---
title: "CCD: Follow the Charge Interactive Lab | Sandbox Physics"
description: "Why does a bright CCD pixel leave a trail when charge transfer is imperfect? An imperfect clock leaves some electrons behind. Later clocks move them into following samples, spreading a sharp bright pixel into a tail. Permanently trapped charge lowers the total signal; amplifier noise adds fluctuations after the transfer."
canonical_url: "https://sandboxphysics.com/labs/ccd-charge-transfer/"
language: "en"
page_type: "lab"
alternate_languages:
  zh-CN: "https://sandboxphysics.com/zh/labs/ccd-charge-transfer/"
last_modified: "2026-10-24"
---

C-X2 · From a pixel to the amplifier

# CCD: Follow the Charge

**Why does a bright CCD pixel leave a trail when charge transfer is imperfect?** An imperfect clock leaves some electrons behind. Later clocks move them into following samples, spreading a sharp bright pixel into a tail. Permanently trapped charge lowers the total signal; amplifier noise adds fluctuations after the transfer.

Watch three clock phases move charge toward a readout amplifier. Compare the original exposure with the digitized stream, then separate delayed charge, trapping and read noise.

> Interactive simulation: open this page in a web browser to change the parameters and run the model.

This synthetic sixteen-cell serial CCD starts with a fixed two-peak charge exposure. Each clock transfers a prescribed fraction toward the amplifier; deferred charge remains for later clocks, and an optional fraction is permanently trapped. Read noise is independent Gaussian amplifier noise in electrons. The replay uses the recorded cell states. Gate timing is schematic: no Poisson electrostatics, trap release kinetics, dark current, blooming, full-well saturation or photon shot noise is modeled. Poor transfer is deliberately exaggerated; scientific CCD transfer efficiencies can exceed 99.99 percent. Layer thickness and charge-cloud size are enlarged.

Synthetic acquisition

## The amplifier’s readout stream

Default record

> Interactive graph of synthetic acquired samples.

Saved amplifier samples

Complete-transfer reference

The dashed reference reads the initial charge in reverse cell order. The acquired stream includes transport delay, trapping and amplifier noise.

Nobel · Condensed matter

## What did the original experiments establish?

Follow the apparatus, observations and interpretation in the historical evidence. Each article explains where this teaching model fits.

[Read the historical evidence](https://sandboxphysics.com/stories/condensed-matter/ccd-charge-transfer/) [All six reading paths](https://sandboxphysics.com/stories/condensed-matter/)

Physics tutorial

## A pixel reaches the amplifier one clock at a time

**Background** A CCD stores an exposure as electrons in potential wells. Clocked gates move that charge to a shared output amplifier.

**Why it matters** A bright feature can spread into later samples even when the optical image is sharp. Follow where each electron goes before blaming the lens.

Start with the essentials

- **Focus question**: Why does a bright CCD pixel leave a trail when charge transfer is imperfect?
- **One-sentence intuition**: Deferred electrons arrive late; permanently trapped electrons never reach the amplifier during the readout.

### Core mathematical model

#### Charge in a cell after one clock

$$
q_j^{(k+1)}=(1-\ell)\left[(1-\eta)q_j^{(k)}+\eta q_{j-1}^{(k)}\right]
$$

The cell on the left supplies the incoming charge. The first cell has no incoming packet. The last cell transfers charge into the amplifier.

#### Account for every electron

$$
Q_{\rm initial}=Q_{\rm collected}+Q_{\rm deferred}+Q_{\rm trapped}
$$

This equality applies before amplifier noise. The finite readout keeps the remaining tail explicitly.

#### Amplifier sample

$$
y_k=q_{{\rm out},k}+\epsilon_k,\qquad \epsilon_k\sim\mathcal N(0,\sigma_r^2)
$$

Independent electronic noise can make a faint digitized sample negative after baseline subtraction. It does not create or destroy the transported charge.

### Common difficulties

#### A tail can conserve charge

**Typical misconception** Every electron absent from the main peak has been destroyed.

**Better mental model** Deferred charge moves into later samples or remains in the array when the finite readout ends. Only the explicitly trapped fraction is removed from mobile charge.

#### A teaching inefficiency is exaggerated

**Typical misconception** A transfer efficiency near eighty percent is typical of a scientific CCD.

**Better mental model** Real scientific CCDs can exceed 99.99 percent per transfer. The degraded preset makes cumulative delay visible in a short sixteen-cell register.

### Run the experiment

1. 01

   #### Follow a clean readout

   Choose Clean transfer. Follow the bright packets to the amplifier and compare the two peaks with the saved output.

   **What to observe:** The closest pixel is read first. With almost complete transfer, the stream preserves the reversed spatial order of the input profile.

2. 02

   #### Delay some of the charge

   Choose A long tail and inspect the later readout clocks.

   **What to observe:** Some electrons stay in each cell and arrive in later samples. The tail follows from those same recorded cell states.

3. 03

   #### Separate noise from missing charge

   Compare Faint exposure with Charge trapped. Export their samples.

   **What to observe:** Read noise fluctuates around the transported signal. Trapping reduces the collected charge and appears in the separate trapped-charge count.

Physical conclusion

Readout changes both the shape and the noise of a CCD signal. Count the remaining and trapped electrons when checking charge conservation.

Fields & matter · Starter path

## From a field to an electron microscope

Map an electric field and steer a charged particle. Then find the electron's wavelength and use electron beams to see a crystal and a surface.

[Start here → **Make an Electric Field Visible** Move the probe around a dipole and check that the field always points downhill on the potential landscape. Observe · 8 min](https://sandboxphysics.com/labs/electric-field/)

1. [1 Make an Electric Field Visible](https://sandboxphysics.com/labs/electric-field/)
2. [2 A Charge Bending through Fields](https://sandboxphysics.com/labs/charged-particle/)
3. [3 Electron Wavelength & Voltage](https://sandboxphysics.com/labs/electron-wavelength/)
4. [4 Electron Diffraction & Reciprocal Space](https://sandboxphysics.com/labs/electron-diffraction/)
5. [5 TEM Bright Field, Dark Field & SAED](https://sandboxphysics.com/labs/tem-mode-selector/)
6. [6 SEM Secondary-Electron Topography](https://sandboxphysics.com/labs/sem-topography/)

[View the full path →](https://sandboxphysics.com/topics/fields-and-matter/#starter-path)