SUNSCREEN in UV

One thing I'm surprised went unmentioned:
"SPF" is defined by a testing method, where the SPF number is the threshold of how much UVA/UVB doesn't make it through the product (applied in a standardized layer). It's a very simple bit of math!
SPF 4 = 1/4 or 25% of the radiation is not blocked (absorbed/reflected)
SPF 8 = 1/8 or 12.5%
SPF 15 = 1/15 or 6.7%
SPF 30 = 1/30 or 3.3%
SPF 50 = 1/50 or 2%
SPF 100 = 1/100 or 1%
SPF 150 = 1/150 or 0.67%
So it makes good sense that you can't readily tell the difference at higher SPF's because the difference is getting tinier and tinier. The difference between SPF 15 and 30 is 3.4% radiation getting through, but the difference between 50 and 150 is only 1.3%. This is also interesting in terms of evaluating the UV camera's receptivity, because SPF doesn't discriminate between how it gets results (absorbing vs reflecting), but the camera is registering the UV radiation emitted/reflected, so as you observed absorption darkens and reflection may be invisible or actually lighter. So if you had two different brands achieving the same SPF value by different combinations of the two mechanisms, they may appear different on the camera even though they have the same efficacy from the skin's point of view.