Quote:
Originally Posted by KTM250 /forum/post/19518230
how does this compare to a non APL window?
In a typical window pattern you have two on-screen video levels. You have the center measurement area, and you have the black background. The pattern marking indicates the video level of the center area.
If someone really wanted they could calculate an average brightness, or average level, for all the on-screen pixels. There really is no reason to do this, because typical video material offers a vast range of video levels, from a full screen of black to a full screen of white. The only relevant item to note about window patterns is that a 100% window will overall be brighter than a 10% window. A 100% video level should output more light than a 10% video level, and since the areas are the same in a window, the 100% window has a higher average screen brightness than the 10% window.
The APL measurement patterns are intended to output the same amount of light across the entire screen for a series of measurements. If someone actually wanted to add up the couple million pixels they would find that both the 10% and 100% APL patterns have the same overall screen brightness. While typical window patterns vary overall brightness along with the measured video level, the APL measurement patterns hold screen brightness constant. Only the measured video level changes in the APL measurement patterns as the overall light output remains the same. The primary difference with the APL measurement patterns is that the average brightness of the screen does not vary, unlike typical windows that vary average light output as the video level changes.
Some displays will vary light output depending on the signal they receive. For example with some displays you may find that the 100% video level will measure a different Y depending if you measure a window or if you measure a field. Both the 100% window and the 100% field share the same video level, but the overall brightness of the pattern is lower for the window (because of the black background) than the field. It's already been noted that overall screen brightness also varies when measuring a series of typical windows. Where you may run into issues is if you are using a procedure that assumes Y remains constant on a display that tends to vary Y depending on overall screen brightness. While there are some displays that can measure a consistent Y regardless of the overall screen brightness, many digital displays (and certain settings) tend to vary Y for a video level depending on overall screen brightness. The point of the APL measurement patterns is to hold overall screen brightness constant in order to defeat variation from the display.