Light pen on LCD monitor?

[DennisCNC]

Will a light pen work on a LCD monitor?

Thanks

[Al_The_Man]

I don't see why it wouldn't just as a touch screen will work with LCD, the only reservation is that a LCD monitor does not normally have a protection screen.
Al.

[Brandano]

actually, there's plenty of reasons why it shouldn't work. A light pen works by measuring the time between the moment the electron beam on a CRT monitor starts its scan of the screen and that when it falls under the light pen sensor. An LCD monitor is scanned almost the same way, but the backlight is always on, and I believe the single pixels just change state when the scan goes over them. That's why you can keep an LCD screen at a low refresh rate without getting an headache within minutes: there's no flashing. I think a light pen just won't work, maybe you should look into buying a Wacom Cintiq? Mind you, they are rather expensive. Or you could try to buy a cheap tablet and rig it behind the LCD screen, with a sheet of perspex in front of it to take the pen pressure. I think I saw such an hack on the net before.

[MicroMill]

No! A light pen is an input device. A LCD screen is an output device. A tablet device will perform both functions.

It's like yelling at the TV during a football game. The players just don't react to your commands!

[Brandano]

Ok, here's what Wikipedia has to say on light pens:
http://en.wikipedia.org/wiki/Light_pen
note it says it won't work on LCD's.
This is what I call a tablet, got a couple of these: http://en.wikipedia.org/wiki/Graphics_tablet
MicroMill, I think you are referring to a "Tablet PC", a specialized notebook that includes a graphic tablet in its display: http://en.wikipedia.org/wiki/Tablet_pc
The Cintiq (http://www.wacom.com/cintiq/index.cfm) that I mentioned earlier is a bit different. It's not a full tablet PC, though it costs as much, but it does have the same sort of graphic tablet/screen hybrid. The justification for the higher cost is the accuracy and graphic quality of the display, since it's mainly used for artistic applications (and, I daresay, as a "boss toy")

[MicroMill]

The guy wasn't asking about a Cintiq or some wikipedia tangent. The question was will a light pen work with a LCD monitor. The answer is still NO.

[bobs bots]

Hi ,
I'm an electronics engineer with 30+ years experience. The other posters are correct, the light pen relies on a the flying spot of light that paints the CRT screen.

LCD screens are row/column multiplexed , so even if it wasn't for the LCD persistence being much longer than the refresh rate, you might just be able to determine your approximate position in one direction only. The video stream into your LCD monitor is converted to frame buffer (it has to do this as the LCD resolution is different to the video resolution), the frame buffer is then displayed. It is theoretically possible to do the LCD multiplexing in such a way that a modified light pen could be used, but I'm not aware of any LCD manufactures doing this.

LCD touch screens are a fairly mature technology, and don't require an ugly pen with a wire hanging out of it. I'm fairly certain that touch screens can be purchased that can be placed on the front of existing LCD monitors.

You could just steal an ATM, they have very tough touch screens and a color LCD, and as a bonus you get extra cash to finish your project.

PS To answer another related question before someone asks it, NO you can't use a bar code wand to read a bar code displayed on your monitor (LCD or otherwise)
bobt

[Brandano]

Another way to get a light pen to work would be possible:
IF the pen was able to tell apart colors on the screen, then with a proper software (working by successive approximations) you could blank half the screen, then if the pen is in that half blank half of that (otherwise us the other half) and so on until you reach the required accuracy. However, you'd have to do this every 2 or 3 frames in order to keep a visible image on the screen for the user. You'd need really quick response times on the LCD. And it would probably introduce a lot of flicker. If the program was smart enough it could track the tip of the light pen and only work in that area, but there should be a way for the pen to tell the computer that it's been lifted from the screen so that the initial scan can be repeated once it's been reapplied to it. I agree, not practical, but a nice hack to try for a laugh

[Doug L]

Won't work, for reasons already mentioned. A light pen relies on the intensification of the phosphor as it's repainted 60-85 times per second by a dot scanning left to right then top to bottom. Each pixel on an LCD panel maintains the brightness and color information it has been assigned, until it's assigned a new color/intensity. There is nothing changing on an LCD panel for the pen to detect.

My background for this comes from being president of FastPoint Technologies, which was the largest light pen company in the world, until we closed our doors in August. Primary reason -- light pens only work on CRTs, and the number of CRTs going into new systems is pretty much zero.

We did try some successive approximation techniques as a work-around, with little success. The problem is that to get a resolution of even 256x256 requires 8 patterns, and with a 20ms time required to change some pixel and then put them back, we're looking at 1/6 of a second. Any movement of the pen during that time renders the measurement invalid. Please believe we were REAL motivated to make it work, and it wasn't for lack of trying that we decided the approach wasn't practical in the real world.

A good substitute for a light pen on an LCD is a touchscreen. A capacitive touchscreen with a stylus gets very close to the same experience.

[KOzOK]

Just my two cents. Touch screen LCDs work great with CNC machines in my opinion. I worked with lots of them, and it's lot nicer than a mouse/trackball in a dirty conditions. Just make sure to use a stylus, and it's all good. I even have one in my car, hooked up to my little car PC.

[DennisCNC]

Thank you all for the input!
It looks like a touchscreen with a stylus is the way to go.

[MicroMill]

Originally Posted by Doug L Won't work, for reasons already mentioned. A light pen relies on the intensification of the phosphor as it's repainted 60-85 times per second by a dot scanning left to right then top to bottom. Each pixel on an LCD panel maintains the brightness and color information it has been assigned, until it's assigned a new color/intensity. There is nothing changing on an LCD panel for the pen to detect. My background for this comes from being president of FastPoint Technologies, which was the largest light pen company in the world, until we closed our doors in August. Primary reason -- light pens only work on CRTs, and the number of CRTs going into new systems is pretty much zero. We did try some successive approximation techniques as a work-around, with little success. The problem is that to get a resolution of even 256x256 requires 8 patterns, and with a 20ms time required to change some pixel and then put them back, we're looking at 1/6 of a second. Any movement of the pen during that time renders the measurement invalid. Please believe we were REAL motivated to make it work, and it wasn't for lack of trying that we decided the approach wasn't practical in the real world. A good substitute for a light pen on an LCD is a touchscreen. A capacitive touchscreen with a stylus gets very close to the same experience.

Thank You Doug,

Be interested in hearing what irons you've got on the fire. I know you must have some great ideas cooking.

J