Browse Prior Art Database

Portable Self Synchronization Spot Wobbler for Evaluation of Color Cathode Ray Tubes

IP.com Disclosure Number: IPCOM000053194D
Original Publication Date: 1981-Sep-01
Included in the Prior Art Database: 2005-Feb-12
Document File: 5 page(s) / 185K

Publishing Venue

IBM

Related People

Morrish, AJ: AUTHOR [+2]

Abstract

To obtain a better contrast and an increased purity margin many color CRTs incorporate the use of a black matrix; this is the term used when the area between adjacent phosphor dots on the screen is filled by a black non-emitting coating.

This text was extracted from a PDF file.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 49% of the total text.

Page 1 of 5

Portable Self Synchronization Spot Wobbler for Evaluation of Color Cathode Ray Tubes

To obtain a better contrast and an increased purity margin many color CRTs incorporate the use of a black matrix; this is the term used when the area between adjacent phosphor dots on the screen is filled by a black non-emitting coating.

The purity margin is defined as the distance between the edge of the collimated electron beam falling centrally on its phosphor dot and the edge of an adjacent, different color, phosphor dot; i.e., distance X in Fig. 1.

Thus, the purity margin can be written as: x = L - (d + D) over 2 (1) where L = phosphor dot spacing d = phosphor dot diameter D = electron beam diameter

Now, if the electron beam and phosphor dot are not concentric, as is usually the case (Fig. 2), the effective purity margin is decreased by the distance, a, between the dot and beam. It is this distance, a, which is termed the beam landing error (BLE). BLEs are undesirable because: (i) if the BLE is such that: a > D - d over 2 (2) then non-uniform illumination of that phosphor dot occurs, as in Fig. 3a, resulting in loss of brightness of that color. (ii) if the BLE is such that: a > L - (D + d) over 2 C3) then the electron beam intended to illuminate phosphor dot A in Fig. 3b partially illuminates a different color phosphor dot B, causing a change in hue (i.e., loss of purity). That is, in this case: a > x That is, the BLE is greater than the purity margin. (iii) if L > D then case (i) above is encountered before (ii) above (i.e., loss of brightness before loss of purity). (iv) if L = D (as is often the case) then a change in hue and loss of brightness occurs simultaneously. Note that then equation (1) becomes: x = D - d over 2 and equations (2) and (3) become equivalent statements. (v) if d < D then an increase in purity margin occurs but at the cost of restricted brightness due to smaller phosphor dots. (vi) if L = d > D then, as is the case for non-black matrix tubes, a gain in the purity margin can be realized for smaller D, but there is a loss in contrast.

The above discussion determines the need for a black matrix. However, there is a drawback in using it in that it is impossible, using direct observation, to determine or correct any BLEs unless they are so large as to be seen as a loss of purity. Thus, the indeterminate position of the electron beam can erode some of the purity margin that the black matrix gains unless some method of direct observation is used. It is for this purpose that the Spot Wobbler was developed.

The Spot Wobbler Principle:

If an electron beam passes through a perpendicular vertical magnetic field, it is deflected to the left or right, depending upon the field direction. Likewise a perpendicular horizontal field produces a deflection in the vertical direction. Fig. 4 shows the effect of an electron beam passing through the shadowmask being deflected by a local horizontal field (a) upward and (b) downward.

In either c...