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'[EE]: Safe viewing colors? POV'
2005\04\23@003035 by Robert Rolf

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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}

2005\04\23@023633 by Jose Da Silva

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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}

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.

2005\04\23@133224 by Robert Rolf

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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

2005\04\23@141842 by Jonathan Hallameyer

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On 4/23/05, Robert Rolf <spam_OUTRobert.RolfTakeThisOuTspamualberta.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

2005\04\23@150502 by Peter

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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

2005\04\23@150740 by Peter

picon face

On Fri, 22 Apr 2005, Jose Da Silva wrote:

> 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.

Current LCDs are advertised for 50ms response times and 20ms is
available as an extra. Some home theater devices are advertised as such.

Peter

2005\04\23@162151 by William Chops Westfield

face picon face

On Apr 23, 2005, at 10:32 AM, Robert Rolf wrote:

> 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.
>
But as far as I know, active matrix LCD display leave the pixel at the
last
state until it's actually changed; it's not like a CRT where the
electron
beam has passed on and all that's keeping the pixel there is phosphor
persistance.  At least, the "decay" time is MUCH longer than the
"update" time.

For that matter, the problem with CRTs is not so much that people can
perceive
the 60Hz flicker, but that you get "beat" effects with 60Hz lighting...

BillW


2005\04\24@093045 by olin_piclist

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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.


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(978) 742-9014, http://www.embedinc.com

2005\04\25@201657 by Howard Winter

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Bill,

On Sat, 23 Apr 2005 13:21:46 -0700, William "Chops" Westfield wrote:

> For that matter, the problem with CRTs is not so much that people can
> perceive the 60Hz flicker, but that you get "beat" effects with 60Hz lighting...

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.

Cheers,


Howard Winter
St.Albans, England


2005\04\26@130437 by Herbert Graf

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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/

2005\04\26@143724 by Jose Da Silva

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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}

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

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