Dead Pixels Test

Text and pictures © 2003-2024 Guillaume Dargaud
Last updated on 2023/10/26

"There is unexpected beauty hidden everywhere in this world — one just has to be open to seeing it. Remember that the next time you sneeze on your monitor."    — Nathan Walton.

Dead LCD screen pixels

So you just bought a superb ultra-flat 25¾" LCD monitor, but it looks like there are a few 'spots' on it. Depending on how many and it'll mean return to the vendor or live with it. Or you could be about to buy one in a store and want to make sure there aren't any dead pixels. Just come to this page !

Testing for dead pixels is very simple, there's nothing to install. All you need to do is click on all the links below to load the various test pages and look carefully at the images for any sign of dead (black) pixel, lit (white) pixel or also colored pixel (here yellow and blue, even if it's difficult to tell the colors).

The links below will open a JavaScript window full screen. If your browser cannot do JavaScript or won't scale full screen either, you still have the option, once you are on one of the pages below, to right click it, do [Set as wallpaper] and hide all your windows (Press [Win][D]). If you still don't see the dot pattern covering the full screen, you need to 'tile' the wallpaper with the following method in Windows: right-click on the Desktop, chose [Properties] and then chose [Tile] as the [Picture Display option]. Once you have tested all the images, go put some better looking wallpaper.

Now don't confuse a dead pixel with a fleck of dirt on the screen or a piece of dried snot. Wipe gently your screen with a dry clean rag before using the test procedure. How can you tell the difference ? Easy: move your head to the side of the screen: if the error moves to the next pixel, it's dried snot ! If you notice dead pixels, sometimes you can massage them back to life: just rub the screen by pressing a finger gently through a rag around the pixel. No guaranty but sometimes it works; LCD monitors are fairly fragile and easily scratched, so take it easy.

So even if you are lucky enough to not have any dead pixels on your monitor, you can still come back to this page when you are cleaning it in order to check that all the specks are gone. Some people pretend that you can also bring a pixel back to life by alternating its color very fast. I'm not convinced but you can always try to leave the 'cycle them' window open overnight... It may do the job if it doesn't kill your video card first or if your monitor doesn't go in sleep mode from sheer boredom.

Some advice about getting the best out of your screen:

• Use it at its highest color mode (32 bits) to avoid dithering (display options of the control panel). 24 bits is okay, but avoid 16 bits modes.
• Use it at its highest frequency to lower flickering (advanced display options of the control panel), although the frequency is a lot less important for LCD than for normal monitors. Note that pivot monitors sometimes have lower frequencies in vertical position than horizontal and the driver will refuse to rotate the image if the horizontal frequency is set too high.
• Unlike classic monitors, LCD screens have only one optimal resolution: the highest. If your screen is given for 1200x1024, don't try to use it at 1024x768 even if characters get too small to read, instead change the default system font size in the advanced display options of the control panel; you'll get better quality and readability this way.
• If both your video card and your monitor have classic analog (VGA) and digital (DVI) outputs, always use the digital cable.
• The images above can also be used to test the bleeding of the pixels onto one another. If you look closely you should see a fine tiny mesh. Otherwise if you see a grey mess or very flickery lines, then it's a sign of lack of quality from the screen or the video card. There can also be a lot of quality difference from one video card to the next, even on identical models.
• Note that CRT screens don't exhibit 'dead pixel' problems. It's specific to LCD or plasma screens.

"I'm a retard. I was trying to use Windows Magnifier to enlarge the dead pixel area to look at it better. I had it open for like 5 seconds, then like 'wait a minute'..."    — RainmakeR.

Dead digital camera pixels

LCD, plasma or OLED screens are not the only high-tech equipment to exhibit wrong pixels. A similar thing happens to digital cameras. A pixel of the CCD sensor that doesn't react to light is always black (dead pixel), while one with a short circuit always shows white (hot or stuck pixel), with the possibility that one of the 3 base colors only is dead or hot. Most recent cameras have special circuits that detect and remove such pixels from the final image, often as part of the noise reduction function applied during the creation of the image file. Note that a dead pixel may look very similar to a speck of dust on the sensor although a dead or hot pixel will stay in the exact same position between pictures while a speck of dust may move somewhat. A speck of dust will also often cover more than just one pixel.

How can you find if your digital camera has dead or hot pixels ? Well, it depends on the camera, but here is a general procedure for hot or stuck pixels:

• Set the camera to save images as RAW. If your camera doesn't create RAW, then you are mostly out of luck.
• Put the tap on the lens and if you have a reflex, block the viewfinder (as some parasite light may go through).
• Set the mode to [M]anual, with the aperture as closed as possible: f9, f22, f32, as high as your lens can go.
• Take one image with a short exposure time (1/8000s or as fast as your shutter goes) and another one around 1 second (stuck pixels stay the same no matter the exposure time while hot pixels become more visible when the exposure time increases due to increased leakage).
• Many camera have a process called 'black subtract' or 'dark removal' when taking images above a certain exposure time (usually 1 to 15 seconds). Make sure this mode doesn't kick in or it will remove the defective pixels.
• Now transfer the RAW file to the PC and load it up in your RAW editor of choice. The image should look perfectly dark. If not then there is parasite light going though.
• Use the default setting for most options, but make sure to NOT use the noise reduction and neither the sharpness. Look around the options and make sure there's no dead/hot pixel removal option in the software (some will automatically detect them on the first images you submit and then remove them on all subsequent images).
• Now export the RAW files as TIF (or PNG but not not JPG) and transfer them to an image editing program such as Photoshop or Paint Shop Pro.
• In PSP you can use [Effects][Edge Effects][Find all Edges], but functions such as [Image][Negative] + [Effects][Edge Effects][Enhance Edges] will produce a similar result.
• Now look closely at the resulting white picture at 100% zoom (or more), panning around the whole image: there will be a few black specks such as shown on the right. Those are hot pixels (the image is shown in negative for readability).
• If you see a LOT of them with all kinds of colors, then they are most likely not hot pixels but electronic noise in the sensor. Try again making sure no light arrives at the sensor and shorten the exposure time. Some cameras like the Ricoh GRd or special astronomy cameras have a special noise reduction function called 'dark removal' which handles this kind of issue (but also gets rid of the dead/hot pixels in the process).
• BTW, when you perform this procedure, make sure your monitor is clean as you don't want to stack dust on the screen with dead screen pixels with dead camera pixels with sensor dust with dandruff on your glasses... C;-)

The opposite procedure of taking an all-white image to see dead sensor pixels doesn't work as well because of light diffusion and bleeding between the pixels. A light grey defocused image would work better.

Left: Four dead pixels off a real digital camera CCD, the image is shown at 100% zoom, but because of the Edge effect, they show up bigger. I've grouped them to make them more visible; they all have similar shapes probably due to the demosaicing alogithm used.