[jhund]

I used to own a B&W enlarger and made prints from my own negatives. I remember the bulb in the enlarger having significantly more power (photons emitted/sec) than a smart phone display has.

I wonder by how much this will increase exposure times, and if it works at all...

[nwh]

I'm not sure that's true of all enlargers. The one I used in school was quite dim; the brightness of my iPhone screen would definitely out power it.