Backlight and contrast

Discussion in 'Displays' started by ThomasL, Jan 14, 2008.

  1. ThomasL

    ThomasL Supporting Actor

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    I have a Samsung LNT4061F which I've been tweaking using Avia II.

    What are the pros and cons of lowering the backlight/raising the contrast? I currently have the backlight at 5 (default) and contrast at 70/brightness at 39 along with Movie/Warm 2 modes. I've noticed that there are number of settings that affect brightness and/or contrast as well as overall grayscale: Contrast, Backlight, Energy Savings, Gamma - it seems that there are many paths to getting the same general visual image. So, I'm curious to know if it's better to lower the backlight and up the contrast to compensate - e.g. does it extend the life of the backlight? - or does it not really matter that much. For example, the CNET settings during their review/test of the 46 inch model of this tv show a Contrast of 98/Brightness of 60 with a Backlight of 1 as their optimal settings in a darkened room.

    Also, I don't seem to suffer from visible light bleed/clouding at the edges due to excessive backlight bleeding through. i.e. if it's there, I don't seem to notice it.

    FYI, I currently have Gamma set to the default (0) and the Energy Savings mode is Off.

    Thanks for any thoughts anyone has on this.


    --tom
     
  2. GeorgeAB

    GeorgeAB Second Unit

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    The primary benefit of LCD backlight adjustment is in compensating for varying room lighting conditions.
     
  3. ThomasL

    ThomasL Supporting Actor

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    Thanks George,

    As you can guess, this is my first LCD tv and backlights are new to me.

    I wasn't sure if by lowering the backlight a few notches and upping contrast I would achieve the same intensity of white yet perhaps improve black level by increasing the overall contrast ratio. But I also know that upping the contrast too much might affect white balance/grayscale.

    Unfortunately, I only have my eyes as my instrumentation at the moment but I'm fairly well versed in the Avia test patterns.

    cheers,


    --tom
     
  4. GeorgeAB

    GeorgeAB Second Unit

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    It may help to understand human vision and perceptual factors on this point. Firstly, the TV should be calibrated to perform in a linear fashion. In other words, picture and signal processing settings should be set according to imaging standards (gray scale tracking at D65, color decoding for proper balance, gamma close to 2.2, contrast at max short of clipping- with color stability, black level set so blacks aren't crushed, etc.). The backlight setting shouldnt change any of these elements noticeably.

    As room lighting is reduced, the backlight can be reduced as well to diminish the contamination of illumination leaking through the LCD panel. This works just like an iris does in a lamp based projector. Our vision then becomes dark adapted. The pupil opens to let more light in. Eyesight then becomes more sensitive to available illumination. This makes the lowered bright portions of the picture appear lighter or perceived as brighter.

    Changing the backlight level doesn't theoretically alter measured contrast ratio, because the dark areas of the image get darker at the same time, and at the same rate, as the light areas. In an ISF day/night setup in the display's memories, the day settings would include a higher backlight setting, and the night setup would include a lower backlight setting. This is another example of how integral viewing environment conditions are to picture quality.
     
  5. ThomasL

    ThomasL Supporting Actor

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    Thanks much George,

    Everything you said makes sense. So, I guess my more practical question would be is there anything harmful to having a backlight set to maximum as opposed to having a lower setting? Does it lessen the life of the illumination device itself at all? My guess would be that it the user level controls that Samsung gives one would not appreciably affect the expected life span of the backlight. My current setting of 5 is really a compromise for both normal tv (with the light on) and dvd movie viewing since this set does not allow me to set two sets of settings for the Movie mode.

    cheers,


    --tom
     
  6. GeorgeAB

    GeorgeAB Second Unit

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    The cold cathode fluorescent tubes used for the backlight will gradually lose brightness over time, as with any product using phosphors for light output. A higher setting will age the phosphors faster but it's difficult to say how much faster than a lower setting. Personally, I wouldn't be too concerned about the difference. I still recommend viewing any TV in a darkened viewing environment for the best image, for many reasons.
     
  7. ThomasL

    ThomasL Supporting Actor

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    Thanks for the reply George.

    I actually stuck Avia II in the dvd player again tonight and decided to fiddle with the backlight setting. I lowered it from 5 to 2 and it made a marked difference on how "black" blacks were to my eye. Of course, it dimmed the entire white balance range so 100 IRE "white" was too gray looking. I bumped up contrast from 70 to 85 using the Needle Steps and Log pattern and that got white looking to my eyes as it had before I had lowered the back light. I tried putting the backlight to 1 but at that point, it seemed like contrast would have been at max. On this set, 100 contrast does not induce clipping - all 3 moving white bars are always visible on the test pattern but the Avia documentation says that even in that case, setting contrast too high can affect the grayscale. Since I have no way of measuring that affect other than looking at the step pattern, I'm leery to crank contrast all the way. As expected, changing the backlight did not change the Brightness/Black Level setting at all using the Moving Bars pattern in Avia. All it did was make black itself darker.

    I then stuck in a movie I had been watching using the old settings and at first it appeared a bit darker - no surprise there but after about 10 seconds of viewing a particular scene, I started noticing more detail than I had before. To my eyes, it seemed that the contrast ratio was much better. Scenes with bright parts and dark parts looked better with more detail. Subjective obviously but overall I think better than before. I also used the settings for normal HD viewing with the table lamp turned on and they looked fine.

    I dunno if I'll try again tomorrow night and see if I can set it to 1 and still get the contrast setting looking as "white" as it does now. Calibration tinkering can become addictive. [​IMG]

    cheers,


    --tom
     

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