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Your eyes actually don't react to "brightness". Your eyes react to heat-radiation from the Sun or any other radiation source. That radiation is "invisible" until it is perceived by a mind.

Your eyes are sensitive to "invisible radiation" not to "brightness". An optometrist or ophthalmologist is not going to explain that to you and may have never even consider it that way. Even if he understands it, it would confuse most people to explain it that way. That's why you won't usually find that type of explanation in typical information. If you dig for it you can find it.

Here is another simple way to view it:
Look at someone sitting next to you. Notice you don't see streams of color or anything else flowing into the person's eyes. That's because the Sun's radiation does not contain color or brightness as a property. If it worked the way you think all your eyes would see is a myriad of color flying around everywhere. When radiation reflects off an object it does not contain color or brightness. If it worked that way you would see color streaming into the person's eyes next to you. Assuming you could see the person at all with all that color flying around in the atmosphere. It just doesn't work the way you think at all.

Your eyes and mind process this colorless radiation into color and brightness. Different wavelengths are processed into different colors and the amplitude of the "invisible" waves determine the amount of "brightness" that your mind creates. The radiation from a "light" source is "invisible" much like the radiation/signal from your cell phone is "invisible". Your cellular system and phone process "invisible", "inaudible" information into sound in a similar way that your eyes and mind process radiation in the visual spectrum into a visual image. Sound is not a property of your cell phone signal in the same way that "brightness" is not a property of light. I assume you can't hear your cell phone signal without a phone. If you can you should go get checked out by a psychiatrist.

Your mind uses a high percentage of it's power to process this "invisible" information into visual images. It's the most single demanding function of your brain. Your body also uses a high percentage of it's calorie intake just to create the energy it takes to process visual images. Processing brightness and visual images is a very demanding task for your mind and body to perform.

The same part of your mind that creates dreams also creates your visual perception of the world around us. Our eyes are not like windows that we look through to view the world. They are more like a satellite dish LNB that receives "invisible" information. The satellite dish is like our retina, the coaxial cable is like our optic nerve (The optic nerve and retina are an extrusion of our brain/mind), the cable box is like our mind and the television is like our "mind's eye" where we view the processed visual information. Note that color and brightness is not a property of your incoming TV signal via "air" or cable.

It's believed that the images of the world that form in our mind are formed from "past stored memories" and not from what we are actually looking at in that moment. Our eyes and mind processes these small fragments of "Invisible" information into brightness and a visual color image that plays in our mind similar to a dream.