White Level (Contrast)

Greetings

When setting contrast, you follow the three rules.

1. No clipping
2. No Discoloration
3. No eye fatigue

Now is eye fatigue the same for you and every other member of the human race? If yes, then there can be one contrast setting that accounts for eye fatigue. Last I looked, we don't live in that universe.

Hence it is open ended.

What fatigues me today may not fatigue me tomorrow.
Will a hockey game be more fatiguing to look at versus a jungle movie?
Is the fatigue level the same if I turn on the room lights?

regards

Many LCDs have backlight controls, and other types of displays have items such as iris adjustments. Rather than using contrast to set light output, on these types of displays it may make sense to turn up contrast so long as you do not get into issues such as clipping or discoloration, gamma drop-off, and banding. Some manufacturers even limit the negative impact the contrast control can introduce, so maximum contrast may be reasonable on some models. With displays that use an adjustable light source, my opinion is that it's generally preferable to try to maximize contrast and then use backlight or iris controls to adjust light output.


Less light in the room will tend to make a dimmer white more reasonable. Personally I doubt if my TV reaches 30 ftL (it might after the bulb change), but I tend to watch in a dim room, so the lower output is fine. Typically backlight or iris controls affect the entire light output from black to white. Since my TV doesn't have the sorts of black levels that some other displays offer, I consider it reasonable to trade some 100% light output for darker blacks.

On an LCD you don't use contrast to set luminance, that's what the backlight is for.

For an LCD you set contrast based on clipping. You don't want to clip. You want it as high as it goes without clipping.

That might be 100, or 89 or 50.

Unless you're calibrating to some sort of military spec where you need to be within a % of the luminance target, just set the back light first (by the numbers if you have the hardware) and then forget about it. Eye fatigue is a factor backlight on an LCD (where as plasma or CRT is still contrast based).

Then when you do contrast, only worry about clipping.

Greetings

Rules 1 and 2 apply only to the contrast control.

Rule 3 applies to the backlight control and the contrast control. (Let's say you take backlight to 0 and it still hurts your eyes ... then you take contrast down even further from where ever that rule 2 point got you. (Highly unlikely that this would happen, but still))

regards

The roots of this discussion still go back to SMPTE recommended practice for reference viewing conditions of video monitors. The seminal work for monitor calibration best practices, viewing conditions, and human factors resulted in two companion technical documents: SMPTE RP166 'Critical Viewing Conditions For Evaluation Of Color Television Pictures'; and SMPTE RP167 'Alignment Of NTSC Color Picture Monitors.' Other standards bodies feature very similar recommendations, such as from the ISO and ITU, up to and including HDTV monitor use. Recommendations from the ISF and THX over the intervening years have been based upon SMPTE's seminal work.

Since consumer display calibration's goal is to emulate professional reference video standards and practices, practical wisdom suggests following the same fundamental best practices as our guide. Achieving a peak white reference of 30 to 35 fL was in "dim surround" viewing conditions. Such conditions were specified as maintaining ambient lighting behind the frontal plane of the monitor screen at 10% of peak white (bias lighting), with a white point of D65 to match video white, to preserve correct color perception. Brighter ambient lighting will require a brighter display. This has the limit of not exceeding room lighting that would begin to interfere with the image on the screen, due to veiling haze and reflections. Such interference tends to wash out the picture, especially in darker scenes. Human factors should never be left out of the equation.

Best regards and beautiful pictures,
G. Alan Brown, President
CinemaQuest, Inc.
A Lion AV Consultants Affiliate

"Advancing the art and science of electronic imaging"

May I ask a "dumb" question? (I treat it as "dumb" as I think this is quite fundamental to video calibration)

For direct-view display, the recommended ftL range is 30-35 ftL (in a dim-surround environment)
For front projection display, the recommended ftL range is 12-16 ftL (also in a dim-surround environment)

Why is there a difference?

As is commented above, the major aim of calibration is to match the set-up (and environment) used when grading was performed.

This is the standard SMPTE spec.

There was a long discussion on this on another thread.

At the end of the day the best calibration is to perfectly match the SMPTE spec, but realise that the actual white level used today is 23FtL (about 80Nits).

Also, most grading rooms are not that dark in reality - they aim to match standard home environments, but that is not direct sunlight, etc.

The bets way to set-up a display is to initially use a couple of shadow and bright level test frames (download from here: http://www.lightillusion.com/downloadcalimages.htm).

Depending on the display controls set brightness to the point you can 'just' see all the black patches, and contrast so you can just see all the bright patches.

Use back-light (if you have it) to set the black point to be black, not liftet grey.

The peak white value then have is the best you can use without clipping, or lifting black with the back-light...

We obviously then use a 3D LUT for accurate calibration within these parameters.

This process is what we use to calibrate grading rooms, so is as good as it gets.

But, if you have light falling on the screen, or too much ambient room lighting you will never be able to calibrate correctly.

Greetings

Field of view ... the more the image covers your field of view, the less bright it has to be. OF course a FP image better be in a totally light controlled room.

regards

You are mistaken. Front projection is to be viewed ideally in a room with zero to minimal ambient light. There is a SMPTE cinema spec for maximum recommended ambient light that I don't have the reference for right now. The Digital Cinema Initiative also has one specified. Front projection screens are even more readily contaminated by any illumination in the ambient environment.

The darker the viewing environment, the less a dark-adapted human visual state can tolerate elevated screen brightness without discomfort in extended viewing sessions. Joe Kane told me he has trouble handling a front projected image much brighter than 20 fL. Gary Reber insists he has never needed bias lighting. There will always be exceptions to a standard recommendation, because humans vary. The recommendations made by standards bodies endeavor to provide general guides for the majority of users, and to promote unity and consistency of program technical quality.

"Dim surround" is an imaging industry term used to describe a reference environment that attempts to emulate end user viewing conditions. Most consumers realize intuitively that their TV looks better in subdued room lighting. This is sensed by reason of a reduction in screen interference and the inherent perceptual advantages of a more dark-adapted viewing state.

Screen brightness recommendations also relate to the general light output limitations of the associated display device technologies. If a specific sample of display has problems operating in a linear fashion at recommended levels, a lower screen brightness should be used. The reverse is true as well- up to a point. That point is defined by the limitations of the human visual system, coupled with the ambient light level required for use of the room. A class room will be significantly different than a movie theater.

Image field of view is somewhat less of a factor in affecting viewing comfort than screen brightness. Many humans want pat answers and rigid formulas. Motion imaging industry recommendations are a guide, and can be used as a starting point. Deviation from the standard recommendations is best done when the underlying principles are understood. Such deeper understanding of the whys and wherefores helps avoid consequences when departing from best practices.

For front projection it might make sense to use typical room lighting during measurements, since you're measuring reflected light. With a TV the intent is usually to measure light from the display, so less light in the room will tend to avoid contaminating the measurements. One area where it may make sense to use typical room lighting is when adjusting black-level (brightness control), because light in the room may limit the detail you can see near black.

You should calibrate to how you watch. That is why some have 2 sets of calibrations, one for a day and one for night.

There are several reasons for the difference in luminance levels for panel displays and front projection... perhaps the most critical is that with front projection, light from the screen bounces back into the room, reflects off of objects (walls, furniture, floor, ceiling, you, etc.) and some returns to the screen, contaminating images. fL measures the intensity of light reflected from the screen... but the screen can be much larger than a panel display and the total AMOUNT of light reflected back into the room can be much larger than the amount of light emitted by a panel display set to 30-35 fL for 100% white.

So there's a battle between making the front projection screen brighter and brighter and having so much reflected light from the room getting back to the screen and contaminating images.

AND even though, say 15 fL, SEEMS like it would be a lot less bright, when you see that much light being reflected from a screen that's 2 or 4 or 6 times larger than a 55" or 60" panel, the images don't SEEM dark at all.

The type of screen you use is a big factor also - different types are more and less sensitive to reflected light from the room getting back into images. Some of the very best reference screens are very sensitive to room reflections and it is recommended that everything in the viewing area be black... black carpet, black curtains, black walls, black ceiling, black furniture. Other screen types can tolerate more reflected light without impacting the viewed images much -- but the images are never quite as good as the reference-type screens.

So the issue is really more complex than a single measurement/specification/guideline.

Backlight will also alter the black level - setting it is a real compromise between good blacks and peak brightness.

Use with care.

hauptm, can you explain more how you came up with your backlight setting?

And which tests are the Blackclipping and Whiteclipping tests?

Thanks.

This covers the black & white settings - http://www.avsforum.com/avs-vb/showp...&postcount=504

Thanks. I thought he was referring to somethng other than contrast and brightness.

Thanks, but now I'm more confused. Is this how everyone sets the backlight without special equipment?

Greetings

On the thx side, people are taught to set back light to not exceed the eye fatigue threshold. (Which varies based on person, environment, programming)

They are not taught to aim for a specific number.

regards

On the TVs I've looked at, a backlight or iris setting usually does not affect where a digital display clips. I use the first Black Clipping pattern to adjust the brightness control to the lowest setting where everything below 17 clips. In doing this it may be easier to spot the flashing in the pattern by setting the backlight high, and it may be more difficult to notice the flashing if the backlight is set low. Usually the backlight will not actually affect where the TV clips, so I consider it acceptable if you want to use a high backlight or walk up to the display to make the flashing easier to see.

Once you find the lowest brightness setting the TV is capable of using I would suggest moving on. Leave the TV at that brightness setting, go to the second APL Clipping pattern, set backlight to minimum, use typical room lighting, and view the TV from a location where you might usually watch. The white in the image and room lighting will probably make the flashing near black difficult to notice on most decent TVs. Start turning up the backlight control, and at some point you will likely be able to see 19 flash from your usual viewing location without touching the brightness control. Anyway, my point here is that clipping (brightness control) and light output (backlight control) usually are two different functions that don't necessarily interact. Room lighting can make near-black levels difficult to notice, and generally I would suggest turning up the backlight before increasing the brightness control.