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Discussion Starter #1
Given the high importance placed on contrast ratio for front projection - I am curious as to what is typical for RPTVs.


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most common RPTV's are CRT based, hence contrast ratio's are not a major concern .. they have true blacks.


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One fact often overlooked about RPTVs: they are more popular than FPTVs because they do not require a darkened room, due to the "gain" inherant in the fresnel lens, which allows them to be used in semi-darkened rooms. A lot of people like this feature because one or more persons in the room (not particularly interested in the current movie) can still be present, while reading, sewing, etc. under a lamp.


What I cannot stand about both direct-view CRTs and RPTVs is the reflected glare from such activity if you ARE interested in the movie. The refected image is even worse than ambient light because it's just that - a reflection, another complete image overlaid over the screen.


The "blacker blacks" part is only true if you are watching the RPTV in a darkened room without glare.


Gary

 

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Discussion Starter #5
OK let's try the question this way.


So if RPTVs or CRTs can display true blacks what would one say the contrast ratio of such a device would be?


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The question actually needs to be asked in several parts to better understand how a display behaves in terms of luminance display range.


1. How bright is a full field black on the display? This gives you an idea of how much background light "noise" is always overlaid on the picture. CRT based RPTV's should be very low (good) on this measure. Any light pollution from poor performance on this parameter will tend to make dark scenes less convincing and less transparent. The effect of too much light leakage is a veiled appearance to dark scenes.


2. What is the ratio of light between the black and white rectangles of a an ANSI contrast test pattern? The higher the number here the better. This differs from the first parameter because it is also affected by light scatter in the display and is a RATIO. This value alone does not directly address the degree of brightness of black. A higher contrast number here is desirable.


3. What is the peak white for both 1% and ANSI test patterns. While the ANSI peak white is a better estimate of average light output capability, the smaller areal peak white is also important to know because that affects perceived brilliance of the picture. Digital projection should have very similar values for large area (ANSI) and small area peak whites. CRT's can exhibit quite a difference between the two. This difference is one reason a CRT with low total light output can appear to produce a more brilliant image than an equal light output display with constant peak white.


4. How does the gamma curve of the display match the transfer characteristic of a gamma 2.5 CRT? If the display does not gamma match correctly, midtones will be misrepresented as too bright or too dark. Information about this characteristic of displays is rarely available but can completely alter the correct representation of luminance values in an image. If gamma is wrong, there will always be some portion of the picture which is displayed as too dark or too light relative to what was intended.


These numbers are all hopefully take with the display operating inside the cutoff range wherein all gradations of gray can be represented. In other words, one could cheat and crank up the contrast beyond clipping levels to achieve an artificially high contrast ratio.


So who polices the measurement of these parameters?????? Do you believe the numbers you get?




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Guy Kuo
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Ovation Software, the Home of AVIA DVD


 

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Hey Guy, why not give an actual MTF curve? That's what I would like to see. What would the MTF for a typical RPTV HDTV be at say 1000 lines of resolution?

 

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Discussion Starter #8
Hmm - question must be tougher than I thought.


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That would be yet another, totally separate set of issues for looking at display resolution and sharpness. Now try funding an attempt to measure that degree of detailed data. Frustrating impasse.


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Guy Kuo
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Ovation Software, the Home of AVIA DVD


 

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The contrast ratio for rated for CRT is very strange. I read that (sorry cannot remember the article) That CRT is between 1000:1 to 1200:1 ANSI. This is alot because most projectors are rate 125-400 ANSI, different from the full-on/off measurement manufactures state i.e My DLP projector is rated 500:1 on/off and 250:1 ANSI. However, it got confusing since the article claims there are no true way of measuring acurately the contrast ratio of CRT since display true blacks (This is because contrats ratio is the measurement between true black and the actuaal black levels in the picture). Take this with a grain of salt since it was until recently I have been searching for the answer myself.


Colin Lopez
 

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"One fact often overlooked about RPTVs: they are more popular than FPTVs because they do not require a darkened room, due to the "gain" inherant in the fresnel lens, which allows them to be used in semi-darkened rooms."


This is only a small part of it, and not in fact unique to RP; FP also enjoys better ambient light rejection with directional, high-gain screens.


The most important factor is the very principle of RP. The screen is transmissive, not reflective. Most ambient light is transmitted through the screen to the lightbox and absorbed, whereas ambient light is almost completely reflected from a FP screen.


"The "blacker blacks" part is only true if you are watching the RPTV in a darkened room without glare."


Per the above a darkened room is precisely where RP enjoys little advantage over FP.


Glare is only an issue with the ill-advised shiny front screen that mfgr's use to make their sets look like giant tube sets; it's intrinsic to RP.



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Noah
 
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