Hi Guys,
I was just reading someone's comments about the rainbow effect on DLP projectors and had an idea of what might be causing this and why some people see it and others don't. Please note that I might be totally out to lunch on this, but here goes:
Apparently the rainbow effect appears as multi-colored artifacts on the edges of moving objects. The fact that it seems to appear only on moving objects is an important clue.
My understanding is that a single panel DLP projector constructs a given video frame by flashing each of the primary color components on the DLP panel in sequence as each of the red, green, and blue segments of the color wheel pass in front of it. The refreshing of the DLP panels must be in perfect sync with the color wheel segments in order for the whole thing to work properly.
Assuming a video frame rate of 30fps, the color wheel should be spinning at 90 revolutions per second so it can pass three color segments in front of the panel in 1/30th of second for a single frame. Also, the DLP panel will need to be refreshed at the same rate (90 images/sec).
So assuming the above is correct, then here's what might be causing the rainbow effect:
1) The DLP projectors on the market are spinning the color wheel and refreshing the DLP panels at speeds less than 90rps or at rates that are not a multiple of the video frame rate. If the projector is refreshing the panels less then 3x the video frame rate, or in a non synchronous manner, you could have a situation when the video frame changes in the middle of an R,G,B color wheel sequence (ie, you get say the red and green images from frame N, followed by the blue image from frame N+1). On a stationary image, this wouldn't be that noticable, but on a moving object you would see color artifacting along the trailing edges of moving objects.
2) The persistence of the DLP panel might be longer than 1/90th of a second. In other words, the DLP panel might not be able to fully reset and refresh at 90fps. Since the DLP panel is a solid state device, I doubt this would be the problem, but if it was, you might see a strange blending of colors, especially on the edges of moving objects. Actually, the max refresh rate of the DLP panel would dictate the speed of the color wheel and might be the underlying reason for #1 above.
As for the reason some people see the rainbow effect and other don't: the sensitivity of the HVS (human visual system) to the above is kind of a perceptual thing. For example, I imagine some people might not be able to tell the difference between video at 20fps and 30fps. People's sensitivity to things like color and contrast also vary by some margin. Thus the "perceptual persistence" of some people's eye might be high enough to mask the rainbow effect completely.
If my assumptions are correct, I would expect that the rainbow effect would be most noticable on B&W video, especially in high contrast seqeuences. Also, DLP projectors with no color wheel (and 3 DLP panels) would not exhibit the rainbow effect.
I don't own a DLP projector and have actually never seen the images they produce. I do have some background in image and video compression, and have messed around with this kind of stuff quite a bit. Perceptual coding seeks to exploit the imperfections of the HVS and I've learned that the eye can be fooled quite a bit. Its possible the engineers that have worked on the DLP systems out there built their systems on the assumption that most people's HVS will mask the rainbow effect, or won't find it that objectionable.
Anyway, just some food for thought. If there are any engineers out there that have worked on the development of DLP projectors, I'd love to hear your thoughts on this. Its funny that the industry has been pretty quiet on the rainbow effect and its causes - they don't really seem to acknowledge its existence.
Anyone else out there have any thoughts on this?
Thanks!
Mike
I was just reading someone's comments about the rainbow effect on DLP projectors and had an idea of what might be causing this and why some people see it and others don't. Please note that I might be totally out to lunch on this, but here goes:
Apparently the rainbow effect appears as multi-colored artifacts on the edges of moving objects. The fact that it seems to appear only on moving objects is an important clue.
My understanding is that a single panel DLP projector constructs a given video frame by flashing each of the primary color components on the DLP panel in sequence as each of the red, green, and blue segments of the color wheel pass in front of it. The refreshing of the DLP panels must be in perfect sync with the color wheel segments in order for the whole thing to work properly.
Assuming a video frame rate of 30fps, the color wheel should be spinning at 90 revolutions per second so it can pass three color segments in front of the panel in 1/30th of second for a single frame. Also, the DLP panel will need to be refreshed at the same rate (90 images/sec).
So assuming the above is correct, then here's what might be causing the rainbow effect:
1) The DLP projectors on the market are spinning the color wheel and refreshing the DLP panels at speeds less than 90rps or at rates that are not a multiple of the video frame rate. If the projector is refreshing the panels less then 3x the video frame rate, or in a non synchronous manner, you could have a situation when the video frame changes in the middle of an R,G,B color wheel sequence (ie, you get say the red and green images from frame N, followed by the blue image from frame N+1). On a stationary image, this wouldn't be that noticable, but on a moving object you would see color artifacting along the trailing edges of moving objects.
2) The persistence of the DLP panel might be longer than 1/90th of a second. In other words, the DLP panel might not be able to fully reset and refresh at 90fps. Since the DLP panel is a solid state device, I doubt this would be the problem, but if it was, you might see a strange blending of colors, especially on the edges of moving objects. Actually, the max refresh rate of the DLP panel would dictate the speed of the color wheel and might be the underlying reason for #1 above.
As for the reason some people see the rainbow effect and other don't: the sensitivity of the HVS (human visual system) to the above is kind of a perceptual thing. For example, I imagine some people might not be able to tell the difference between video at 20fps and 30fps. People's sensitivity to things like color and contrast also vary by some margin. Thus the "perceptual persistence" of some people's eye might be high enough to mask the rainbow effect completely.
If my assumptions are correct, I would expect that the rainbow effect would be most noticable on B&W video, especially in high contrast seqeuences. Also, DLP projectors with no color wheel (and 3 DLP panels) would not exhibit the rainbow effect.
I don't own a DLP projector and have actually never seen the images they produce. I do have some background in image and video compression, and have messed around with this kind of stuff quite a bit. Perceptual coding seeks to exploit the imperfections of the HVS and I've learned that the eye can be fooled quite a bit. Its possible the engineers that have worked on the DLP systems out there built their systems on the assumption that most people's HVS will mask the rainbow effect, or won't find it that objectionable.
Anyway, just some food for thought. If there are any engineers out there that have worked on the development of DLP projectors, I'd love to hear your thoughts on this. Its funny that the industry has been pretty quiet on the rainbow effect and its causes - they don't really seem to acknowledge its existence.
Anyone else out there have any thoughts on this?
Thanks!
Mike