Question

In an image intensifier, the factors contributing to brightness gain are:

• A. Electronic gain and voltage gain
• B. Geometric gain and power gain
• C. Electronic gain and geometric gain
• D. Current gain and voltage gain

Hint:

To remember this easily: Brightness is increased by focusing the image down to a smaller size (Geometric layout) and by kicking up the speed of the inner electrons using high voltage (Electronic flux).

Answer 1

The Correct Option is C

Concept: An image intensifier is a complex device used in fluoroscopy to convert low-intensity X-rays into a visible light image that is bright enough to view clearly. The total enhancement of brightness achieved by this system is known as the total Brightness Gain.

Step 1: Defining Total Brightness Gain.
The total brightness gain of an image intensifier tube determines how much brighter the image at the output phosphor is compared to the original illumination at the input phosphor. It is structurally the product of two distinct components: \[ \text{Brightness Gain} = \text{Minification Gain} \times \text{Flux Gain} \]

Step 2: Explaining the component mechanisms.

• Minification Gain (Geometric gain): This occurs due to the geometrical difference in surface area between the large input phosphor screen and the very small output phosphor screen. Compressing the same number of emitted electrons onto a smaller output surface concentrates the brightness geometrically. It is calculated as: \[ \text{Minification Gain} = \left( \frac{\text{Diameter of Input Phosphor}}{\text{Diameter of Output Phosphor}} \right)^2 \]
• Flux Gain (Electronic gain): This gain is achieved electronically via high-voltage acceleration fields within the tube. Electrons emitted from the photocathode are driven by an electric potential of about \( 25 \text{ to } 30 \text{ kV} \) toward the output screen. This acceleration causes each electron to strike the output phosphor with massive kinetic energy, producing a significantly larger number of light photons than would occur otherwise.

Step 3: Synthesizing the options.
Since minification gain represents the physical/geometric scale reduction and flux gain represents the electronic kinetic energy enhancement, the total brightness is governed by the combination of electronic gain and geometric gain. This maps precisely to Option (C).