while other rooms using natural light are cool and gray because it’s raining outside. Some people are fine with that, some are not.
(BTW, Apple HomeKit has “adaptive lighting” (their name for circadian lighting) built into HomeKit. So if you happen to have an iPad you could easily set this up with a Philips hue bridge and associated bulbs that you also used in SmartThings. No coding required. But it won’t match the real outside condition and you can’t use the circadian light information in SmartThings. But it’s a quick and easy way to get those lights to adjust, and if you already have a hue bridge and an iPad or HomePod mini, there’s no added cost.)
WHAT IF I REALLY WANT AN EXACT MATCH BASED ON ACTUAL CONDITIONS, NOT A PROJECTED SIMULATION?
Then you’ll need a physical RGB sensor AND you’ll need to integrate the sensor to SmartThings AND you’ll need to write code to read its values AND you’ll need to write code to set your color temperature changing lightbulbs to match it.
The first two are pretty easy but, as I said, I don’t know if anyone else who has done steps three and four yet. And that’s not a basic out of the box SmartThings function.
I’m going to tag both @ogiewon and @bcopeland because both might have some more ideas.
I don’t know if you yourself have a coding background or are familiar with arduinos, but you can see why at this point some people just stop and say, “You know what? A time of day simulation is just fine with me. “
But if you do want the real thing…
LIGHT BULB CHOICES
I’m sure you’ve already thought of this part, but just for completeness, the lightbulbs themselves have to be able to change color temperature when instructed to do so. The ones available to you will depend on what country you’re in, but Philips Hue has a fairly wide range available in multiple regions, including some already built into lamps and even some of those rated for bathroom use. These are expensive, but the engineering is great, and they can definitely work for the project.
There are a few other brand choices that might work as well, but again it depends on the country you are in.
One note on this step: many bathroom light fixtures are fully sealed in order to make them waterproof but most smart bulbs are not designed to work in a fully sealed fixture and will overheat. That’s why you have to be a little careful about the specifications. If it’s an all in one specifically designed for a bathroom environment, like the Philips hue models mentioned in their article, then there’s no issue, of course.
Also, look for the “lumens“ rating to determine how bright it will be, not just watts, since these will be LED bulbs. For example, one of the Phillips bathroom lights is a 40 W draw, but 2400 lm, which is an equivalent brightness to 200 W of incandescent bulbs.
SENSOR CHOICES
Most of the cheap ones, under $20, will be parts that you add to an arduino. So they won’t have a radio themselves to communicate with SmartThings. Here’s one candidate, but they’re fairly easy to find.
Personally, though, I usually start at Adafruit for projects like this, and this caught my eye.
Interestingly, some people are using that one with Home Assistant for some similar projects. That would be a pretty long integration path to bring it into SmartThings, though, but if you happen to also be interested in Home Assistant (in some ways an open source competitor to SmartThings, but there are certainly people who use both), here’s that link:
NEXT STEP: CONNECT THE SENSOR TO AN ARDUINO
Where was I? Oh, yeah…whatever sensor on a chip you use, you have to connect it to an arduino or something similar to get the ability to communicate to SmartThings. Here’s a link to a project about adding a sensor to an arduino. We’re still not at actual SmartThings integration yet, though. Just setting up the hardware so we COULD integrate with SmartThings if we wrote code on the ST side to handle the incoming data.
So here’s a project utilizing the Arduino color temperature sensor libraries.
OK, THE SENSOR IS READY TO SEND ITS DATA TO SMARTTHINGS. NOW WHAT?
What a great question! seriously, it’s a great question, and one that other people will have to help with the answer for. (I’m now quadriparetic and dependent on voice recognition and no longer code myself.) in addition to the two folks I’ve already tagged, I’m sure there will be other developers in the community who may be interested. Hopefully some will chime in.