Both settings increase the output of an X-ray tube, which leads people to treat them as two ways of doing the same thing. They are not. One controls how energetic the photons are. The other controls how many there are, and no amount of one substitutes for the other.
What is inside the tube
An X-ray tube contains a cathode and an anode inside a sealed vacuum envelope. On the cathode side is the filament, and on the anode side is the target.
The vacuum is essential. Electrons cannot cross from filament to target through gas, so any loss of vacuum ends the tube’s useful life immediately.
The filament works like the filament in an incandescent bulb. Passing current through a tungsten wire heats it until electrons gain enough energy to escape the metal surface, a process called thermionic emission. Those free electrons are then available to be accelerated.
A high voltage, measured in thousands of volts, is applied between filament and target. The electrons see that potential difference and accelerate across the gap toward the target.
How the X-rays are produced
Two distinct mechanisms operate when those electrons strike the target, and they produce different parts of the output.
Bremsstrahlung, German for braking radiation, is the dominant one. An incoming electron passing near a target nucleus is deflected and decelerated by the nucleus’s electric field. The kinetic energy it loses is emitted as an X-ray photon. Because electrons lose varying amounts depending on how closely they pass, this produces a continuous spectrum spanning a range of energies.
Characteristic radiation is the second mechanism. An incoming electron knocks an inner-shell electron out of a target atom entirely. An electron from an outer shell drops down to fill the vacancy, and the energy difference between the two shells is released as a photon. Because those shell energies are fixed properties of the element, this produces sharp peaks at specific energies determined by the target material.
The output of any tube is bremsstrahlung continuum with characteristic peaks superimposed on it. The photons exit through a window in the tube wall.
Voltage sets photon energy
Tube voltage determines how fast the electrons are traveling when they arrive at the target, which determines how much energy is available to convert into photons.
The relationship has a hard ceiling. An electron accelerated through 100 kV arrives carrying 100 keV of kinetic energy, so the most energetic photon that tube can possibly produce is 100 keV. Raising the voltage raises that maximum and shifts the whole spectrum toward higher energies.
Photon energy is what determines penetration. More energetic photons have a higher probability of passing through dense or thick material rather than being absorbed by it. If a sample is too thick or too dense for the energy being used, the photons do not reach the detector, and the image contains no information about what is inside.
Current sets photon quantity
Tube current is the number of electrons crossing from filament to target per unit time, and it is governed by how hard the filament is driven.
More electrons striking the target produces more photons per second. It does not produce more energetic photons. The spectrum keeps the same shape and the same maximum energy, with more photons at every energy within it.
Photon quantity determines signal. More photons reaching the detector means a stronger signal relative to noise, which means a cleaner image or the same image quality in less exposure time.
Why one cannot substitute for the other
This is where the practical consequence lives.
If a sample is too dense to penetrate at the voltage being used, increasing current will not fix it. More photons of insufficient energy are still insufficient. They are absorbed by the same material, in greater numbers, and the detector still receives nothing useful. The image gets no better and the target gets hotter.
Running the other direction has a different cost. If an image is noisy and voltage is raised instead of current, more photons will reach the detector and the noise will improve. But higher energies penetrate everything more readily, including the features being looked for, which reduces the difference in absorption between materials. The image gets brighter and flatter, and contrast is usually what the inspection depends on.
So voltage is chosen for the sample, high enough to penetrate it and no higher than necessary to preserve contrast. Current is then adjusted for signal quality within whatever headroom the tube allows.
What limits current
That headroom is thermal. Nearly all the energy in the electron beam becomes heat at the target rather than X-rays, and the target can only shed so much heat from a given area.
This is why tube current cannot simply be increased indefinitely, and why higher power operation generally requires spreading the beam across a larger focal spot to manage the heat load. A larger focal spot produces more geometric unsharpness, which costs resolution.
Voltage, current, focal spot size, and resolution are all connected through that constraint, which is why the settings on an X-ray system are a system of tradeoffs rather than a set of independent dials.
