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[tachitoscope] How to show a pic for exactly 12.5ms?

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[tachitoscope] How to show a pic for exactly 12.5ms?

Postby calliefrance » 28 Aug 2019, 03:50

I want to present a pic(human face) last only for 12.5ms or even shorter time period. But after read some of the articles that posted here, I've got really lost.
Could anyone help me on this problem, what should I do to show a pic on screen for exactly 12.5ms(and disappears)? Or what should I have/buy?
Does 120HZ or 144HZ monitor only matters? Or I should also consider on other special technology such as use black frame to remove the persistence?
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[tachitoscope] How to show a pic for exactly 12.5ms?

Postby Chief Blur Buster » 02 Sep 2019, 21:22

Hello!

This thread should in the Area 51 Display Science, Research & Engineering Forum, so I have moved this thread there.

Firstly, one needs to understand how LCD Monitors refresh, see high speed videos at http://www.blurbusters.com/scanout ... You will realize that there is a massive unfortunate error margin that makes what you want to do impossible without monitor modification because
(A) Not all pixels refresh simultaneously and synchronously.
(B) Because of pixel response, each pixel "fade in and out". This is the GtG pixel response time.
(C) Pixel changes can only be done at granular refresh intervals, dictated as the refresh rate.

You need to concurrently solve (A)(B)(C)
You can solve (C) by creating a custom refresh rate that uses refresh rate of (1000ms/target). For your case, (1000ms/12.5ms) = 80 Hz, creatable using custom resolution utility.
You can solve (A) and (B) by obtaining a blur-reducing strobe-backlight monitor, but most only have adjustable flashes from ~0.25ms to ~2.00ms. In many of them, the pulse width is independently adjustable though, so as long as you use a refresh rate higher than target, it can be doable if within the common range of 0.25ms to about 2ms.

You could use software-based black frame insertion but you won't be able to fix (A), unless you only use a tiny horizontal strip in the middle of your screen as a crude workaround -- and you will still have (B) as the error margin (pixels fading in and out when you flash). 1ms GtG, 5ms GtG, etc. Pixel response is an error margin in your case.

Your requirement of 12.5ms is a problem for off-the-market solutions; you likely will have to open up your screen.

Your last option is to probably open up the monitor and wire a custom circuit to the monitor's backlight.
Basically a trigger wire that will turn on the LEDs upon receiving a signal from an Arduino or Raspberry PI or similar circuit.

Once the screen hardware is modified to allow an Arduino to turn on/off the illumination, do as follows:
1. Continuously display image (in total darkness)
2. Wait at least a few milliseconds, but preferably for multiple refresh cycles (in total darkness) to allow LCD to finish fully transitioning the image.
3. Use an Arduino to turn on the backlight or edgelight for 12.5ms ...and then off

This eliminates pixel response from being an error margin, since the panel is refreshed in the dark before you flash the backlight (edgelight) which is LED based and practically instantaneous.

Marc Repnow of Display Corner could help you build a tachitoscope.
https://display-corner.epfl.ch/index.ph ... scopeSetup

Also, out of curiosity — why exactly 12.5ms? If you follow Tablot-Plateau law, a 125 microsecond flash 100 times brighter, will also be equally visible to a human. It’s simply equal number of photons crammed into an instant. For all sufficiently short flashes — Brightness times flash length makes all flash length look equally instantaneous and equally bright. Photo flash bulbs for cameras are still seen even if they flash less than 1 millisecond because they are super bright when they flash.

Also.... Make sure to verify experiment design includes the flash brightness variable: Same as steady state? And are you flashing black-image-black, or flashing grey-image-grey, with grey representing average picture brightness level? These variables can affect certain experiments.
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