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I think that CRT monitors create a rather strong unvisible flicker, even for a vertical frequency equal to or above 80 Hz.
1. How large is the measurable luminance variation for a uniform gray rectangle on a CRT monitor ? The most simple model is L=Lo*exp(-t/T) What's the average time constant T for CRT monitors ?
2. Does anybody know an investigation about the damage of the neuronal system, if the operator is exposed over many years to this CRT flicker ? The neuronal system has a bandwidth in the region of 40 Hz. This can lead to rather strong aliasing effects, and it cannot be expected, that eye+brain are prepared to handle these signals without confusion.
> I think that CRT monitors create a rather strong > unvisible flicker, even for a vertical frequency equal to > or above 80 Hz.
> 1. How large is the measurable luminance variation for a > uniform gray rectangle on a CRT monitor ? > The most simple model is L=Lo*exp(-t/T) > What's the average time constant T for CRT monitors ?
> 2. Does anybody know an investigation about the damage > of the neuronal system, if the operator is exposed over > many years to this CRT flicker ? > The neuronal system has a bandwidth in the region of > 40 Hz. > This can lead to rather strong aliasing effects, and > it cannot be expected, that eye+brain are prepared to > handle these signals without confusion.
I've corrected the headline. Besides the aliasing effect it might be interesting to investigate the luminance control system in the eyes. We're perceiving the average luminance. The peak luminance is probably much higher.
Flicker of the nature that you describe would be a problem if the uniform gray were made by field sequences of white and black but that tends to not be the case for modern graphics monitors. Gray is produced my modulating the individual pixels not by modulating the sequential fields. The pixel scan rate is much higher - in the megahertz region.
The parameter that you need for your comparison is the critical flicker fusion frequency - the speed at which the visual system cannot distinquinsh a change in light because the photoreceptors cannot change state quickly enough. This is relatively low for chromatic differences around the 40Hz that you cited from Andrew Stockman. But the critical flicker fusion frequency is much higher for luminance contrasts. One can sense flicker at refresh rates up to 100 Hz if the contrast is high enough. Such high constrast images are rare in natural scenes so there is generally no need for concern. Older radar screens with a black background and a bright white pattern would be the main display where some concern might be expressed. Fortunately, modern radar screens now have a large number of images in the scan and often include chroma contrasts rather then just luminance contrasts.
Danny
"Gernot Hoffmann" <hoffm...@fho-emden.de> wrote in message
>I think that CRT monitors create a rather strong > unvisible flicker, even for a vertical frequency equal to > or above 80 Hz.
> 1. How large is the measurable luminance variation for a > uniform gray rectangle on a CRT monitor ? > The most simple model is L=Lo*exp(-t/T) > What's the average time constant T for CRT monitors ?
> 2. Does anybody know an investigation about the damage > of the neuronal system, if the operator is exposed over > many years to this CRT flicker ? > The neuronal system has a bandwidth in the region of > 40 Hz. > This can lead to rather strong aliasing effects, and > it cannot be expected, that eye+brain are prepared to > handle these signals without confusion.
> Flicker of the nature that you describe would be a problem if the uniform > gray were made by field sequences of white and black but that tends to not > be the case for modern graphics monitors. Gray is produced my modulating > the individual pixels not by modulating the sequential fields. The pixel > scan rate is much higher - in the megahertz region.
> The parameter that you need for your comparison is the critical flicker > fusion frequency - the speed at which the visual system cannot distinquinsh > a change in light because the photoreceptors cannot change state quickly > enough. This is relatively low for chromatic differences around the 40Hz > that you cited from Andrew Stockman. But the critical flicker fusion > frequency is much higher for luminance contrasts. One can sense flicker at > refresh rates up to 100 Hz if the contrast is high enough. Such high > constrast images are rare in natural scenes so there is generally no need > for concern. Older radar screens with a black background and a bright white > pattern would be the main display where some concern might be expressed. > Fortunately, modern radar screens now have a large number of images in the > scan and often include chroma contrasts rather then just luminance > contrasts.
> Danny
Danny,
a non-interlaced CRT monitor 'flashes' a uniform gray area like any other image once per frame. The luminance of the activated phosphors is not constant (it depends on the phosphor 'persistence', remember long persistance vector monitors). I'm expecting a behaviour like L(t) = Lo*exp(-t/T). First question: the typical value T for CRT monitors.
Additionally we have a blanking period during the vertical refresh. Both types of flicker are existing physically, even for a uniform gray image (background). Has nothing to do with patterns. Second question: where can I find a diagram L(t) ?
The display is written line by line not field by field in a non-interlace CRT. So the scan line ramps up then fades while the next scan line ramps up and so on. The transitions are thus more uniform and continuous than on/off unless the vertical refresh is very slow. Some older vector displays would load the entire memory plane and then switch the display.
The most useful information that I had ever found was supplied by the old Conrac corporation who made broadcast monitors. I do not know if they still around and/or have good technical documents. The other place to check is with the web site of the Society for Information Display (SID). They used to have several good reference link on their web site. After that I would check the NEC display site - though today most of the new documents are about LCD or plasma displays and not about CRT displays.
"Gernot Hoffmann" <hoffm...@fho-emden.de> wrote in message
>> Flicker of the nature that you describe would be a problem if the uniform >> gray were made by field sequences of white and black but that tends to >> not >> be the case for modern graphics monitors. Gray is produced my modulating >> the individual pixels not by modulating the sequential fields. The pixel >> scan rate is much higher - in the megahertz region.
>> The parameter that you need for your comparison is the critical flicker >> fusion frequency - the speed at which the visual system cannot >> distinquinsh >> a change in light because the photoreceptors cannot change state quickly >> enough. This is relatively low for chromatic differences around the 40Hz >> that you cited from Andrew Stockman. But the critical flicker fusion >> frequency is much higher for luminance contrasts. One can sense flicker >> at >> refresh rates up to 100 Hz if the contrast is high enough. Such high >> constrast images are rare in natural scenes so there is generally no need >> for concern. Older radar screens with a black background and a bright >> white >> pattern would be the main display where some concern might be expressed. >> Fortunately, modern radar screens now have a large number of images in >> the >> scan and often include chroma contrasts rather then just luminance >> contrasts.
>> Danny
> Danny,
> a non-interlaced CRT monitor 'flashes' a uniform gray area like > any other image once per frame. > The luminance of the activated phosphors is not constant > (it depends on the phosphor 'persistence', remember long > persistance vector monitors). > I'm expecting a behaviour like L(t) = Lo*exp(-t/T). > First question: the typical value T for CRT monitors.
> Additionally we have a blanking period during the vertical > refresh. > Both types of flicker are existing physically, even for a uniform > gray image (background). Has nothing to do with patterns. > Second question: where can I find a diagram L(t) ?
> The display is written line by line not field by field in a non-interlace > CRT. So the scan line ramps up then fades while the next scan line ramps up > and so on. The transitions are thus more uniform and continuous than on/off > unless the vertical refresh is very slow. Some older vector displays would > load the entire memory plane and then switch the display.
> The most useful information that I had ever found was supplied by the old > Conrac corporation who made broadcast monitors. I do not know if they still > around and/or have good technical documents. The other place to check is > with the web site of the Society for Information Display (SID). They used > to have several good reference link on their web site. After that I would > check the NEC display site - though today most of the new documents are > about LCD or plasma displays and not about CRT displays.
Danny,
I've done a fast test, using an 'unspecified' photosensor. a) measure dark level in a black rectangle (near zero). b) measure signal in a white rectangle. The signal is a peak, approximately like a Gaussian bell, but with slightly faster rise time. The decay time T is about 1ms, the whole pulse width about 2ms. The repetition time is Tv=(1/75)s, approximately Tv=13ms.
It's indeed necessary that T is much smaller than Tv. Otherwise the monitor would be on top much brighter than at the bottom.
I'm going to buy a qualified sensor and repeat the experi- ments. So far I'm feeling confirmed that eye+brain are exposed to strong & unhealthy flicker.
Thanks for the informations - so far no useful information in the Web.
Gernot Hoffmann wrote: > Danny Rich schrieb: > > The display is written line by line not field by field in a non-interlace CRT.
It is done by pixel by pixel in all CRT tubes.
> b) measure signal in a white rectangle. > The signal is a peak, approximately like a Gaussian bell,
Color CRT tube has three electron beam "guns" (R, G and B) and the three beams are addressing each pixel individually on the screen, that is the three beams are electro-magnetically deflected from pixel to the next pixel on a line until the end of the line, then the same is done on the next line etc all the way down to the last line. Only one pixel at any given time is "refreshed", so, in order to measure the form of the luminance peak you'd need to measure just one white pixel with the screen otherwise at RGB=0.
> The decay time T is about 1ms, the whole pulse width about 2ms.
The persistency of the phosphors affect to these a lot but your sensor is seing many many pixels on different lines.
> The repetition time is Tv=(1/75)s, approximately Tv=13ms.
In other words the monitor is being driven at 77Hz frame rate. The repetition time is not affected even if you measure many pixels on many lines, but the form of the luminance peak is, stronly. E.g. a 1280 x 1024 pixel screen has total of 1310720 pixels, if the screen refresh rate (non-interlaced) is 75Hz then there will be (less than) 10.17ns for refreshing each pixel.
I'm assuming that a CRT monitor with phosphor P4 has a decay time 0.330 ms, down to 10%. The decay time down to 50% may be T=0.150 ms. For a monitor with 1000 lines, each frame updated in 13ms (76.923076 Hz in your nomenclature), one line consumes 0.013 ms. Considering the decay time T, one can say: about 10 lines are evenly lit. This band goes from top to bottom, and when it passes my detector, then I get the mentioned pulse. There is nowhere any need for investigating the pulse shape for a single monitor 'pixel'.
The eye is exposed to this moving band of illuminated lines. The peak value is much higher than the measured average luminance of 100 cd/m2. The retina is exposed to peak values. Because of eye movements these bands and peaks are projected onto different areas. If the eye's visual axes were fixed, then the foveas would perceive an extraordinarily strong perio- dical flicker, and the other regions would perceive the same flicker with phase shifts. The fovea area is most sensitive. Roughly some millimeters diameter, projected by angles onto the monitor.
I'm mainly interested in an answer, whether this can affect the neural system (it's not necessary to explain basic principles of CRT monitors).
The light of your LCD display is generated by fluorescent tubes that operates by repeated very rapid electrical discharges. My understanding is that the flicker that Gernot refers to (peak luminance) is far higher with fluorescent tubes than with CRTs.
Gernot Hoffmann wrote: > Considering the decay time T, one can say: about 10 lines > are evenly lit. This band goes from top to bottom, and when > it passes my detector, then I get the mentioned pulse. > There is nowhere any need for investigating the pulse shape > for a single monitor 'pixel'.
OK, you are correct. I took some pictures of my screen at f/2.8 and 1/1000s and at 100 ASA. The (about) evenly lit band is underexposed by one f/stop and has height of about 50/1500 of the screen height.
Taking that the band fully exposes at f/2.8, 1/500s and 100 ASA the luminance of this band hardly is damaging.
It would be rather interesting to know if the pupil diameter is the same when the eye is exposed to:
A) a 100cd/m2 CRT white B) a paper at 100cd/m2 when the paper is illuminated by a direct current driven lamp.
> > Considering the decay time T, one can say: about 10 lines > > are evenly lit. This band goes from top to bottom, and when > > it passes my detector, then I get the mentioned pulse. > > There is nowhere any need for investigating the pulse shape > > for a single monitor 'pixel'.
> OK, you are correct. I took some pictures of my screen at f/2.8 and > 1/1000s and at 100 ASA. The (about) evenly lit band is underexposed by > one f/stop and has height of about 50/1500 of the screen height.
> Taking that the band fully exposes at f/2.8, 1/500s and 100 ASA the > luminance of this band hardly is damaging.
> It would be rather interesting to know if the pupil diameter is the same > when the eye is exposed to:
> A) a 100cd/m2 CRT white > B) a paper at 100cd/m2 when the paper is illuminated by a direct current > driven lamp.
> Timo Autiokari
Timo,
interesting test. I found about 15 bright lines of 768 lines for 1/4000 s:
Hello Gernot, so in your case the bar is about 2% of the height of the monitor. Then we could approximate that the luminance of that bar is 100/2 = 50 times the average max luminance of the screen, so in the range of 50x100cd/m2 = 5000cd/m2.
> The doc shows as well the frequency response for the pupil control system.
I was thinking that:
When looking at a CRT the eye is subjected to something like 5000cd/m2 flicker that will average to something like 100cd/m2. This CRT flicker is so very rapid that the pupil reflex (in the usual/common sense) can not be activated due to it.
So, when we look at a paper that has 100cd/m2 (illuminated by direct current lamp) then the pupil is certainly adjusted naturally, it will be wide open or close to that.
But when we look at a CRT that has 5000cd/m2 flicker that averages to 100cd/m2 then how is the pupil adjusted in this case? In case it is wide open similarly as above then there will be somewhat higher load to the optic nerves. It would also be an un-natural case that possibly could affect to the way we preceive the tonal range (especially in the dark end) on the CRT compared to the same situation on the paper.
> When looking at a CRT the eye is subjected to something like 5000cd/m2 > flicker that will average to something like 100cd/m2. This CRT flicker > is so very rapid that the pupil reflex (in the usual/common sense) can > not be activated due to it.
> So, when we look at a paper that has 100cd/m2 (illuminated by direct > current lamp) then the pupil is certainly adjusted naturally, it will > be wide open or close to that.
> But when we look at a CRT that has 5000cd/m2 flicker that averages to > 100cd/m2 then how is the pupil adjusted in this case? In case it is > wide open similarly as above then there will be somewhat higher load to > the optic nerves. It would also be an un-natural case that possibly > could affect to the way we preceive the tonal range (especially in the > dark end) on the CRT compared to the same situation on the paper. > Timo Autiokari
Timo,
the Web offers little about this issue. 5000cd/m2 is rather bright. Can we interprete this as the luminance of a certain bulb or tube ? The linear averaging (here to 100cd/m2) seems to be correct, see the chapter about flicker in W+S.
Ref 19 concerns a publication by Berman, Greenhouse and Bailey. This publication is often quoted but not available on-line. The authors had proved, that flicker is transmitted by eye+brain for frequencies above the Critical Flicker Frequency or Fusion Frequency (max. 45Hz), up to more than 142 Hz.
>> > The display is written line by line not field by field in a >> > non-interlace CRT.
> It is done by pixel by pixel in all CRT tubes.
Yes but the drive electronics for a single scan line is separate from the drive electronics for one line to the next. a 640 x 512 will have approximate a 31kHz pixel rate but 60 Hz line rate. There is a horizontal blanking period in which the gun magnets are reset to pixel column 1 without striking the phosphors. This is easiest to visualize in the Sony "stripe" technology. There are no pixels in this only Red, Green, Blue stripes of phosophor. The shadow mask slots define the pixel locations and act as field stops for the electon beam. The beam is continuous and is swept horizontally and veritically - not fired pixel by pixel.
>> b) measure signal in a white rectangle. >> The signal is a peak, approximately like a Gaussian bell,
> Color CRT tube has three electron beam "guns" (R, G and B) and the > three beams are addressing each pixel individually on the screen, that > is the three beams are electro-magnetically deflected from pixel to the > next pixel on a line until the end of the line, then the same is done > on the next line etc all the way down to the last line. Only one pixel > at any given time is "refreshed", so, in order to measure the form of > the luminance peak you'd need to measure just one white pixel with the > screen otherwise at RGB=0.
>> The decay time T is about 1ms, the whole pulse width about 2ms.
> The persistency of the phosphors affect to these a lot but your sensor > is seing many many pixels on different lines.
>> The repetition time is Tv=(1/75)s, approximately Tv=13ms.
> In other words the monitor is being driven at 77Hz frame rate. The > repetition time is not affected even if you measure many pixels on many > lines, but the form of the luminance peak is, stronly. E.g. a 1280 x > 1024 pixel screen has total of 1310720 pixels, if the screen refresh > rate (non-interlaced) is 75Hz then there will be (less than) 10.17ns > for refreshing each pixel.
> White paper under sunlight can have far higher luminance.
> > The linear averaging (here to 100cd/m2) seems to be correct, > > see the chapter about flicker in W+S.
> Indeed, so why is it that the alleged non-linearity of the tonal range > of the vision does not come into play at all in this situation?
> Timo Autiokari
Timo,
I'm understanding your point: luminance vision is linear if the adaptation state is fixed. But the flicker test is perhaps useless as a proof - if the averaging happens by optical-chemical processes instead of neuronal 'calculations'.
About the lightness of the horizontal bar on a CRT monitor: maybe 5000 cd/m2 for an averaged luminance of 100 cd/m2. White clouds on a sunny sky: 6700 cd/m2, blue sky 4000 cd/m2 according to René Bouillot, La pratique du reflex argentique & numerique. But this is misleading, because eye+brain are adapted to the average monitor luminance.
The next part is a simplified description for a test, using a SoLux bulb 4700K and the matt side of Chromolux carton, probably a Lambertian reflector.
L = (1/pi) R E cos(Phi)
L: Luminance in cd/m2 E: Illuminance in lx R: Reflectance factor, perhaps 0.8 Phi: Angle between light direction and reflector normal, zero
Monitor averaged: E = 400 lx L = 100 cd/m2
Monitor peak: E = 20000 lx L = 5000 cd/m2
The SoLux bulb delivers approximately E1 = 1000 lx in distance d1 = 1m, as measured by Spectrocam for the reflected light.
For E2 = 20000 lx the distance d2 is calculated by d2/d1 =Sqrt(L1/L2), which delivers d2 = 0.22m . This spot is perceived as 'very bright' for monitor adapted eyes.
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