Post by thread:[EE]: Safe viewing colors? POV

I have 'normal' persistence so low refresh rates don't bother me. But I can also spot LED lights or other strobed lighting (LED sign boards) because they make distinct spots of light as I pan my viewpoint across them. Some people do appear to have much shorter persistence than normal. One friend of mine, who was a military pilot, can't stand to look at my CRTs because they flicker too much for him. Having shorter persistence, and so lower blur when flying near ground, is probably an advantage to a pilot. I wonder what the chemistry difference is. E... has no problem looking at laptops since their LCDs are very slow to respond (80msec IMS), so I rather doubt that laptops can be causing headaches from the REFRESH. They may be causing headaches because of seating posture or other mechanical issues (close focus etc.). I think the important issue about reading LEDs is to have a clear contrast between light and dark states, and broader spectrum (yellow green rather than pure green) to address color blindness issues. Ideally, you have indicators that are UNIcolor and only light for ONE state, rather that trying to multiplex the function. e.g. Power led on monitors, green=on, yellow=powersave, red=standby/off A R-G color blind person would have a difficult time distinguishing red and green states. Robert Jose Da Silva wrote: {Quote hidden}> On April 22, 2005 09:33 am, Robert Rolf wrote: > >>Simple way to get LED Xmas lights to double flicker rate is to put a >>bridge rectifier in front of them. Also makes them brighter. I am >>surprised that the manufacturers don't already to this. I rather >>doubt that a normal viewer could perceive flicker above 70Hz since >>persistence of vision is typically 40 msec. >>www.pisavisionlab.org/downloads/motion_blur_97.pdf > > > Yes, 40mSec is typical (25Hz), but since a friend of mine notices the > street XMAS lights, I can say that there are still cases where some > people are still sensitive to 60Hz. > > You can either design scissors or cameras to be used by right-handed > people (9 out of 10 people), or you can put a little more thought into > the process to include that 1-out-of-10 as well. > Since the question seemed to apply, I thought it was worth suggesting > this as well. > Hopefully, my suggesting pushing above 60Hz helps influence your > thoughts to include people in a minority, or in other words, include > the left-handed scissor or camera users too ;-) > > cheers! > > > >>Jose Da Silva wrote: >> >>>On April 21, 2005 08:05 pm, PicDude wrote: >>> >>>>Anyone have a link or doc that indicates which colors of (LED) >>>>light are generally better for displays -- easier on the eyes, >>>>quicker to read, etc? I'm trying to find some facts on this, >>>>rather than just "generally known" info. >>> >>>Nothing to do with colour, but I had to repair the fluorescent in a >>>laptop, and while digging for information, I recall reading that >>>the frequency can cause headaches. I lost the article, but >>>hopefully one of these helps. >>>www.google.ca/search?hl=en&ie=ISO-8859-1&q=laptop+fluorescen >>>t+headache&btnG=Google+Search&meta= >>> >>>Some people are sensitive to the 60Hz Xmas lights that seem to be >>>common this last Xmas, so you may want to increase the refresh rate >>>somewhere above 60Hz. you may notice some computer monitors are 75 >>>or 80Hz, so perhaps those are good numbers to work with to avoid >>>headaches or sensitivities, yet low enough to allow you to use more >>>generic driver circuitry. >>>You will also want to avoid multiples of 60Hz, like 120Hz, 180Hz in >>>countries with 60Hz power, or multiples of 50Hz in countries with >>>50Hz power. >>> >>>Otherwise, traffic safety seems to know about this type of stuff, >>>which also brings up the subject of colour blindness too. >>>Here's something: >>>www.tc.gc.ca/tdc/summary/14000/14030.htm

On April 22, 2005 09:30 pm, Robert Rolf wrote: > I have 'normal' persistence so low refresh rates don't bother me. > But I can also spot LED lights or other strobed lighting (LED sign > boards) because they make distinct spots of light as I pan my > viewpoint across them. They also lack persistence, which you may note if you were to compare, let us say, taillights and turn signals on cars, tucks, buses, etc. {Quote hidden}> Some people do appear to have much shorter persistence than normal. > One friend of mine, who was a military pilot, can't stand to look > at my CRTs because they flicker too much for him. Having shorter > persistence, and so lower blur when flying near ground, > is probably an advantage to a pilot. I wonder what the chemistry > difference is. > > E... has no problem looking at laptops since their LCDs are very > slow to respond (80msec IMS), so I rather doubt that laptops can be > causing headaches from the REFRESH. They may be causing headaches > because of seating posture or other mechanical issues (close focus > etc.). The article (which I couldn't find) was referring to the refresh rate of the fluorescent tube. Some laptops use PWM to brighten/dim the screen. The refresh rate of the LCD itself is rather slow because you are working on a liquid. Liquid has mass, therefore rather slow. I would not expect an LCD screen to refresh any higher than, let us say, 15Hz, and even that may be an optimistic figure.

Jose Da Silva wrote: > On April 22, 2005 09:30 pm, Robert Rolf wrote: >>But I can also spot LED lights or other strobed lighting (LED sign >>boards) because they make distinct spots of light as I pan my >>viewpoint across them. > > They also lack persistence, which you may note if you were to compare, > let us say, taillights and turn signals on cars, tucks, buses, etc. I was referring specifically to AC powered or strobed LEDs. DC powered LEDs leave a streak trail, and yes, they certainly turn on/off instantly. >>causing headaches from the REFRESH. They may be causing headaches >>because of seating posture or other mechanical issues (close focus >>etc.). > The article (which I couldn't find) was referring to the refresh rate of > the fluorescent tube. Some laptops use PWM to brighten/dim the screen. Hadn't thought of that since every laptop whose upconverter I can hear has been in the khz range. They'd need a much larger transformer to run at rates low enough to get flicker (10's of Hz). > The refresh rate of the LCD itself is rather slow because you are > working on a liquid. Liquid has mass, therefore rather slow. I would > not expect an LCD screen to refresh any higher than, let us say, 15Hz, > and even that may be an optimistic figure. The response time is very much depends on the type of LCD chemistry. TN (twisted nematic) STN (super..) etc. http://www.perfectlcd.com/productlcd_type.htm Newer LCD monitors are now specifying response time in addition to contrast ratio and pixel count/pitch. I see adverts for 'super fast 8 ms' displays since gamers want a fast responding display, so 125Hz refresh seems possible now. Robert

On 4/23/05, Robert Rolf <spam_OUTRobert.RolfTakeThisOuTualberta.ca> wrote: ... > I think the important issue about reading LEDs is to have a clear > contrast between light and dark states, and broader spectrum > (yellow green rather than pure green) to address color blindness > issues. Ideally, you have indicators that are UNIcolor and only > light for ONE state, rather that trying to multiplex the function. > e.g. Power led on monitors, green=on, yellow=powersave, red=standby/off > A R-G color blind person would have a difficult time distinguishing red and > green states. > > Robert Ive seen some red-green LEDs that I have a hard time telling the difference between green and yellow states. And some others are more orange than yellow, and you cant tell if it is in red or yellow state. And I have "normal" vision other than some near sightedness. So yes, avoid R-G tricolor LEDs if at all possible. Jonathan

On Fri, 22 Apr 2005, Robert Rolf wrote: > I have 'normal' persistence so low refresh rates don't bother me. > But I can also spot LED lights or other strobed lighting (LED sign > boards) because they make distinct spots of light as I pan my > viewpoint across them. Try watching the computer screen while munching a sandwich (or something crunchy). > Some people do appear to have much shorter persistence than normal. > One friend of mine, who was a military pilot, can't stand to look > at my CRTs because they flicker too much for him. Having shorter > persistence, and so lower blur when flying near ground, > is probably an advantage to a pilot. I wonder what the chemistry > difference is. It's very likely not chemistry but nervous system responsivity. > E... has no problem looking at laptops since their LCDs are very > slow to respond (80msec IMS), so I rather doubt that laptops can be > causing headaches from the REFRESH. They may be causing headaches > because of seating posture or other mechanical issues (close focus etc.). Laptops with faulty (or dimmed) bakcklights are known to have all sorts of annoying flicker. It could be that. The discharge path in a undervolted tube takes the shape of a moving or meandering spiral. The frequency of the spin depends on a number of factors but it can be in flicker range. I have read somewhere that LED and VFD displays must be muxed at 70+Hz frame rate to avoid flicker (probably more for cockpit applications). You can often see the flicker with the corner of your eye. It 'helps' if the display is quite dark and *not* red. Apprently the rods have higher speed response than the cones but the cones can be really resistive to flicker in certain ranges. Peter

Jonathan Hallameyer wrote: > Ive seen some red-green LEDs that I have a hard time telling the > difference between green and yellow states. And some others are more > orange than yellow, and you cant tell if it is in red or yellow state. > And I have "normal" vision other than some near sightedness. So yes, > avoid R-G tricolor LEDs if at all possible. This is somewhat of an aside, but it illustrates the problem. Yesterday I was assistant-reffing a soccer game. One team wore darkish green shirts and the other darkish red. This field is in a large open area with the horizon being the tops of trees about 300 meters away. It was raining, and at the beginning of the second half it was coming down hard with dark clouds above, but a lighter patch just above the trees on the other side of the field from me. I was tracking the second to last defender, and was keeping my eye on an attacker that was is off sides position. When she received the ball I raised my flag, and then realized that her shirt was in fact green, not red. I put my flag down right away and felt like an idiot. Even after realizing this was a problem and paying extra careful attention, it was sometimes not easy at all to distinguish the two shirt colors. ***************************************************************** Embed Inc, embedded system specialists in Littleton Massachusetts (978) 742-9014, http://www.embedinc.com

On Tue, 2005-04-26 at 01:16 +0100, Howard Winter wrote: > Not for me! I can't stand CRTs running at less than about 65Hz because I *can* see the flicker - and it's > much worse when I look away and see it in peripheral vision. And here we have 50Hz mains, and it happens > whether there are lights on or not. Upping the refresh rate to 70Hz stops the problem. Might I also add that one doesn't have to actually NOTICE the flicker to be bothered by it. 60Hz is pretty good at causing most people eye strain, yet many don't notice the flicker at all. Personally 75Hz is a bare minimum, it is sometimes annoying but usually OK for my eyes, 85Hz is what I run if I can. That's one really nice thing about LCD, although LCD has it's own set of problems (aliasing at non native resolutions, "ghosting", diminished viewing angles and contrast, etc.). TTYL ----------------------------- Herbert's PIC Stuff: http://repatch.dyndns.org:8383/pic_stuff/

On April 23, 2005 10:32 am, Robert Rolf wrote: > Jose Da Silva wrote: > > On April 22, 2005 09:30 pm, Robert Rolf wrote: > >>But I can also spot LED lights or other strobed lighting (LED sign > >>boards) because they make distinct spots of light as I pan my > >>viewpoint across them. > > > > They also lack persistence, which you may note if you were to > > compare, let us say, taillights and turn signals on cars, tucks, > > buses, etc. > > I was referring specifically to AC powered or strobed LEDs. > DC powered LEDs leave a streak trail, and yes, they certainly > turn on/off instantly. This should help you understand how the light in a laptop is supplied. The light in the laptop created by upconverting the battery power to something the fluorescent can use, so it has nothing to do with the AC mains. I suspect that the desktop models run on the same principle too, because several models also have brightness/dimness features on them too. http://computer.howstuffworks.com/question580.htm ...speaking of the dark blue LEDs (and light blue LEDs), it appears there is an interesting blurb on this page too: "Human eyes are not sensitive to ultraviolet light, but human skin is" {Quote hidden}> >>causing headaches from the REFRESH. They may be causing headaches > >>because of seating posture or other mechanical issues (close focus > >>etc.). > > > > The article (which I couldn't find) was referring to the refresh > > rate of the fluorescent tube. Some laptops use PWM to brighten/dim > > the screen. > > Hadn't thought of that since every laptop whose upconverter I can > hear has been in the khz range. They'd need a much larger transformer > to run at rates low enough to get flicker (10's of Hz). The weblink above links to the article I had trouble finding, so here is the article which may be worth reading concerning refresh rates and headaches: http://computer.howstuffworks.com/framed.htm?parent=ARTICLE-FILENAME.htm&url=www.cloanto.com/users/mcb/19960719lcd.html