What is the name of the principle that allows a large area of focal spot heating while keeping the effective spot small?

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Line focus principle explains how you can heat a larger area of the anode while producing a small effective focal spot at the patient. By angling the anode, the actual focal spot is longer, but the projection of that spot onto the image receptor is smaller. This keeps geometric unsharpness low to preserve sharpness, while spreading the heat over a wider area on the anode so the tube can handle higher currents without overheating. The other terms don’t achieve this combination: the anode heel effect describes intensity variation across the field due to the anode angle, not the focal spot size; the focusing cup relates to shaping the electron beam at the cathode, not heat distribution on the anode; beam filtration is about removing low-energy photons and doesn’t govern focal spot geometry.