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-   -   Grayhawk, HC DaMat, or KBKs paint? (http://www.avsforum.com/forum/23-screens/19303-grayhawk-hc-damat-kbks-paint.html)

kelliot 01-03-2001 09:02 PM

It seemed to me time for a post comparing gray screen options.

So I thought that I'd express some opinions and see what others have to say.

The most obvious options are:
1) the Stewart Greyhawk
2) the DaLite HC DaMat
3) KBK's paint
4) other DIY options

1) The Stewart Greyhawk. I wasn't a beta tester but my biggest concern is the 85% gain. I wish that I had a better idea of what it looks like without a projector, but my Kodak 90% card is pretty close to matte white and its hard to see how an additional 5% improvement is going to change much over a white screen. Certainly for those with 300 lumen/m^2 setups, this is better than the white versions IMO. I just worry that its not enough for those with 500 lumens/m^2.

2) The DaLite HC DaMat- I got my second sample today and guess what, its the same as the first one and seems way too dark. For reference I'm using a Kodak 90%/18% set of gray cards. The color of the DaMat is much closer to the 18%. It may be great for 1000 lumens/m^2 but it will be too dark for most users IMO.

3) KBKs paint. I haven't sprung for the $50 beta liter (is it Canadian $'s?). KBK's paint should at least permit the user to get the desired gray level. The only problem is that one needs to know how to paint very, very uniformly which is beyond the capbility for many of us.

4) Other DIY options- Don't rule out dyed bed sheets until you've seen one. I haven't but my experience with gray fabrics has been positive. The alternatives are endless, but a choice of backing material is important and I have seen some moire effects even with a tight weave.

Don Stewart 01-03-2001 09:24 PM

Kelliot.
You never took me up on a factory tour.
Bring your projector and you can try one.
By the way...The gray is much darker on the
Grayhawk than the gray scale card that you mentioned.
How about after CES?
Regards,
Don

KFung 01-03-2001 09:26 PM

Keep in mind that the GrayHawk has a gain less than 85% at the dark end of the grayscale. From this thread http://www.avsforum.com/ubb/Forum9/HTML/000583-3.html the brightness numbers for "black" indicate a gain closer to 60% (or 0.6), from the 0.303 FL for the 1.0 gain sample and the 0.191 from the GrayHawk screen. So for the dark part of the picture, the gain is a highly desireable 0.6 or so, and for the lighter portion it is the quoted 0.85 (which is desireably light, so we don't lose as much light).

Regards,

Kam Fung

kelliot 01-04-2001 07:57 AM

Don,
I am negligent in that respect but still intend to take you up on it. Its just far less time consuming to do things at home. If the Grayhawk is effectively grayer I should be much happier with it. I base my statements on the published and anecdotal data.

KFung,
The only way that measured gain can differ between the white and dark ends of the scale at these light intensities is if ambient is playing a role. I think that Noah Katz and some other posters have explained this. Its also been pointed out that dark or black frames present the toughest case.




[This message has been edited by kelliot (edited 01-04-2001).]

KFung 01-04-2001 07:54 PM

kelliot,

If you can point me to that thread, I would love to read it over. I don't remember it very well (I might have missed it).

However, I don't think their discussion quite covers this situation. Noah was probably talking about comparing the contrast ratios of regular and gray screens. Don's test was not comparing regular and gray screens though, it was comparing a small sample of a regular screen and a gray screen. The situation is a little different, instead of measuring values off a full size regular screen, the values now come off a very small sample. If you have time please read my post in this thread http://www.avsforum.com/ubb/Forum9/HTML/000583-3.html after Don posted his results. It explains why I think that Don's use of a small sample instead of a full size regular screen makes all the difference. The short of it is that with a small sample the ambient light would be the same for both the gray screen and small sample measurements.

I'll attempt to elaborate here:
Mr. Wiggles' post in the same thread shows how a gray screen can increase ANSI contrast ratios. This is because there is less ambient light bouncing around the room, which makes sense right? Less ambient light to pollute the black level. The same decrease in ambient can be achieved if you make everything in the room darker instead of using a gray screen, this likewise decreases ambient light. In fact, if you made your room very light absorbant you could increase your ANSI contrast without a gray screen. I believe the consensus that Noah's thread probably reached was that, while ANSI contrast is enhanced by a gray screen, actual contrast in the abscence of ambient light is not. Is that right? The increase in ANSI contrast comes from the difference in ambient light from a regular screen and a gray screen.

However, if you finagled it so that the regular and gray screen both had the same amount of ambient light when you tested them, you would find that their ANSI contrasts would be the same. The is what Don did when he used a small sample in his test. What cannot be explained by ambient light is why the ANSI contrast ratio was different for the small sample and the GrayHawk screen when the ambient level of light was the same for both measurements.

Not the best explanation, I know.

Regards,

Kam Fung

kelliot 01-04-2001 09:28 PM

The crux of the problem is the uncertainty in measurement of very low light levels. The balck level can easily be off by 30% if a strong white is present as with an ANSI contrast measurement.

A full on/full off measurement is more straightforward because projector related ambient is minimized. This is also the case when most of us notice that the black screen is closer to gray tahn black. It is very difficult to get different contrast ratios with a full on/full off measurement because the balck and white intensities don't mix.
However, the ANSI contrast has an ambient term that is quadratic in the gain of the screen. Hence very modest gray levels (matte gain=.8) can produce excellent reductions in black level. (.64) with an aparent improvement in contrast of (1/.8) or 125%. This only effects the screen-based ambient and not the direct contrast term(projector) or other ambient terms(lights,windows). It also changes with environment (like black walls) so its tough to quantify in a standard way.

I myself prefer a gain of .65-.7 at >500 lumens/m^2.



[This message has been edited by kelliot (edited 01-05-2001).]


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