EEVblog #1346 - How An Infrared Optical Touch Screen Works - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: November 08, 2020, 09:35:19 am
content: Dave tears down and investigates a Dell infrared IR optical touch screen monitor found in the dumpster.Infrared touch screen are not often used in consumer computer monitor applications compared to resistive and capacitive touch screen technologies.

author: Ranayna
author_rank: Super Contributor
timestamp: November 10, 2020, 09:23:24 am
content: I think the large SMART Whiteboards and SMART Displays use the same or a similar technique. They also have that inner rim, but with the large boards this rim is significantly wider, i think it's more than a centimeter.I don't know when the next time will be that i use one of them, due to corona induced homeoffice, but i will see if i can find out how many sensors those use.

author: ludzinc
author_rank: Unknown
timestamp: November 12, 2020, 01:15:42 am
content: *YouTube* is currently not playing videos.Finally decided to watch on Lbry!https://lbry.tv/@eevblog:7/eevblog-1346-how-an-infrared-optical:4

author: thm_w
author_rank: Super Contributor
timestamp: November 12, 2020, 08:40:51 pm
content: I'm surprised this display style was not more popular, it seems incredibly cheap to produce.Sure on consumer gear now touch displays are getting cheap, but industrial displays are not. Although I think this display would be confused if it were raining, causing issues, so it likely wouldn't work outdoors.

author: Ranayna
author_rank: Super Contributor
timestamp: November 13, 2020, 07:36:14 am
content: I don't think this would work well in an industrial setting. As Dave mentioned, and i noticed myself, those screens are quite susceptible to dirt accumulating on that reflective surface. Especially the lower lip and the lower corners are where the crud accumulates if the screen is not regularly and properly cleaned.

author: twospoons
author_rank: Frequent Contributor
timestamp: November 30, 2020, 03:36:21 am
content: The analysis is largely correct.The LEDs are lit sequentially, so each camera can take a snapshot. Its can't be done simultaneously as the cameras would see each others LEDs, resulting in saturation and false peaks from surface reflections.Using three cameras is the minimum to resolve two touches unambiguously. One of the designs I worked on had 6 cameras and could handle more than five simultaneous touches. Most of our screens used four cameras.The cameras will be taking two images, with the LED on and off, in order to remove ambient lighting. Flashing the LED also lets it be overdriven beyond its DC current limit, to get more light out during image capture.The strange video waveform is the result of two factors: distance attenuation, and angular attenuation. Retroreflective tape loses reflectivity as the light angle moves away from the normal, as the effective aperture of the microprisms reduces.Quote from: thm_w on November 12, 2020, 08:40:51 pmFor anything bigger than a small tablet it is far cheaper than other technologies - the price scales with perimeter length rather than area. We managed to get the camera price down to ~2USD - custom camera chip and single element aspheric moulded lens. We would have made about 3 million screens by the time the company was shut down.Touch isn't that useful on a desktop monitor though. Its ergonomically awkward to use. Its much better used on whiteboards, and information kiosks.Although dust and dirt are obviously a problem for optical touch, the system we had was sufficiently adaptive to handle a significant amount of contamination.There's a few hidden things that might be of interest, though I'm not entirely certain thats one of our touchscreens. The camera is a linescan cmos camera, but internally each pixel had a number of subpixels that could be enabled/disabled. A bit like an area camera 1000 pixels wide and 10 pixels high. This allowed electronic alignment of the camera to the screen bezel, turning off those subpixels that were looking outside the border.There was also a fair bit of math going on to correct the lens distortion and get accurate touch locations.

author: Poe
author_rank: Frequent Contributor
timestamp: December 24, 2020, 06:12:20 pm
content: twospoons.I'm currently working on an industrial optical project which uses LEDs, lens, and retroreflectors.Any chance you could recommend component suppliers and/or engineering firms?Thanks

author: jancelot
author_rank: Regular Contributor
timestamp: January 21, 2021, 06:55:34 pm
content: You were trying to pass light with both 405 and 510 which are far off, while you were expecting 405 & 410 or 505 & 510. At 5:32.Optical Filter Followup

author: twospoons
author_rank: Frequent Contributor
timestamp: February 13, 2021, 05:32:37 am
content: @Poe .. Oof, now you're testing me!Our LEDs came from OSRAM, very helpful with questions not answered by the datasheets.The camera was a custom design, spec'd by me and made by ST Microelectronics.The lens was designed in-house by our optical engineer and manufactured in Singapore - can't recall the name of the company, one of the many optical precision molding co's in Singapore.The retroreflector was a microprism array that came from the US - again I can't recall the name of the company, but they did work extensively with us to tailor the prism angles to suit our application. There's a lot of this stuff used in road signage, thats a good place to start looking for suppliers.