Étendue & Throughput Calculator
Calculate source and monochromator étendue, identify the limiting element, and design relay optics for optimal coupling.
Étendue (also called throughput or optical invariant) G = A × Ω is the product of source area and solid angle of emission; it is conserved through any lossless optical system and sets a fundamental upper bound on how much light can be transferred between a source and an instrument. When coupling a fiber or extended source into a monochromator, the throughput is limited by whichever element — source or entrance slit — has the smaller étendue. This tool calculates source étendue from core diameter and NA (for optical fibers or circular sources) or from width and height (for rectangular sources), and monochromator étendue from slit dimensions and f-number. It identifies the limiting element, computes the étendue ratio, estimates coupling efficiency, and calculates the relay magnification and image size needed to match f-numbers between source and monochromator.