Question 1

What is photon flow rate and how is it measured?

Accepted Answer

Photon flow rate (also called photon flux) is the number of photons passing a point per unit time, expressed in photons per second (ph/s). It describes how many individual quanta of light are emitted, transmitted, or detected each second. In photonics and quantum optics, it's the fundamental quantity reported by single-photon detectors, laser power meters (when combined with photon energy), and light source specifications.

Question 2

What is an Einstein and why is it used in photochemistry?

Accepted Answer

An Einstein (E) is one mole of photons - exactly 6.02214076×10²³ individual photons (Avogadro's number). It was named by Robert Luther and Theodor Förster to honour Einstein's photon concept. Photochemists use Einstein/s (or µE/s, mE/s) because chemical reactions are driven by molar quantities: one Einstein of photons at a given wavelength delivers exactly one mole of photon energy. Plant physiologists measure PAR (photosynthetically active radiation) in µmol photons/m²/s, which equals µEinstein/m²/s.

Question 3

How do I convert photons/s to photons/ns or photons/µs?

Accepted Answer

Divide by the number of nanoseconds in a second (10⁹) to get photons/ns, or by 10⁶ to get photons/µs. Example: 1 Gph/s = 10⁹ ph/s ÷ 10⁹ = 1 ph/ns. This is useful in TCSPC (time-correlated single-photon counting), pulsed laser characterisation, and optical communication where sub-nanosecond timing matters.

Question 4

What photon flow rates do common light sources produce?

Accepted Answer

A typical 1 mW, 500 nm laser emits about 2.5×10¹⁵ ph/s (2.5 Pph/s). A single-photon avalanche diode (SPAD) detector saturates around 10⁸–10⁹ ph/s (100 Mph/s–1 Gph/s). A photomultiplier tube (PMT) in single-photon counting mode handles ~10⁷ ph/s (10 Mph/s). Sunlight at AM1.5 delivers ~4.4×10²¹ photons/s per square meter of surface. A faint star through a 10 cm telescope might produce ~10⁴ ph/s.

Question 5

What is TCSPC and which units does it use?

Accepted Answer

TCSPC (Time-Correlated Single-Photon Counting) is a technique for measuring fluorescence lifetimes and photon arrival time distributions. It processes photons one at a time, with detector count rates typically in the range of 10⁵–10⁷ ph/s (0.1–10 Mph/s) to avoid pile-up distortion. Results are binned in nanosecond or picosecond time channels, so ph/ns and ph/ps are natural units for raw histogram counts.

Question 6

How is photon flow rate different from photon flux density?

Accepted Answer

Photon flow rate (ph/s) is the total number of photons per second passing through any cross-section - an extensive quantity. Photon flux density (ph/s/m² or ph/s/cm²) is photons per second per unit area - an intensive quantity. To convert between them you need the beam area. This converter handles only photon flow rate (ph/s and compatible time/molar scales); if you need flux density, multiply ph/s by the reciprocal of your beam area in m² or cm².

Question 7

How accurate are the conversions?

Accepted Answer

All molar conversions use the exact 2019 SI-defined Avogadro constant: N_A = 6.02214076×10²³ mol⁻¹. All time-scale conversions are exact (1 minute = 60 s, 1 hour = 3600 s, 1 day = 86400 s). Results are accurate to IEEE 754 double-precision limits (~15 significant digits).

Question 8

Is my data private?

Accepted Answer

Yes - all conversions run entirely in your browser using JavaScript. No data is ever sent to any server.

Photon Flow Rate Converter

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