How important is screen iridescence to a DLP PJ?

[Harpmaker]

This topic started in another thread, but it was a bit off-topic there so I'm starting this one.

In the other thread I said "Perhaps Texas Instruments is simplifying the matter too much, but their explanation of how a single-chip DLP projector works ( http://www.dlp.com/tech/what.aspx ) says, in both text and graphic, that the DLP chip is struck by only red, green and blue light (the light that passes through the color filters on the color wheel), thus only red green and blue light hits our screens using such a PJ, and only one color at a time. It is the "persistence of vision" of our eyes that makes us see the full visible light spectrum. As I'm sure you know, it is the same phenomenon that lets us watch a CRT television without seeing the even and odd scan fields, but rather one image that doesn't flicker.

If what T.I. says is true, I don't understand how iridescence can come into play with such a PJ. "

Bill misunderstood me (unless I'm misunderstanding him ) and thought I was talking about a screen, such as the one by Sony (Chromavue?), that is comprised of transparent red, green and blue layers on a reflective substrate. I actually made a mini-version of such a screen using layers of red, green and blue transparent plastic and it simply doesn't work for DIY.

For those coming late to the party (), it is a known fact that the mica particles commonly used in paints to create a metallic appearance act is prisms and break up white light into multiple colors. If a projector that shoots white light is used with a screen that was painted with a mica-based paint, color-shifting of the image will occur due to this prismatic effect.

What I'm getting at is that a simple one-chip DLP projector NEVER shines white light on the screen (see link above). Such a PJ shines red, green and blue light on the screen, and only one color at a time, never two or three. These also are the only colors reflected from the screen (except for any ambient light present). The reason we see the full visible spectrum of light is because of eyes "mix" the colors from the RGB light. I find that concept fascinating!

So, this means that even though the screen may be very prismatic under white light, when used with the PJ described above, the screen can, at best, break up the green light into a very narrow spectrum; and the same for the blue light. The red light from a PJ is almost monochromatic (one single color). There is a chart someone did of the individual spectra of their PJ's output, but I forget where I saw it.

It would be interesting to create a very prismatic screen on purpose and then see how the images looked when it was used with DLP and LCD PJ's!

[mechman]

Harpmaker wrote:

This topic started in another thread, but it was a bit off-topic there so I'm starting this one.

In the other thread I said "Perhaps Texas Instruments is simplifying the matter too much, but their explanation of how a single-chip DLP projector works ( http://www.dlp.com/tech/what.aspx ) says, in both text and graphic, that the DLP chip is struck by only red, green and blue light (the light that passes through the color filters on the color wheel), thus only red green and blue light hits our screens using such a PJ, and only one color at a time. It is the "persistence of vision" of our eyes that makes us see the full visible light spectrum. As I'm sure you know, it is the same phenomenon that lets us watch a CRT television without seeing the even and odd scan fields, but rather one image that doesn't flicker.

If what T.I. says is true, I don't understand how iridescence can come into play with such a PJ. "

Bill misunderstood me (unless I'm misunderstanding him ) and thought I was talking about a screen, such as the one by Sony (Chromavue?), that is comprised of transparent red, green and blue layers on a reflective substrate. I actually made a mini-version of such a screen using layers of red, green and blue transparent plastic and it simply doesn't work for DIY.

For those coming late to the party (), it is a known fact that the mica particles commonly used in paints to create a metallic appearance act is prisms and break up white light into multiple colors. If a projector that shoots white light is used with a screen that was painted with a mica-based paint, color-shifting of the image will occur due to this prismatic effect.

What I'm getting at is that a simple one-chip DLP projector NEVER shines white light on the screen (see link above). Such a PJ shines red, green and blue light on the screen, and only one color at a time, never two or three. These also are the only colors reflected from the screen (except for any ambient light present). The reason we see the full visible spectrum of light is because of eyes "mix" the colors from the RGB light. I find that concept fascinating!

So, this means that even though the screen may be very prismatic under white light, when used with the PJ described above, the screen can, at best, break up the green light into a very narrow spectrum; and the same for the blue light. The red light from a PJ is almost monochromatic (one single color). There is a chart someone did of the individual spectra of their PJ's output, but I forget where I saw it.

It would be interesting to create a very prismatic screen on purpose and then see how the images looked when it was used with DLP and LCD PJ's!

I think you're splitting hairs! lol! But I do understand your thinking. The question is, at what rate does wheel spin? And if it appears white is it not white?

It brings up an interesting point though and one that could be expanded upon. Such as if the prism (mica) is of a certain quality that it refracts red faster than blue, then what?

One thing that I can say for certain is that I will delve a bit deeper into the experimentation of this. My microscope would be fully capable of capturing an image reflected off of a screen. And I have a DLP projector. I'm off to see if this is possible...

Good stuff Harp!!

mech

[mechman]

I didn't start on this yet as I was unsure whether or not my HC3000 had the clear on it's color wheel. Which, if it did, would invalidate the test. It appears that it does not. From what I've been able to gather it is a six segment wheel of RGB-RGB. I'll get on this Monday!

mech

[jimwhite]

Yeah, users of higher-lumen DLP's such as the HC1000, HD1500, and HD-70 need not apply.... they have white segments in the wheel....

[mechman]

I'm not having much luck with this! I upgraded my laptop to Vista last week and I cannot find a driver for the QX3 microscope. I'm gonna keep looking...

mech

[Harpmaker]

mechman wrote:

I'm not having much luck with this! I upgraded my laptop to Vista last week and I cannot find a driver for the QX3 microscope. I'm gonna keep looking...

mech

Didn't anybody tell you? Vista isn't a upgrade, it's problems-in-a-box (and does it's job well)!

[mechman]

Believe it or not, I actually like it! But I got a deal on 4gb of ram before I did it.

But I may have found something...

mech

[mechman]

While I work on this you can take a look at this shot of S-I-L-V-E-R, which has a high concentration of mica in it. This white image is from the 'Get Gray' calibration disk. I can see blues, greens, yellows... this with just my camera.



That image is also zoomed in 200%.

mech

[mechman]

I loaded the driver and got it working. Now the problem is the shadow of the microscope in relation to the projected light. I got a couple:





It was really a pain to do as the microscope uses distance as the focusing factor. And I had to hold it and adjust with my hands. Regardless, there is a prism effect with a six segmented color wheel lacking a clear lens. I could see it with my eyes only standing up near the panel.

mech

[Harpmaker]

Very, very interesting! (BTW, very glad Vista is treating you nice - you're in the vast minority)

I don't see the prism effect in the photos, but if you say you saw it I believe you.

Your last statement "I could see it with my eyes only standing up near the panel" brings up another question; if the effect can only be seen very near the screen, does it really make a difference in the image at normal viewing distances?

[mechman]

I circled the areas of concern.





These images combined with the original magnifications and the spectral curves make this one an easy case for the judge and jury!

About the only question that needs to be resolved in my mind is the question of higher quality mica. Ben has proven to me that it is worth a look. It's definitely not something that can be achieved with popsicle stick paints! Where have we heard that before?!?! However, it'll be up to someone else to test it (nudge nudge - wink wink). Everything I've seen so far keeps me very interested in the aluminum.

mech

[Harpmaker]

mechman wrote:

These images combined with the original magnifications and the spectral curves make this one an easy case for the judge and jury!

Playing devils advocate (), I have to say that even when I zoomed into the problem spots I see no prism effect, only clues of specularity (sparklies). If the DLP worked like I thought it might, and I say might, I would expect any screen sparklies to still show up (which I see in the photos), but not be broken up into the colors of the rainbow, but stay the color of the projected light (in this case white). This is what I see in the photos. BUT, I know how hard it is to capture what you see on a PJ screen with a camera, so if you say you see a prism effect I truly do believe you.

About the only question that needs to be resolved in my mind is the question of higher quality mica. Ben has proven to me that it is worth a look. It's definitely not something that can be achieved with popsicle stick paints! Where have we heard that before?!?! However, it'll be up to someone else to test it (nudge nudge - wink wink). Everything I've seen so far keeps me very interested in the aluminum.

I was in a Michael's today looking for Auto-Air Aluminum (no joy) and picked up some mica-based paint to play with. One bottle is clear flakes that have a VERY prismatic effect, and the other is a very dark gray paint that has large flakes that show NO prismatic effect at all. Gonna do some experimentin'...

Pleas understand that I NEVER intended to take the focus off using Aluminum flakes. Al is much better than mica all the way around for PJ screens.