[krasmuzik]

The press release for the Lumagen series processors say that you can do gamma calibration. However I was unable to find it in the manual or on the menus.


I just recently calibrated a CRT and the gamma result was 3.0 - I prefer 2.5.


So how does one calibrate the gamma on the Lumagen?

 

[oferlaor]

I don't think they implemented it yet.


The objective is not to create a simple gamma curve, but to create a completely customized curve that will let you ensure D65 throughout the entire range and with a perfect shape.


Silly question. Why do you like a gamma curve of 2.5. If I remember correctly, the objective for TVs is to create a 2.2 gamma curve in order to undo the inverse curve that cameras and broadcasting systems implement.

 

[1st on the Block]

Email [email protected] and they will send you the factory codes to access the grayscale and gamma calibrations. I had mine ISF'd, and the guy was astonished how much control this feature gave him across 5 different IRE (100, 80, 50, 30, 20). I ended up with a nearly perfect D65. The CMTP menu is broken down to Luminance, Red, Green, Blue values for each IRE.

 

[krasmuzik]

You mean I have to charge to ISF the processor as well as the display?


Oh what fun.


I was just hoping for a simpler gamma selector to lighten things up a bit that my customer can choose their preference. That would make sense for the user menu - then you could still have the ISF capability in the service menu. So are there plans to add simple gamma to the user menu? Even the cheap DVD players have gamma selectors now!


Do you know how many bits the gamma curves are using? This is connected to a CRT so I don't want to cause any digital banding by mucking in the menu.


Is the gamma curve also unique per each input?


gamma preferences are subjective and depend on the individual and the room and the source material. Just depends on how much black detail they like to see and can see with their display technology.

 

[patrick harkin]

To adjust for gamma, press "menu 0910" to enable extra items within the menu. Under the "IN->CONFIG->COLR" menu you'll see a CTMP (for Color temperature) selection.


This allows the grayscale color-temperature to be calibrated at input levels of 100, 80, 50, 30, and 20 IRE.

This feature is intended for fine-tuning the grayscale after the projector, or HDTV, grayscale color temperature has been calibrated. We recommend that the display be calibrated at 100 and 30 IRE using its internal grayscale calibration controls, before using this feature. This calibration feature operates like the grayscale calibration in projectors, except there are five points of calibration (100, 80, 50, 30, 20), rather than the typical two points. If the projector’s or HDTV’s service codes are not known, this feature allows the system grayscale to still be calibrated.

Keep the "LUMA" value at the respective level (i.e. 100, 80, 50, 30, and 20) for a standard (linear) response through the Vision. For example, if Green were increased by two clicks to correct color-temperature, select the LUMA entry and down click it once to put the LUMA value back to the correct level. As another example, if 100 IRE must be corrected, the “LUMA†level out can no longer reach 100 IRE since it is a weighted average of red, green and blue.

If the calibrated “100 IRE†point has a “LUMA†value of 2% less than 100 (98), then reduce the LUMA of each of the lower four points (80, 50, 30 , and 20) by 2% to keep a linear response through the Vision (in this case resulting in 98, 78, 49, 29, and 20 as the “LUMA†settings in the menu).

You can also use the "LUMA" value to change the gamma curve of the Vision. We recommend the LUMA response curve be left as linear, unless a trained technician with the proper equipment is available to calibrate gamma. Improper gamma settings can easily degrade picture quality.

 

[krasmuzik]

I am a trained ISF tech - so can you explain the interaction of the Luma and RGB a little bit better? This is for a customers Vidikron (Electrohome) - of course he may end up liking the "contrast" of a 3.0 gamma and not want it changed.


I use ColorFacts to calibrate with - and it does the gamma curve plotting and I can calculate what the luma should be.


I guess it is this complicated then the ISF additional input rate applies!


patrick - are you with Lumagen - I am across the bridge if you want to give me a training session!

 

[oferlaor]

Patrick is the highest ranking techie at Lumagen and he's amazing!


Patrick,


I didn't know this feature was already implemented. Excellent stuff. This is truly a first and a brilliant feature at that!!

 

[JohnPM]

Quote:

Originally posted by krasmuzik I am a trained ISF tech - so can you explain the interaction of the Luma and RGB a little bit better?

I would expect that the Luma value is mainly shown as a reference to let you know the effect your R/G/B tweaks have had on overall brightness at the point you are adjusting. The calculation will depend on the colour space for which the luma value is calculated, e.g. for Rec 601 Luma (Y) = 0.299*Red+0.587*Green+0.114*Blue. I don't know what colour space the Lumagen gamma menu uses, no doubt Patrick can fill in that gap. The coefficients for the various colour spaces are:

Code:

+----------------+---------------+-----------------+----------------+
| Standard       | Coef. for red | Coef. for Green | Coef. for Blue |
+----------------+---------------+-----------------+----------------+
| Rec 601        | 0.2989        | 0.5866          | 0.1145         |
| Rec 709        | 0.2126        | 0.7152          | 0.0722         |
| ITU            | 0.2220        | 0.7067          | 0.0713         |
+----------------+---------------+-----------------+----------------+

 

[krasmuzik]

I think I see - the Luma is calculated as a reference while you are doing the RGB greyscale. But the luma itself can be edited so that you can adjust for gamma - and it will recalculate the RGB points for that luma point - keeping them balanced.


So Luma control changes the RGB control, and RGB controls change the luma control?

 

[1st on the Block]

Quote:

Originally posted by krasmuzik So Luma control changes the RGB control, and RGB controls change the luma control?

That's correct.

 

[patrick harkin]

Quote:

I am a trained ISF tech - so can you explain the interaction of the Luma and RGB a little bit better?

The Rec 601 equation in JohnPM's post above demonstrate the interaction.
Quote:

patrick - are you with Lumagen

Yep, I'm a design engineer at Lumagen.

 

[krasmuzik]

Very cool!


Wish more displays had calibration controls like this!