Image quality, the most important feature of a monitor, is also the hardest to judge. Good is a subjective term, so relying on the judgment of reviewers (such as CNET) may not get you exactly what you want. Further, like a snowflake, every monitor is unique. There can be significant differences from one monitor to another--even among the same make and model. It's unrealistic to expect to run a comprehensive diagnostic on a monitor before you buy it, but here are some of the most important qualities to look for when choosing a CRT or an LCD. We've also included a few simple tests you can do in the store to assess image quality.

Quality control: LCDs: what to look for | Tips and tricks: assessing LCD image quality | CRTs: what to look for | Tips and tricks: assessing CRT image quality 

Choose a monitor that automatically adjusts to the timing of an analog (VGA) signal. You want to see stable gray and patterned images with no pixel jitter. Find a monitor with good viewing angles. Colors and brightness should not change in the corners of the screen, where the viewing angle is the greatest. Pick a monitor that looks uniformly bright when viewing both dark and light images.

Check for dead pixels, ones that are always off. It's not uncommon for monitors to have one or two defective pixels--especially larger models--but you want as few as possible. Open Internet Explorer, in the address line type about:blank, and press Enter. Then press the F11 key to make it full-screen. Look for small black specks that are not dust or debris on the screen. (Press F11 again to see the Explorer toolbars.) Check for stuck pixels, which are always on. Right-click the Windows Desktop, pick Properties, then choose the Screen Saver tab. Choose Blank from the list of screensavers, then hit the Preview button. Look for any glowing red, green, or blue spots. (Press any key to return to the Display Properties window.)

Find a monitor with good convergence (clean white lines with no colors on the edges), as this increases image sharpness. Choose a monitor with good screen geometry, especially if you intend to use it for design or drawing tasks. Pick a monitor with good image regulation; you want an image that does not change its dimensions or distort when the content of the image changes. Poor image regulation can be a problem with lower-priced models and displays that are operated at the upper limits of their specifications.

Check the convergence of the display. Open Windows Paint and start a new image. Choose Image, then Attributes, and change the size of the image to the same number of pixels as the current resolution (to check the resolution, right-click anywhere on the desktop, select Properties, then choose the Settings tab). Use the Fill tool to fill the screen with black. Then choose the Rectangle tool, set the drawing color to white, and draw a box that fills the screen, starting in the upper-left corner. Scroll to the bottom-right corner of the image and draw another box that fills the screen. Now choose View, then View Bitmap to see the image on the full screen. You should see a black screen with two intersecting white rectangles. Check the white lines closely; you should not see any areas where the lines separate into colored lines. A slight color tinge along the edges of a line is acceptable, but a pure white line is best. Check the screen geometry. Look to see that the lines drawn in the previous step are straight. You can also draw a circle in the center of the image using the Circle tool (hold Shift down to draw a perfect circle), then copy and paste the circle in the corners and along the edges. Look to see that they are round--not squashed or egg-shaped.

Good geometry

Bad geometry