By curiosity, I tried to measure the correlated color temperature of different light sources in my house by taking a photo of a white object (a piece of paper) lit by a single source at a time. The temperature is then measured in Lightroom by selecting the part of the paper with no reflections.
While a gray card will give more exact results, the results I got are quite weird:
7400K for a Nikon SB-600 flash,
2800K for a LED spot,
2400K to 2650K for different fluorescent light bulbs.
I don't understand the 7400K for a flash. Isn't it expected to be closer to 5400K? I would understand if it was, say, 6000K, given that the measure is not done with a gray card, but the measure I get looks too extreme.
Is there a cleaner way to measure the correlated color temperature of a light source without a gray card?
Note: I'll try the next week to take similar measures with studio light bulbs which are at 5400K according to the specs, as well as do the tests with a gray card. I still ask the question, because I think it may be interesting for people who don't have a gray card and don't want to spend $20 for one.
Update: I used the same technique with a studio light bulb rated at 5400K. I obtain 5500K in Lightroom, which corresponds to what I expected: there is some imprecision due to the lame way of measuring the temperature, but not too much. What is surprising is that the photo taken with an SB-600 appears much bluer compared to the same scene lit by studio light bulbs (white balance being set manually and kept the same across the shots). I'll take more photos with different settings and update the question again later.