In raster scan colour CRT monitors, how is colour intensity for each primary typically controlled to produce accurate images?

Introduction / Context:
Raster scan colour CRTs render images by scanning three electron beams corresponding to red, green, and blue primaries. This question asks how intensity is modulated for each primary to achieve the desired colour and brightness at each pixel location.

Given Data / Assumptions:

• Three primaries are driven independently.
• Phosphor triads emit light when excited by corresponding beams.
• Raster scanning proceeds line by line across the screen.

Concept / Approach:

To form arbitrary colours, the display must independently control the intensity of red, green, and blue. Therefore each electron gun receives its own drive signal so that the resulting combination at the phosphor triad produces the target colour through additive mixing.

Step-by-Step Solution:

Verification / Alternative check:

Block diagrams of colour CRTs show three video amplifiers and three gun drive signals, confirming separate control.

Why Other Options Are Wrong:

Option a is a geometric arrangement that does not control intensity. Option b prevents independent primary control. Option c describes physical structure but not the control method. Option e is invalid because a correct mechanism is given.

Common Pitfalls:

Confusing mechanical alignment features with electronic drive control, and assuming a single composite signal can encode independent per primary currents without separation hardware.

Final Answer:

By controlling each electron gun with a separate signal