AAT Bioquest

How to determine the brightness of a fluorescent molecule?

Posted August 5, 2019

The brightness of a fluorescent molecule depends on two factors:
  1. Extinction Coefficient (ε) - how strongly a fluorescent molecule absorbs light at a particular wavelength.
  2. Fluorescence Quantum Yield (Φ) - how efficiently the absorbed light is converted into emitted light.
To calculate a fluorescent molecules 'brightness' use the following formula:
Brightness = Extinction Coefficient (ε) x Fluorescence Quantum Yield (Φ)
For example, let's use the above formula to compare the practical brightness of Phycoerythrin (PE), Allophycocyanin (APC) and Cy5.
  • PE = 1,960,000 cm-1M-1 x 0.84 = 1,646,400
  • APC = 700,000 cm-1M-1 x 0.68 = 476,000
  • Cy5 = 250,000 cm-1M-1 x .20 = 50,000

Therefore, PE > APC > Cy5, in brightness


To view extinction coefficient or fluorescence quantum yield data for a particular molecule, use our:

Additional resources

Fluorescence Fundamentals