He is saying that it is an approximation (see in comments): "I looked at what I thought were comparable regions. I checked several frames of the video and the f

He is saying that it is an approximation (see in comments): "I looked at what I thought were comparable regions. I checked several frames of the video and the frame at 0:51 in the article is what I used. The method is admittedly very approximate as you have to guess the “gamma curve” of the video capture device to back out the linear value (I used a gamma of 2.2)."