Keep coming back to this. (I I like a breather in between!)

Is there any electronic sensor that mimics the eye's 'response' curve out there that I can buy & put a scope on its output? What I'm thinking is that if I put such a sensor directly over the LED being driven with PWM then 'shoot' for a linear slope, then that's gonna be the way forward here - else I'm sort of stabbing in the dark!