I made this write up to clarify some of the misinformation and misunderstandings that exist with how projector screens affect lighting conditions and contrast ratio.
Example 1) A screen can only change both the black level and white levels together,
hence even if increasing the brightness by a factor of 2x, the difference between them (ON/OFF Contrast) are not changed from a ratio standpoint,
here we have a 2:1 ratio regardless (100|50 = 2:1, 200|100 still = 2:1).
This essentially means the contrast starts and ends at a different part of the contrast spectrum (a brighter or darker range), but the overall Total ON/OFF contrast ratio remained unchanged. So although the perceivable contrast will appear differently at different levels to our eyes, the total ON/OFF contrast ratio cannot be increased in this case. Also note that these levels should be calibrated to standards, not blown out to make some content look better, while making other content look worse.
One of the simplest definitions for contrast ratio that I have seen is shown below:
For instance, if a display claims a contrast ratio of "500:1," what this is saying is that the white areas
(the brightness when the video inputs are at their maximum value) are 500 times the brightness of the
"black" (all video inputs at their minimum).
Example 2) HOW a screen can help black levels without INCREASING ON/OFF contrast
If the black levels become darker from the beneficial effects of a screen, then this is only possible if the screen helped in making the room itself darker or redirected the ambient light to not hit the screen itself as much, fairly simple here. This is because of how our eyes work (the darker the room, the darker a black will appear). So since the screen made the room darker from less reflection, the whites will also be darker. That is what people really mean by perceivable contrast, it's just allowing you to lower the starting range of the total contrast range (the black level part of the contrast range) and still seeing it without the ambient light washing it out. When you did this, the maximum white level also decreased at exactly the same rate. Therefore, this did not increase the projector's TOTAL ON/OFF contrast by even a single number, what it did was allow you to see the lower levels in the contrast range visibly within the limitations of your eyes given a certain amount of light in the room, hence allowing your eyes to view lower levels of the contrast spectrum.
Getting Darker Blacks = ENHANCED PERCEIVABLE ON/OFF CONTRAST might just be called DARKER BLACK LEVELS and DARKER WHITE LEVELS at the same time.
So if a screen improves the appearance of how dark a black is in a given setup, it just demonstrates the fact that some people have their projectors calibrated incorrectly or are running them in a poorly light-controlled room (or a white ceiling or something is wrong). Keep in mind though that since no room can be considered a perfect bat cave, high gain or gray screens or any screen that changes light reflectivity characteristics could potentially improve the image, even in a room thought to be a bat cave. Howver, also note that overall what people are really seeing as far as improvements from a SCREEN are what could have otherwise been acheived simply by calibrating for maximum contrast in a perfect BAT CAVE on any screen regardless of the type of screen material or reflective nature (excluding the one exception being if their image is too bright or dim beyond what their projector can correct, then we need a + or - gain effect on the screen itself to correct the issue).
Reflectivity Characteristics of the Screen affect Contrast Passively not Actively
I have white ceilings like many, so any screen that can reduce reflections will help improve black levels and ANSI contrast overall, but that doesn't mean the screen did it directly, it means the screen did it only by reducing ambient light by redirecting some of the light to a better place or just absorbing / reflecting less light in the case of a gray screen. So in this case, contrast isn't improved directly by the screen ACTIVELY, but its only because the room is not as bright did the ANSI contrast improve, that is how we can still call the screen a passive device. Yet in this situation, the ON/OFF contrast did not improve at all, it just allows you to adjust it to a darker level and for your eyes to see the darker colors at the lower end of the spectrum. Hence the blacks and whites can now be darkened while still allowing you to comfortably view the image without the image appearing to be too dim (thanks to the now darker room).
The only time a Screen can assist the projector to INCREASE the Total On/Off Contrast is in this one scenario below
So the only situation where the screen can affect the ON/OFF contrast as opposed to just changing the white and black levels proportionately together, is by allowing your projector to be calibrated to get back to best mode when the lamp is not bright enough (or too bright), or possibly if your projector just has calibration issues at certain levels of brightness. This is where the HIGH POWER can help so much, or any high gain screen for that matter. Again, this is only referring to the screen allowing you to calibrate within the optimal range, but it is not the screen changing the actual contrast number, it simply allowing your projector to be adjusted at or near a BEST mode, when previously you purposefully had to use incorrect/non-BEST mode values to obtain the best possible picture either due to brightness limitiations of your lamp or the room conditions.
A High Power or High Gain Screen does NOT ruin black levels
As long as the high power does not raise the light so much that it makes the image too bright for your projector to compensate, then the HP will not cause a problem in black levels. On some projectors, sure it can cause a problem by making the black levels start too bright, but that means you need to move the projector off-axis or correct the problem so that the calibration range can be achieved. It is a falsehood that the HP screen will ruin black levels, in the case of white ceilings it may make them better by keeping the room darker overall from less reflected light off the ceiling. Now to acheive the best black levels in a bat cave with a high gain screen, the room has to be absolutely 100% perfectly dark, don't go watching the movie in a white jumpsuit or a silver leotard (don't ask), and your furniture better be black or close to it.
I believe the above issues are why people get so confused on this.
If you have a bunch of ambient light, then it just means you have to calibrate the black level / starting point of the ON/OFF range to a brighter point to compensate, hence the reason the black levels worsen or rise. Now you could have to make this so bright to compensate for the ambient light that the contrast is clipped at the upper ranges, so in this sense the contrast was changed, but this only applies to a non-optimal lighting scenario. The same goes at the lower scales, you could set the lower range of contrast starting so low that the black levels are clipped (at least from what our eyes can see), and the picture is washed out and too dim.
You should NOT try to create a VISIBLE increase in perceivable contrast without checking it against a calibration
This can be a problem because if you perceive an increase in constrast by your eyes alone, then you really have no idea how this affected all movies or scenes as a whole that you will be watching in the future. So the end result is that by trusting our eyes for contrast, we are now straying away from the optimal calibration standards which are there for a purpose, this is to keep movies looking their best on average and how the director intended. Keep this in mind if you are planning on "improving" perceivable contrast by fiddling with stuff or implementing a MAGICAL DIY screen without calibrating back to the correct ranges.
To demonstrate this, start from a THX or ISF mode and go watch Apollo 13, a Pixar cartoon, or Lord of the Rings, or something like this, and notice how most of the scenes are very contrasty looking. It's because the excellent camera work in these films kept most scenes within appropriate ranges to how a projector would optimally display it as an end result. Now go watch a TV episode from the 1970's, you could probably change your projector's settings away from ISF and make the 1970's TV series look better, but then go back and watch Apollo 13 with the new settings, it will usually look worse given all other factors are equal.
Now there is no harm in making an adjustment to the PJ settings temporarily on a really bad quality movie source or poorly lit transfer, but this is why projectors have user stored settings that you store as a calibration, or revert back to after you muddle them up for specific scenarios.