---
tags:
  - science/physics
  - electromagnetic-radiation
  - visible-light
  - sensors
created: 2026-05-16
breadcrumb: "Physics > Electromagnetic radiation > Visible light"
---

# Visible light

> **Mechanism:** Photoelectric & photovoltaic effect

## Items

- **Photodiode** — Silicon p-n junction; absorbs a visible photon, generates an electron-hole pair, drives a reverse current. The cheapest workhorse for "is there light."
- **CCD** — Charge-coupled device array; each pixel collects photo-generated charge, then shuttles it serially to a single readout amplifier. Low noise, slow.
- **CMOS** — Active-pixel sensor; each pixel has its own amplifier and addressable readout. Fast, cheap, dominates phone and consumer cameras.
- **Photoresistor (LDR)** — Cadmium-sulfide film; resistance drops with light intensity. Slow, analog, used in night lights.
- **Phototransistor** — BJT or FET with light-controlled base or gate; gives gain compared to a plain photodiode but is slower.
- **PMT** — Photomultiplier tube; photocathode emits electrons that cascade through dynodes for 10^6–10^8 gain. Single-photon counting in vacuum.

## Related

- [[infrared]]
- [[ultraviolet]]