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Discussion Starter #1
I bought a HS10 last December and recently began to calibrate it after reading many posts in this forum. I use a commercial lux meter (0.1 lux sensitivity) and dichroic filters to measure the three colors. I use 80IRE from the Avia dvd (played on an xp-30) and the low temperature settings to calibrate the meter. The original gain settings are R145, G 89, and B 72. I then add a cc20r optiflex filter and adjust the RGB gain and bias to R165, G150, B149 to get back the original RGB ratio at 30 and 80IRE. The pictures generally look good but black looks blue.


When I measure the contrast ratio using 0IRE and 100 IRE from the avia, I only get around 470. So I checked the contrast ratio for each color using the filters. The red is 550, green is 650, but the blue is only 280. Even when I crank the blue gain all the way up, I still only get around 320 (my picture contrast is 80). 0IRE looks distinctly blue. I remember reading a post saying he/she had contrast for all three channels above 600. Has anyone seen such a low contrast ratio for the blue channel before? Is there any adjustment to correct this problem? Or I have a defective LCD panel for the blue channel?
 

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I have just completed the first part of my calibration (color balance is within limits) and did a quick calculation on on the contrast ratios (contrast=79):


R=531

G=534

B=361


So mine also appears quite low on the blue panel, although not quite as low as your's. I still need to bring down my contrast as Green is maxed out.


Btw, my factoray default settings for Low temperature was:

Gain - R145, G78, B75

Bias - R124, G127, B117


I'm still digesting exactly what all of this means, but I was watching Blade II last night and the black levels were terrible. I'm going to try a grey screen tonight.


Xander
 

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Discussion Starter #3
Xander,


Thanks for sharing your experience. It would be interesting to see if low contrast for the blue channel is a common problem.


I was looking at some of the default settings in the factory mode. In the panel driver menu there are gain and bias for each color as well. Does anyone have any experience with these adjustments? I cannot think of any reason for the low contrast other than something related to the blue panel. leakage of blue through other channels will not cause this problem
 

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Discussion Starter #5
The problem is that with a cc20r the blue channel gain needed to get the color temperature right at 80IRE is already close to the red gain so I cannot increase blue gain much. But at 0IRE there is twice as much blue as needed.
 

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Just to clarify that my RGB contrast readings above are indeed with a CC40R filter. Also, if you look at the ColorBalance Graph produced by Smart III, the R and G are slightly too low at IRE0 - IRE10 and the B is way too high (as you pointed out). From IRE20 to IRE100 everything is spot on. Once I've completed my contrast setup I might send my spreadsheet to Steve for comment on how one might improve IRE0 - IRE20. Alternatively, if it is a generic question, Steve might want to comment here?
 

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Discussion Starter #7
Xander,


Thanks for the clarification. In principle the contrast for each color is independent of the compensating filter used. Now I begin to suspect that it is the transmittance of a full opened LCD panel that determines the contrast (I suppose that of a fully closed should be similar). An indication of high transmittance for the blue panel might a very low default blue gain (less than 50) for the low color temperature. I remember that Dave's is only 25! My default setting is 72. This could be due to the manufacturing variations. It would be interesting to see if there is indeed a correlation between default setting and contrast ratio.
 

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This is an interesting discussion, but you should all be aware that the sensitivity of the human eye relative to the three primary colours needs to be taken into account.


The eye is most sensitive to changes in luminance of the colour green, with red second and blue a distant third. That means that while the amount of blue in the mix to create a given colour is important for accuracy of the colour, it is NOT important to the perceived brightness of the colour.


To say it another way, a large contrast ratio in green equates to a large perceived contrast ratio for all colours (inlcuding black, grey, and white!), and a smaller contrast ratio in blue will not be perceived as being nearly as important to the overall contrast ratio. Red is somewhat in between.


It seems counter intuitive but that's just the way the sensors in our eyes work.


To get the maximum 'perceived' contrast ratio (not necessarily measured), then make sure you have the maximum contrast ratio in the colour green.


Dave
 

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Discussion Starter #9
Dave,


Thanks for the info. I understand that according to the CIE sensitivity curve the human eye is most sensitive to green light. But once the curve is taken into account when measuring luminance (my lux meter is CIE weighted), shouldn't identical luminance values produce the same brightness?


I can see that if only a small amount of blue is needed for a particular color temperature, then the blue contrast probably does not matter that much. But at 0IRE the measured blue from my PJ is roughly 1.5 time the green and black looks very much blue. It is rather disappointing.


Since you were able to obtain good measured contrast, I wonder if it would be possible for you to post the blue contrast for your PJ?
 

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amateur - I'd be happy to post the contrast of the blue gun. I won't be home until Sunday however - I'll see about doing it then.


Dave
 

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amateur, I haven't gone back to tweaking again yet (will probably do so sometime this long weekend) but wouldn't it help if we lower the Blue Bias a bit?
 

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Poor blue contrast ratio probably comes from blue leakage and unfortunately changing the bias setting won't help, almost by definition. You can certainly lower blue bias to fix excessive blue at IRE 20, but at IRE 0 you will need to either raise the other bias controls (bad for contrast ratio in general) or use a filter to fix the problem. A yellow filter will help with blue leakage, if green is not also in abundance at IRE 0, otherwise a red filter is probably the best choice as it will also likely allow maximizing the contrast ratio.


Steve
 

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Discussion Starter #13
Thanks for the replies.


Dave, I look forward to seeing your contrast ratio.


Xander, I did try to reduce the blue bias and there was no effect, indicating that the blue panel was already full closed at 0IRE.


Steve, Could you explain more about the possible blue leakage? Is it through the blue panel or through some other route? At this point I can only see two possibilities for low contrast ratio: a panel has a high transmittance when fully closed, or it has a low transmittance when fully open. I think my PJ fit the latter case since the default blue gain is relatively high.


With the cc20r I am using, I still have some room for increasing the green gain (10%) and blue gain (20%). I just ordered a cc10Y. I will post my findings next week.
 

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I can’t really say how the leakage gets there. Presumably it is through the panels and the polarization plates, but the point is that it bypasses the controls and so it does not respond to the bias settings. What you might want to do is to lower the brightness a bit so that all colors are at their leakage level at IRE 0, and then raise the contrast so that all colors are maxed out. No measure the contrast ratio of each color. The color with the lowest contrast ratio will determine the limiting contrast ratio (blue in your case?) and the others will need to be lowered to meet it if you wish to have a color balance that is the same at both ends of the IRE range. The question it to achieve the proper color balance while keeping all the contrast possible. You obviously don’t want to turn down the gain on the contrast limiting color. That is where CC filters come in as they allow you to balance colors without turning down the gains. Ideally you will want to do this with a few filter surfaces as possible and use the gains on the non-contrast limiting colors as the final tweak.


Steve
 

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Discussion Starter #15
Thanks Steve. If the leakage is through the panel then a possibility is that the polarizability of the panel is wavelength dependent and decreases with wavelength, resulting in more blue leakage. Therefore all LCD based projectors will have somewhat lower blue contrast ratios. It would be interesting to do a survey to see if this is true.
 

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Discussion Starter #16
I calibrated my hs10 a little more. Now I use a cc20r and a cc10y. The gains are R165 G145 B170. The measured contrast ratio is 520. Ideally I should be using a cc25r and a cc05y so I can increase the green gain to around 160. If I use this gain for my current filters I get a contrast ratio of 560. For these settings the contrast is visibly better and my eyes cannot tell any color change resulted from increasing the green gain.


The optiflex filter has a very low neutral density so you can use two and do not lose much brightness. But I do see reflected light projected onto the screen so I made the filter slanted by about 30 degree and it seems to work.


Lee offers a cc25r filter and I might try it some time. I hope it also has a low neutral density.
 

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Steve/amateur/other calibrators,


I have the new glass CC30R filter on order (hope they come in soon), but I have an initial setup question. I have done a calibration without any filter and got a very good 30ire to 100ire grayscale resulting in a CR of 533 with my HS10. Now, when I install the CC30R filter, where do I start from with the Brightness and Contrast (and to a smaller extent Color and Hue) controls? Should they be set to 50 and then start the calibration, or should I run the basic AVIA test process to set all of these and then hook up the calibration equipment and begin the RGB Gain and Bias adjusting?


Thanks,

dagger
 

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The ideal Brightness and Contrast settings should be the same with and without a filter as long as you do not change the RGB Gains and Biases at the same time as adding the filter.


The G Bias and Gain setting has the biggest effect on the Contrast and Brightness settings. Any significant change in either of these will require redoing the C and B after colour correcting.


If you leave the G Bias at 127, the correct Brightness setting will be 50 plus or minus around 2.


Changing the gain settings will require fine tuning the Contrast setting afterwards - this is the whole point of adding the filter (by maximizing the Contrast setting while keeping the colour temp of 100IRE accurate). You will simply need to experiment.


A simple method to get the Contrast setting close is to put on the gray scale chart on AVIA (vertical gray bars from black to white), crank the Contrast setting up until the bright white is clearly bluer or greener than the next brightest gray bar, then turn down the Contrast a few settings.



Dave
 

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Discussion Starter #20
daggerNC,


Your CR without a filter is already very good. What are the factory settings for the low temperature? Did you measure the blue CR?
 
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