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Display Defects and Artifacts
Identifying Visual Imperfections in the Image
While display designers and manufacturers strive for perfection (at least we hope that's true!), the practical side of the world usually dictates that some level of imperfection must be accepted. Thus you might find some defects in your display, or find some artifacts - those image details or areas that appear artificial and unnatural. Here we discuss some of these issues, and this is particularly appropriate if you are in the learning or evaluation phase before actually purchasing your home theater system. But we do want to offer a quick comment about display defects - if you already have
purchased your display and haven't noticed any defects while viewing normally, you might not want to know about any that are present! Sometimes a defect only becomes bothersome when a viewer becomes aware of it - then he might be drawn to it instead of simply enjoying the entire image. And, unfortunately, frequently nothing can be done about it. Thus the below checks might be most appropriate as part of an evaluation before you buy, or possibly during a warranty period.
If you still want to proceed, the
Full White,
Full Black,
Full Red,
Full Green and
Full Blue patterns can be use to check both for display defects as well as "dust" that may have settled on the panels of a digital display. To evaluate for defects, use the full field patterns and visually scan the entire screen while looking for defects. Most defects can be seen on the full white pattern, while the single color patterns are good to isolate the problem to a specific color.
Digital Display Defects
For digital displays, the most common defect is usually a defective pixel(s), which may be either always OFF or always ON - and if they are ON, they may be on at an intermediate
brightness level. To check for dead pixels, use the full black field to look for pixels that are stuck ON, and use the individual full fields (red, green, blue) to look for pixels that are stuck OFF. Note that the pixels are small, so you have to carefully scan all areas of the image to make sure you catch all defects.
Digital displays can also get dust on the panels, which typically appears as small (usually several pixels per "edge") irregularly shaped area(s) that are dimmer and have some color shading. Usually the best way to find dust is to display the full white field pattern and to then adjust the lens focus slowly to both extremes - this "focusing" adjustment can usually bring any dust into focus - if present, you will see relatively well defined small colored spots (magnified dust). This dust is on the panel of the missing color - so if the spot appears as magenta, the dust is on the green panel.
The other check for dust is to display the full black field pattern, and look for small areas that brighter than the surrounding areas of black - such an area is probably due to dust.
Fixed Pixel Display Artifacts
Aliasing: The images from all fixed pixel displays are made up of discrete dots, each of which is a pixel. When you view the display from a distance, the individual pixels blend together and form an image that appears quite detailed, natural and realistic. When you view from close distances, however, the individual pixels can be seen and this can detract from the image. When image details are made up of just a few pixels, while you can clearly see the pixel edges, it may be difficult to see the exact image edge. When the image has lines that are at only a slight angle to the horizontal or vertical, you will see a stepping from one pixel row or column to the next. This is aliasing - when there are just too few pixels available to
accurately reconstruct the original image.
This aliasing can be seen by viewing the circles in the
Complex Pattern (WV-51), and note how, when viewed from close distances, the circles consists of a series of steps, particularly near the top, bottom, left and right parts. Within each circle locate the small triangle - one side is at 45 degrees, and the other two sides are lines that step down (or over) one pixel at a time. This is the way a line at a slight angle will display as "steps" as it is limited by the number of pixels available. This aliasing effect, which is seen at it's worst here, can also occasionally be seen in small details in normal HDTV images. You can check to see just how far away, measured in screen widths, that you must be to make the effect inconsequential.
[Note that for optimal results with digital, fixed pixel displays, this Complex Pattern requires that the PC graphics output resolution exactly match the display resolution for a pixel by pixel mapping. The display should be set for a "true" display, and not be scaled or resized. If there is any question, check the
Alternating Pixel Pattern (WV-52) - looking up close you should see the pixels making clean alternating ON/OFF column and hatch patterns. It should not appear to be noisy, or appear to be clear in some areas but blurry in other areas - if it is, you may be able to correct it by selecting an automatic alignment mode, or by adjusting Clock and Phase settings in the display.]
Note that the scanning system and relatively "soft" pixels in a CRT display generally
act to hide these aliasing effects.
Screen Door Effect: For digital displays, when viewing from close distances you may be able to clearly see rows and columns of individual pixels, appearing like a "screen door". This effect is easiest to see in the areas without details, such as the solid colors squares in the Complex Pattern. To evaluate how objectionable the effect is, back away from the display to see just how far away you must be so the effect doesn't detract from the image.
A form of the screen door can also be seen on direct view CRTs, as well as on plasma and LCD monitors. In these cases there are separate red, green and blue pixels that are adjacent to each other, since they can't be in exactly the same location. The resulting visual effect is a series of relatively closely spaced vertical lines is seen at the closer viewing distances.
DLP "Rainbow Effect": Single "chip" DLP projectors rapidly display
a sequence of red, green and blue only images, and these images are nicely combined by the eye to make a full color image - almost all the time. The other times, usually when the eye is moving rapidly and when there is a white object on a dark background, the three colors fall on different parts of eye and thus appear separately as a "rainbow" of objects. If you have a DLP projector, check out our
Rainbow Finder test pattern (WV-14) - simply follow the LOOK HERE words and see what happens to the center white bar as your eye sweeps across the screen. You will have to judge for yourself if this effect is acceptable - most viewers don't have a problem here, but a few find it difficult to watch.
CRT Display Defects
For CRT projection displays, there are four primary defect types to look for.
a. There can be "spot" defects in the phosphor surface of one of the CRTs (red, green or blue), and these will usually will show as a spot on the viewing screen, which is a small area where there is no light coming from the tube with the defect. Spots are usually fairly small, usually about 5 mm in diameter, but can range from small dots to about 20 mm. Sometimes you may also see excessive phosphor "grain", which can appear as an unevenness in brightness and make the phosphor surface appear "dirty".
b. CRTs can exhibit an aging effect called "raster burn", where the area of the phosphor where the image has been active over an extended time is aged and no longer is as efficient (bright) as the phosphor areas that have not been used. While this can be present on any tube, usually this is a problem primarily affecting blue and green tubes. If present and viewable, a raster burn will show up as a large darker rectangle, nearly the size of the screen, with brighter areas outside the rectangle.
With a CRT system that have never been moved, the most likely
raster burn to be noticed is on a 16:9 aspect ratio screen, in
which a 4:3 image area has been burned into the center of
the display. Even for projectors that have been
moved/reinstalled, since any burned rectangle is likely to be about as
large as the image, the burn may not be visible, or may only be visible on one, two or three edges.
c. The third type defect is called a "pattern burn", which occurs when a fixed image or pattern has been displayed for an extended time, and has been "imprinted" on one or more of the tubes. This is the effect seen on monitors that are used to repeatedly display lines of text, such as airport arrival/departure displays. If you look carefully while displaying the full field patterns, you should be able to see any pattern burn that is present.
d. The last of the CRT defects noted here is the "scan burn". Normally in a CRT projector the electron beam is rapidly moved (scanned) both left to right and top to bottom to create the image. If there are failures in the electronics that cause this scanning to stop, there is a danger that a horizontal or vertical line can be "burned" through the center of the image. This defect is seen as a line where there is no (or reduced) brightness on one (or more) or the CRTs. |
