[lcaillo]

As the HTShack community matures it will be to our advantage to pursue ever greater depth in video discussions. The leading advocate for video quality in the US and around the world is the Society of Motion Picture and Television Engineers (SMPTE). These engineers deal with the science of motion imaging in all that they do. The science of imaging is founded upon the detailed characteristics of human visual perception. Understanding human perception has led SMPTE to publish recommendations for viewing conditions in critical monitoring rooms. They know that room conditions can potentially interfere with image quality and viewer perception of video programs. Unfortunately, unless you are a SMPTE member, you have to buy the standards and will not have access to many publications. However, one of our members has published a "white paper" type article that does a good job of summarizing such issues. I think presenting this specific area of imaging science and video quality for discussion will be helpful for designing, equipping, and/or calibrating any home theater or video system.

My hope is that we can start some discussions on this area that can be useful to our members and eventually develop a couple of stickies on specific topics such as room color, lighting, and bias lighting. I have been recruiting Alan, the author of the article below and one of the principal evangalists for high standards in these areas, to help us get some discussion started. Hopefully he will be sharing his considerable experience here more.


Here's the links to the article and to SMPTE to start things out:

http://www.smpte.org/home/
http://www.cinemaquestinc.com/ive.htm

 

[Alan Brown]

These videos demonstrate how even subtle changes in ambient lightness can instantly alter our black level perception of an object:

;

;

.

This explains why backing up a TV with a dark wall can be counterproductive in preserving convincing blacks on the screen. Only the darker portions of the screen image are enhanced by a lighter surround. The lighter areas of the image are not affected, therefore increasing perceived contrast ratio as well. These quotes document this characteristic:

"Contrast could be considered to be the most significant quality that impacts not only the perceived depth of an image, but also affects the apparent sharpness.....While the luminance level of a given image affects how the eye perceives contrast and detail, the ambient conditions surrounding the image can also have a dramatic impact. This phenomena was studied by Bartleson and Breneman (1967) to examine the impact of perceived contrast based not only on the luminance level of the image but taking into account the surrounding ambient luminance levels as well. Their results showed that the perceived contrast increased as ambient luminance increased. With the increase in ambient luminance, the eye interprets black levels as being darker while the impact to the white level is minimal. Since the perceived difference in dark areas is greater under the higher ambient luminance conditions, the perceived contrast is higher. It is a natural tendency to want low ambient luminance levels to strive for "better" perceived image quality and what is thought to result in higher contrast. However, in reality, the opposite is true. This tendency may be justified for current direct view CRT televisions due to the issue of glare that results from the glossy surface of the glass tube [also true for certain flat panel displays today]. With less ambient luminance, the glare is reduced- but it may be important to keep some ambient luminance behind the television [as in the case of bias lighting] to keep the perceived contrast higher.....While sharpness can affect the apparent contrast of an image, the converse is true in that contrast can also impact the apparent sharpness of an image. Images that have lower contrast will appear to be not as sharp as an image of the same content, but with higher contrast.....A subjective study was then conducted to verify the impact that ambient lighting has on perceived contrast. Several non-technical (and thus presumably non-biased) and technical observers were asked to compare a series of images with various ALL [average luminance levels] under different ambient luminance extremes in order to understand the impact that ambient viewing conditions might have on the perceived contrast between the two television technologies [CRT and DMD (DLP RPTV)]. Under dark ambient conditions, the result for images with an ALL > 5% was found to be equal between the CRT and the first DMD display. However, under bright ambient conditions (about 250 nits of luminance on the wall behind all of the units), the DMD display was favored over the CRT by 50% of the observers as having higher perceived contrast.....This proved that ambient conditions have the effect of potentially raising the black level threshold of the eye above the actual black level of the television such that the perceived contrast ratio is higher." from the SMPTE Journal, 11/02. 'The Importance of Contrast and its Effect on Image Quality' by Segler, Pettitt and Kessel

"Their experimental results, obtained through matching and scaling experiments, showed that the perceived contrast of images increased when the image surround was changed from dark to dim to light. This effect occurs because the dark surround of an image causes dark areas to appear lighter while having little effect on light areas (white areas still appear white despite changes in surround). Thus since there is more of a perceived change in the dark areas of an image than in the light areas, there is a resultant change in perceived contrast.....Often, when working at a computer workstation, users turn off the room lights in order to make the CRT display appear of higher contrast. This produces a darker surround that should perceptually lower the contrast of the display. The predictions of Bartleson and Breneman are counter to everyday experience in this situation. The reason for this is that the room lights are usually introducing a significant amount of reflection off the face of the monitor and thus reducing the physical contrast of the displayed images. If the surround of the display can be illuminated without introducing reflection off the face of the display (e.g., by placing a light source behind the monitor that illuminates the surrounding area), the perceived contrast of the display will actually be higher than when it is viewed in a completely darkened room." from 'Color Appearance Models,' by Mark D. Fairchild, Ph.D., of the Chester F. Carlson Center for Imaging Science: Munsell Color Science Laboratory

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

"Advancing the art and science of electronic imaging"

 

[lcaillo]

With the trend toward LCD panels with inherently poorer blacks than CRT and PDP, and with the tendency for these thin panels to be mounted on a stand rather than wall mounted, it seems more relevant than ever to consider the useful effect of backlighting. Now, with the interest in 3D, this maybe even more true.

In this industry where so many lament the lack of objective support for many system tweaks, here we have something that has been studied and researched systematically and is well understood, and where there are clear industry guidelines for best practice. Why is it, Alan, that so many at all levels, from the studio engineer to the dealer, will ignore something than can be of immediate and clear benefit to perceived contrast ratio for the cost of an overpriced HDMI cable?

It seems to me that viewing environment might be as important as room acoustics are, yet it may be even easier and cheaper to get decisive improvements than modifications to acoustical properties.

 

[glaufman]

Great discussion to have. Though Alan is clearly the resident expert, I'll add my 1.5¢ ...
I think this is the same problem that causes many of the mediocre (or worse) HT setups out there... people have perceptions, and there are only a precious few who truly embrace science enough to let objective evidence shatter those perceptions. Examples:
The perception that since a movie theater is dark, a home theater must be dark, giving no consideration to the different technology of the display.
Perception: a magazine shows a flat panel mounted above the fireplace as a cool place to put one. Who cares that you can't sit thorugh a TV show without neck strain?
Perception: These are the best speakers for sale at (insert store name here) so they must be the best that are worth paying that price for. Besides, they sell a lot, so they must be the best!
I could go on and on...

 

[Alan Brown]

You and Greg bring up several interesting points.

With the trend toward LCD panels with inherently poorer blacks than CRT and PDP,

Popular trends in consumer electronics have frequently tended toward compromise of real performance quality for the sake of some conflicting sense of:
affordability (think plastic speaker cabinets)
convenience (wall mounting of TVs, or over a fireplace)
"cool factor" (ever thinner TVs, Philips 'Ambilight')
smaller size (Bose, iPhone Movies)
style (glossy black TV frames)
etc.
LCDs are outselling plasmas but not because of superior picture quality.

and with the tendency for these thin panels to be mounted on a stand rather than wall mounted, it seems more relevant than ever to consider the useful effect of backlighting.

If a TV is going to be the primary movie display in the home, I try to talk my clients out of putting it on the wall. Wall mounting does not negate or diminish the need for bias lighting, although it adds a degree of difficulty and additional cost.

Now, with the interest in 3D, this may be even more true.

It looks like the 3D models will have the capability to produce much higher lumen output to compensate for light loss from the required glasses and alternating frame rates. I expect the extra boost in brightness will only be engaged during 3D mode. Even then (when viewing 3D), image brightness may not be as great as during normal viewing. Many viewing comfort factors are yet to be conclusively evaluated with 3DTV.

In this industry where so many lament the lack of objective support for many system tweaks, here we have something that has been studied and researched systematically and is well understood, and where there are clear industry guidelines for best practice. Why is it, Alan, that so many at all levels, from the studio engineer to the dealer, will ignore something than can be of immediate and clear benefit to perceived contrast ratio for the cost of an overpriced HDMI cable?

While I have studied human factors of visual perception in the context of imaging science, my grasp of human nature and psychoanalysis is insufficient to answer that question. The importance of viewing environment conditions has been recognized since the birth of television. The technique of bias lighting has been consistently used by professionals and consumers for well over a half century. I think part of the reluctance to implement ideal viewing conditions may lay with the fact that many people grew up with the TV as an entertainment/informational appliance, rather than a vehicle for exhibiting cinematic art. HDTV and larger screens have made home cinema more of a desirable objective.

It seems to me that viewing environment might be as important as room acoustics are, yet it may be even easier and cheaper to get decisive improvements than modifications to acoustical properties.

Viewing environment conditions are a fully apt parallel to listening environment conditions. As poor room acoustics can interfere with and distort an audio program, so poor room conditions can interfere with and distort a video program. The single most important factor to recognize in this field of study is that the viewer is every bit as important a component in a video system as the display. Human visual perception has performance characteristics and limits, just as any piece of electronic gear.

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

"Advancing the art and science of electronic imaging"