Hacker News Reader

Comment by tecleandor | Hacker News Reader

As of today it should be easy to do a relatively cheap rapatronic-like camera with LCD film.

The basic principle is more or less the same: two linearly (?) polarized sheets at 90° that block all the light going through, and a sheet/material in the middle that rotates 90° the light polarization when an electric current is applied. Some of those glass windows that can obscure instantly work with this technology. https://en.m.wikipedia.org/wiki/Liquid-crystal_display

What I don't know is the speed of polarization rotation for liquid crystals (compared to a Kerr cell) that would define the minimum "obturation" speed available.

View on HN

See https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=81... for examples of modern LCD shutters. They have closing times that are in the hundreds of microseconds, so they are not really replacements for ultra-short exposures. But they are "cheap".

Auto-darkening welding helmets claim 40 microseconds.

That seems to match with a shutter vendor that says "50 microsecond rise time 1.3 millisecond fall time". So I guess 'rising' is going dark, and 'falling' is going clear.

Or 'rising' means changing the natural state of the crystal (thus rotating 90° the polarization), and 'falling'... well, falling back to the natural resting state. It makes some sense that falling back when unenergized is slower than rising.

Now it'd be interesting to know what happens during rise and fall time. Progressive linear polarization? Waiting time until the crystal reacts?

A back of envelope calculation for the shutter speed of a Pimoroni LCD shutter [1] suggests 1/15s is the fastest speed available. [2]

[1] Pimoroni LCD shutter [Not available any longer ] https://shop.pimoroni.com/products/lcd-shutter

[2] Testing the amazing Pimoroni LCD shutter https://youtu.be/i3GCfAtdZYI

I've never researched much about this, but this company claims that TN shutters can do up to 100hz and "liquid crystal pi cells" can do 1000hz.

http://www.liquidcrystaltechnologies.com/products/lcdshutter...

This is interesting, I might expend the afternoon reading about it

Edit:

Related 6 years old EE Stack Exchange post: https://electronics.stackexchange.com/questions/276014/chara...

Also this company sells a pi-cell shutter with "50 microsecond rise time 1.3 millisecond fall time" for ~$200USD. I have to calculate the refresh for that : https://boldervision.com/liquid-crystals/pi-cell-shutter/

Reply on news.ycombinator.com