CRT Video Quality & Calibration Charts

The monitor calibration procedure of image manipulation applications often relies on calibration charts that use 1-bit chess-bard dither. These are not accurate on Cathode Ray Tube (CRT) monitors and/or with poor quality display driver card. 

This includes the gamma swatch in Photoshop 4.01 and earlier, monitor calibration in e.g. Picture Publisher, the Gamma.tif of Paint Shop Pro and Monitor gamma dialog in Umax MagicScan scanner driver.

Calibration Charts and Dithering

The reason for the dithered sections in all calibration charts is to provide a known reference level that is compared against an area with continuous color. 

Monitor gamma does not affect to the black level nor to the white (or full intensity of any color) so dithering  is a convenient way to provide a reference level (50% gray). 

When 1-bit chess-board dithering is used then a large error is introduced into the calibration, the value of the error depends on the quality of both the CRT monitor and the display driver card of the system. Even on highest quality CRT and display driver the error of the 1-bit chess-board dither is 15% or more.

What is wrong with the 1-bit chess-board dither

Please see the gray square in the below chart. It is composed by three different black and white dither patterns. 

• The inner section is a 1-pixel chess board patter, (1 pixel black and white).
• The middle section is composed by black and white vertical lines  that are 1 pixel in width.
• The outer section is composed by black and white  horizontal lines that are 1 pixel in height. 

Each of the three dithered sections should provide the very same 50% gray level since they all contain equal number of black and white pixels per a given area.  Do you see intensity difference between the sections ?  Please remember to view the square at such distance that the dithering can not be discerned.  If you see other colors than gray or moire pattern in the square then the video performance of the system is rather poor.

What is the reason for the intensity difference between the mathematically equal density dither patterns

The problem is due to the video signal quality in the display driver card and/or in the signal amplifiers in the monitor. 

On the CRT monitor the image is drawn horizontally by an electron beam, one line after another. 

When there are black and light pixels next to each other on the same horizontal line and the the signals that drives the red, green and blue guns in the CRT does is not fast enough (or does not not stabilize fast enough) then the intensity value of a pixel affects to the intensity value of the next pixels, this called bleeding or ghosting. 

Photoshop v.4.0.x and earlier Gamma Calibration is Affected

The graph on the left is a portion of a screen capture, the 6x insert is done by nearest neighbor. As can be seen the reference level is composed by 1 pixel black and white checker board.  Other image manipulation applications like the Picture Publisher and Paint Shop Pro also provides gamma images that employs the chess board pattern. This is corrected in Photoshop versions 5.0 and above, Adobe Gamma utility now use horizontal line dithering that is accurate.

Quantify the Quality of the Video System

A better view of this problem is shown below in the Monitor Signal Conditioning chart. Again please view the image at such distance that the dithering can not be discerned. 




Normal view Rotated by -90 degrees.


The above Monitor Signal Conditioning test chart contains 3 swatches, all with 26 levels starting from pure black, mid-range and from the pure white having the upper part with horizontal line dither and lower part using vertical line dither (vertical line dither results the same as 1-bit chess-board).

On a perfect monitor the upper and lower part should appear for the eye exactly similar in luminance or lightness in each of the three swatches. However on a typical CRT monitor there is large error nearly all over the range.

This chart can be used for quality evaluation, view it at such distance that the dither is averaged by the eye and assess the highest step in each swatch where the upper and lower part have equal luminance. Higher result is better.

On a very low quality monitor the lower parts of the swatches will show moire patters and/or color(s) other than gray.

Laptops that have LCD or TFT displays are superior in respect or bleeding / ghosting since in them the image is not converted into analog video signals. On such displays all the sections in the Monitor Signal Conditioning chart match perfectly.

Conclusion

Horizontal line dithering must be used for dithering the reference levels in gamma charts. 

How Dow it Affect Photographic Images

This problem show most strongly in images that have strong vertical edges but it is clearly notable in typical photos. We should demand much better CRT monitors. There is a plug-in for Photoshop available on the FilterFormula/ATS page just for compensating the photographs, with it you can evaluate the effect on your own photos.