Chasing a ghost

I was recently rewatching a title sequence I admire, and noticed, as I often do, the way the white logo­type “detours” through blue as it fades into darkness:

This effect isn’t rare; you see it very often in the title sequences of oldish movies. In fact, the effect was sim­u­lated to lend vin­tage vibes to the opening of Stranger Things. Notice how the text doesn’t fade from white to gray to black, but white to red to black:

I really love the way this looks, so I asked the Assem­bled Brains of Twitter what was behind it. Joel Davis and Rod Bogart pro­vided the answer, which I’ll now attempt to restate myself:

It is an arti­fact of the film stock’s uneven response to light at dif­ferent wavelengths. When the titles are white, the film’s red, green, and blue layers are all totally overexposed, so bright white light shines through the trans­parent film. But then, as the expo­sure ramps down toward darkness, the layers diverge — they become opaque at slightly dif­ferent rates — which gives you the ghost of color. (Whether that color is red or blue has to do, I think, with the process by which the titles have been inte­grated into the overall film. Clar­i­fi­ca­tion is wel­come at robin@robinsloan.com.)

Here is one par­tic­ular film stock’s color response curve:

That’s not motion pic­ture film, and its curve isn’t nec­es­sarily (?) the kind that would pro­duce the pulses of color above, but, the point is just to see the space between red, green, and blue in their swoop from overexpo­sure to underexpo­sure. It’s that gap that pro­duces the ghost we’re chasing.

No film has a “perfect” color response curve. Often, the slight dif­fer­ences between red, green, and blue are what give a par­tic­ular film stock its char­ac­ter­istic (and very appealing) look! But it’s only in this rel­a­tively strange scenario — overexposing the film to pure white, then ramping the expo­sure down smoothly, often con­trasted against black — that you get to see, in effect, only those dif­fer­ences. They are what pro­duce the image.

After learning what was happening, thanks to Joel and Rod, it was easy to sim­u­late the effect. Here you go, in CSS and a scrap of JavaScript; red on the way out, blue if you click again:

April 2020, Oak­land