quantum efficiency
/ QE /
Imagine two buckets left out in the rain: one catches nearly every drop, the other lets most splash away. A light detector is much the same — of all the precious photons that land on it from a faint galaxy, only some get registered. The fraction that actually count is the detector's quantum efficiency.
Quantum efficiency, or QE, is simply the percentage of incoming photons that a detector converts into a measurable signal. The old photographic plate had a QE of only a few percent — it threw away the great majority of starlight. A modern CCD reaches a QE of 80 to 95 percent over much of the visible band, meaning almost every photon counts. QE also varies with wavelength: a silicon CCD is excellent for visible and near-infrared light but blind to ultraviolet and X-rays, which need different detector materials.
QE matters because it directly multiplies a telescope's reach. Swapping a 3-percent plate for a 90-percent CCD is like making the telescope's mirror thirty times larger in light-grasp, without touching the optics — which is why the CCD revolution let modest telescopes outperform giants of the photographic era. When astronomers plan how long to expose, the detector's QE is one of the key numbers, because it sets how many of the arriving photons turn into the signal they need.
Going from a 4-percent plate to a 90-percent CCD multiplies the detected signal more than twentyfold — the same gain a telescope would get by quintupling its mirror diameter.
A better detector can beat a bigger mirror.
High QE captures more photons but cannot beat the randomness of light itself. Even a perfect 100-percent detector still faces photon noise — the unavoidable statistical scatter in how many photons arrive.