CRT Test Screen

Press Go full screen for a checkerboard grid, then step through line patterns, an edge frame and gray steps.

Pattern Pick one, then go full screen.

What to look for

F goes full screen. Escape leaves.

Checkerboard grid, 16 by 12 squares

Checkerboard grid, 16 by 12 squares

Preview · scaled to fit

Sixteen squares across and twelve down, black and white by turns. On a 4:3 tube each should be a true square of the same size, with edges that run straight to the corners. Their edges work as a grid for judging the shape of the picture.

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What to look for

What a picture tube shows

Getting a fair picture

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All 19 patterns and checks

Patterns the page draws 9

  • Checkerboard grid, 16 by 12 squares Sixteen squares across and twelve down, black and white by turns. On a 4:3 tube each should be a true square of the same size, with edges that run straight to the corners. Their edges work as a grid for judging the shape of the picture.
  • Cyan convergence lines The line blocks in cyan, made by the green and blue beams. With the magenta and yellow patterns it covers every pair of beams, so you can tell which color is the one out of place.
  • Dark patches, levels 4 to 40 Ten patches on black at levels 4 to 40, each with its number. In a dim room, find the lowest one you can see. On a tube the background should stay black while the low patches still show.
  • Magenta convergence lines The same blocks drawn in magenta, which only the red and blue beams make. Where those two land apart, a magenta line turns into a red line beside a blue one.
Show all 19 patterns and checks (15 more)

Patterns the page draws, continued

  • Picture-size frame, 5 and 10 percent in A white line on the outermost pixels of the picture and two inner frames, 5 and 10 percent in. A tube television hides the outer line on purpose. The inner frames show how much of the picture is lost behind the bezel on each side.
  • Purity fields, one for each beam Three broad fields of red, green and blue, one for each beam alone. Each should be one even color from top to bottom. A blotch of another color inside a field is a purity fault. Look near the corners and edges as well as the middle.
  • Tube brightness staircase, 10 grays Ten flat grays from level 0 to 255, about 28 levels apart. Every step should be told from the next, the first should be black and none should carry a tint. Steps that merge at either end point to brightness or contrast.
  • White convergence and focus lines Blocks of upright and level white lines on black, one, two and four pixels thick. All three beams draw a white line together, so it shows two things at once: how finely the tube focuses, and whether the three colors land on the same spot.
  • Yellow convergence lines The line blocks in yellow, made by the red and green beams. A yellow line that shows a red edge on one side and a green edge on the other marks a gap between those two beams.

What a picture tube shows 8

  • A patch of the wrong color A beam that strikes phosphors of another color tints that area while edges stay sharp. This is a purity fault. A magnetized shadow mask can cause it, for example after a loudspeaker has stood beside the set.
  • A picture that grows as it gets brighter Show the dark patches, then the checkerboard grid, and watch the edge of the picture. If it swells on the brighter pattern, the set is blooming: the picture grows when a bright image pulls the tube's high voltage down.
  • A two-color line that splits in two Step through magenta, yellow and cyan. The color that appears as the stray line in two of the three patterns is the beam out of place: red shows in magenta and yellow, green in yellow and cyan, blue in magenta and cyan.
  • An outer line you cannot see Tube televisions were built to cut off the edge of the picture, because sets varied in where they placed it. A missing outer line is normal on a TV. Tubes made for computers are set the other way, with a border round the picture, so there the line should show.
  • Colored fringes on white lines A white line with a red, green or blue shadow beside it means the three beams are not landing together there. This is a convergence error. At the center it is called static, and toward the edges and corners dynamic.
  • Edges of the grid that bow in or out Follow the outer rows of squares along each side of the screen. They should run straight. Sides bowed toward the middle are called pincushion distortion, and sides bulging outward barrel distortion.
  • Lines that are thicker in the corners The one-pixel blocks sit at the left of the pattern. Drag the browser window so they lie at the center of the tube, then near a corner. Lines that stay thin and separate in both places mean the beam is focused across the whole face.
  • Squares that change size across the screen Count on the squares being equal. If those on one side are wider than those on the other, or the top row is taller than the bottom, the picture is stretched unevenly in that direction.

Getting a fair picture 2

  • Degauss before you judge color Many sets clear stray magnetism by themselves each time they are switched on, with a thump and a brief shake of the picture. A degauss option is usually in the menu as well. Run it, then look at the three color fields again.
  • Draw the patterns at the tube's own signal Open this page on the computer that drives the tube and go full screen. The page draws at whatever resolution the computer is sending, so one drawn pixel is one pixel of the signal. Browser zoom at 100 percent keeps it so.

How this list was compiled: Written for this page. The levels of the gray steps and the dark patches were computed. Convergence (all three electron beams landing on the same spot), that it is called static at the center and dynamic toward the edges and corners, that poor convergence shows as color shadows along edges, purity (each beam striking only the phosphors of its own color) and that poor purity makes areas look off-color while edges stay sharp, are from https://en.wikipedia.org/wiki/Cathode_ray_tube, which also describes blooming: the picture growing when anode voltage falls. That a magnetized shadow mask distorts and discolors the picture, that a degaussing coil reduces the magnetization, that many sets degauss by themselves when switched on, and that a degauss option is usually in the menu, are from https://en.wikipedia.org/wiki/Degaussing. That tube televisions cut off the edges of the picture on purpose, that the picture could grow with a brighter image, and that tubes made for computers are set to show the whole picture inside a border, is from https://en.wikipedia.org/wiki/Overscan. Pincushion and barrel distortion as names for lines bowed inward and outward are from https://en.wikipedia.org/wiki/Distortion_(optics). That red and blue light make magenta, red and green yellow, and green and blue cyan is from https://en.wikipedia.org/wiki/RGB_color_model. All read 2026-10-02. The page draws patterns to judge by eye and names no repair. Licence: own work

How it works

This CRT test screen draws patterns for a picture tube: a checkerboard grid for geometry, fine lines for focus and convergence, an edge frame for picture size and gray steps for brightness. It has no circle and no crosshatch.

Read the full guide

Shape first, then the three beams

1. Press Go full screen. On the grid, every square should be the same size and the outer rows should run straight. 2. Press the right arrow for white lines. A colored shadow beside a line is a convergence error. 3. Step on through magenta, yellow and cyan to find which beam is out of place.

Picture size, brightness and color

The edge frame shows how much of the picture hides behind the bezel: a tube TV cuts off the outer line on purpose. The gray steps and dark patches set brightness and contrast by eye. On the red, green and blue fields, a blotch of another color is a purity fault.

  • Run the set's degauss option before judging the color fields.
  • Under Show, red, green or blue redraws the white lines in one color.

Questions and answers

What does a CRT test screen check?

What a picture tube can get wrong: the shape of the picture, whether the three electron beams land together, focus from center to corners, color purity, and how much of the picture is cut off at the edges.

What is convergence on a CRT?

A color tube draws with three electron beams, one each for red, green and blue. Convergence means all three strike one spot. Where they miss, white lines show colored shadows. Beams can meet at the center and still stray toward the edges.

Why does my CRT show patches of the wrong color?

That is a purity fault: a beam is striking phosphors of another color. A magnetized shadow mask can cause it. Many sets degauss when switched on, and a degauss option is usually in the menu.

Can I use these patterns on a flat screen?

They will draw, but the checks are for tubes. A flat panel has a fixed grid of pixels and no electron beams, so it has no convergence to check. The TV color bars page has patterns for flat screens.

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