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HDR data is designed for backwards compatibility. TV sets’ video processors do no blindly read data and process it. They look into the data stream and extract what they know. What they do not know about they don’t process.
Now with HDR there is “auxiliary data” for the “expanded color”. A TV that can handle HDR will see a flag that HDR content is will arrive and so it will look for it in the data stream and process it. A TV set that cannot handle HDR, well doesn’t know about the flag. For it the bit is “undefined”, hence the “auxiliary data” is not used at all. It never even looks at it. For the non HDR TV that is just noise.
Now the problem comes in: HDR content is differently mastered. It is not like the additional data is just an extension, and the normal data is just like a normal 1080 Bluray. The data is different,
the “Core Picture” is usually overall darker (sometimes washed out in general), so the additional bright color information pops later. It has to do with the way color on your TV works, without going into technical details here.
As a result you usually have to pump the backlight way up on HDR displays. That is one thing of HDR: For the additional colors the backlight needs to be high, it is usually almost set automatically to maximum on HDR TVs when entering HDR mode, especially on edge lit TVs."
https://discourse.osmc.tv/t/4k-hdr-videos-too-dark-on-non-hdr-tv/54220/12
