Can someone explain contrast ratio?

Discussion in 'Displays' started by Randall Wetmore, Nov 13, 2005.

  1. Randall Wetmore

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    Hi Everyone,

    I see many LCD displays with varying contrast ratios. Is higher always better? Does contrast ratio have anything to do with how deep black levels can be? Exactly what is it a ratio of?

    I have tried researching this topic, but haven't found a clear explanation anywhere.
     
  2. Dick Knisely

    Dick Knisely Second Unit

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    "In reference to computer monitors, the measurement of the difference in light intensity between the brightest white and the darkest black. Contrast ratio is often used in marketing computer monitors, where a high contrast ratio, such as 400:1, represents a better color representation (the better the information will appear against a darker background) on the monitor than a lower contrast ratio, such as 150:1.

    The term is used more frequently in reference to LCD monitors than CRT monitors."

    http://www.webopedia.com/TERM/C/contrast_ratio.html

    Google the phrase and a ton of info sources will come up.
     
  3. ChrisWiggles

    ChrisWiggles Producer

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    First off, rely more heavily on objective measurements of Contrast Ratio especially by a 3rd party. Manufacturers do all kinds of tricks to spec their numbers much higher. When you have a reviewer describing exactly how they got their CR number, you can understand whether it is or is not legitimate.

    All contrast ratios as spec'd are usually a ratio of White:Black, so some light output for white divided by some light output for black. If black is zero light output, you have infinite contrast ratio capability.

    There are two types of common contrast measurements that you will see, and BOTH are very important:

    On/off Contrast: this is the black level measured off a black screen, then the white level measured off a white screen. Notice how black and white are not on the same screen at the same time, so they are separate measurements of black and white. You can see variations of this spec'd sometimes especially with CRT displays or plasmas, as Peak Contrast ratio, which may use smaller patches of white for the white level measurement. CRTs and plasmas can drive a smaller area of white brighter than they can the entire screen all at once, and this is also related to Peak Lumen output. on/off Contrast ratios can be anywhere from a few hundred:1 with much older digital projectors, to more commonly a few thousand:1 with newer digital displays, to tens of thousands:1 or essentially infinite with CRT displays.

    ANSI Contrast: this measures black and white levels by using a 4x4 checkerboard pattern. White blocks are measured, then black blocks are measured off the same pattern. ANSI contrast attempts to quantify how well a display can "hold" black in the presence of bright objects in the picture. Where the on/off capability might be high, ANSI contrast tells you how well shadow details and black will be maintained in mixed APL scenes where there is a mixture of dark and bright objects at the same time. ANSI contrast on CRT displays is fairly low, usually about 100:1 to maybe 250:1. Digital displays tend to have higher ANSI contrast, with DLPs having the highest, maybe 500:1 or so. ANSI contrast is heavily affected by the optics and the behavior of the light etc in the display, as well as VERY VERY heavily by the room itself. To measure ANSI contrast, the best way is to fire the projector in a room COMPLETELY covered in black velvet, and shoot it black velvet not even a screen to really isolate the ANSI performance of the display.

    An ideal display will have both high ANSI contrast, and high On/off Contrast. This ideal display would be able to go very very black in dark scenes, while also maintaining good shadow detail and blacks in bright scenes.

    As it stands now, I will simplify: a CRT display has very high on/off CR (say 20,000:1), which means it can have very very deep blacks in dark scenes or fullscreen black. However, CRTs have low ANSI contrast (say 150:1), which means that as bright objects begin to populate the screen, shadow details and blacks begin to washout as light from the brighter portions spills onto the darker portions.

    Something like a single-chip DLP has lower on/off Contrast (say 3,000:1) but high ANSI contrast (say 500:1). On dark scenes or full black frames, black will be significantly elevated compared to the CRT black, and will be more greyish. However, as bright objects begin to populate the screen, because of the high ANSI contrast, this display will hold shadow details better in mixed APL scenes.

    This contrast calculator is interesting to explore the interplay between on/off CR and ANSI CR, and how they interact to lead to a particular scene's instantaneous CR:
    http://home1.gte.net/res18h39/contrast.htm
     
  4. Jack Gilvey

    Jack Gilvey Producer

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  5. ChrisWiggles

    ChrisWiggles Producer

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    That is a great link, though I have problems with this portion:

    "Conclusions
    So, just how much contrast do you need to see in an image? Empirical data suggests the human eye is limited to a dynamic range of 100:1 at any given instant. That means that if you look at a scene with objects of different luminance values, you won't be able to discern more than a 100:1 difference between the darkest and lightest objects. "

    The 100:1 number is drastically removed from context and incorrect. The actual number is more on the order of 100,000:1 with the iris in your eye not adjusting, and then over time with your iris adjusting many many more times that yet. It is not 100:1, that is a misconstrued number based on CSF measurements which is not at all the same thing.
     
  6. Jay Mitchosky

    Jay Mitchosky Producer

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    Nice work, Chris. [​IMG]
     
  7. Randall Wetmore

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    Thanks, Chris.
     

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