How do you calculate photons per second?
Enter optical power and choose whether you know wavelength, frequency, or photon energy. The calculator converts your inputs into photon energy, photon rate, and total photons over a selected time interval.
Results
Enter your values and click Calculate to see the photon rate.
Premium physics workflow
This calculator is designed for optics, spectroscopy, laser applications, LEDs, and detector calculations where you need a reliable conversion from optical power to photon flux.
- Supports wavelength, frequency, or direct photon energy inputs.
- Converts power into photons per second using Planck’s constant and the speed of light.
- Also reports total photons delivered over your selected time interval.
- Generates a wavelength comparison chart at the same power level to show how photon rate changes across the spectrum.
Ephoton = h × f
Ephoton = h × c / λ
photons/s = P / Ephoton
Photon rate vs wavelength at your selected power
For a fixed power, longer wavelengths have lower energy per photon, so the source emits more photons each second.
Expert guide: how do you calculate photons per second?
If you have ever worked with lasers, LEDs, fluorescence measurements, solar instruments, fiber optics, photodiodes, or quantum efficiency calculations, you have likely asked the question: how do you calculate photons per second? The answer comes from one of the most useful bridges in physics, the connection between the macroscopic quantity of power and the microscopic quantity of photon energy. Once you know how much energy each photon carries, you can divide the total energy delivered each second by the energy per photon. That gives you the number of photons emitted, transmitted, or detected per second.
In practical terms, photons per second is often called photon flux. If a laser outputs 1 milliwatt at a known wavelength, you can convert that 1 milliwatt into joules per second and then divide by the energy of one photon. This tells you how many individual light quanta are involved every second. That number can be enormous. Even a modest optical source can emit trillions to quadrillions of photons per second.
The fundamental formula
The energy of a photon is determined by Planck’s constant and either the frequency or wavelength of light:
- E = h × f
- E = h × c / λ
Where:
- E is photon energy in joules
- h is Planck’s constant, 6.62607015 × 10-34 J·s
- f is frequency in hertz
- c is the speed of light, 2.99792458 × 108 m/s
- λ is wavelength in meters
Once you know photon energy, the photon rate is:
- Convert optical power into watts, where 1 W = 1 J/s.
- Compute energy per photon in joules.
- Divide power by photon energy.
Mathematically, this is:
photons/s = P / Ephoton
Step by step example with wavelength
Suppose you have a 1 mW green laser at 532 nm. First convert the units:
- Power: 1 mW = 0.001 W
- Wavelength: 532 nm = 5.32 × 10-7 m
Now calculate photon energy:
E = hc/λ = (6.62607015 × 10-34) × (2.99792458 × 108) / (5.32 × 10-7)
This gives roughly 3.73 × 10-19 J per photon.
Next divide power by energy per photon:
0.001 / (3.73 × 10-19) ≈ 2.68 × 1015 photons/s
So a 1 mW green laser emits about 2.68 quadrillion photons every second.
Why wavelength matters so much
At constant power, the photon rate depends strongly on wavelength. Shorter wavelengths have higher energy per photon, so fewer photons are needed to make the same power. Longer wavelengths have lower energy per photon, so the source emits more photons each second. This is why infrared sources can have higher photon flux than ultraviolet sources at identical power levels.
That relationship is easy to understand if you think of power as a budget of energy per second. If each photon costs more energy, you can afford fewer photons. If each photon costs less energy, you can emit more photons each second.
| Wavelength | Approximate Color / Region | Photon Energy | Photon Rate at 1 mW |
|---|---|---|---|
| 365 nm | Ultraviolet A | 5.44 × 10-19 J | 1.84 × 1015 photons/s |
| 450 nm | Blue | 4.41 × 10-19 J | 2.27 × 1015 photons/s |
| 532 nm | Green | 3.73 × 10-19 J | 2.68 × 1015 photons/s |
| 650 nm | Red | 3.06 × 10-19 J | 3.27 × 1015 photons/s |
| 850 nm | Near infrared | 2.34 × 10-19 J | 4.28 × 1015 photons/s |
| 1550 nm | Telecom infrared | 1.28 × 10-19 J | 7.80 × 1015 photons/s |
Using frequency instead of wavelength
Sometimes the wavelength is not provided, but the optical frequency is. In that case, use the simpler relation E = hf. For example, if your light has a frequency of 500 THz:
- 500 THz = 5.00 × 1014 Hz
- E = 6.62607015 × 10-34 × 5.00 × 1014
- E ≈ 3.31 × 10-19 J per photon
If the power is 2 mW, the photon rate is:
0.002 / (3.31 × 10-19) ≈ 6.04 × 1015 photons/s
Using electron volts for photon energy
In many optics and semiconductor contexts, photon energy is given directly in electron volts. This is common in spectroscopy, detector specification sheets, and bandgap discussions. To use that value in a photons per second calculation, convert electron volts to joules using:
1 eV = 1.602176634 × 10-19 J
For example, a 2.0 eV photon has an energy of:
2.0 × 1.602176634 × 10-19 = 3.20 × 10-19 J
If your source power is 5 mW, then:
0.005 / (3.20 × 10-19) ≈ 1.56 × 1016 photons/s
Common mistakes to avoid
- Forgetting unit conversion. Nanometers must be converted to meters. Milliwatts must be converted to watts.
- Using total energy instead of power. Power is energy per second. If you only know total energy, divide by time first.
- Confusing wavelength and frequency conversions. They are related by c = λf.
- Mixing joules and electron volts. Keep all energy terms in joules before dividing power by energy per photon.
- Ignoring source bandwidth. Real LEDs and broadband lamps emit over a range of wavelengths, not a single wavelength.
What if the source is broadband?
For a monochromatic laser, the photons all have nearly the same energy, so the calculation is straightforward. Broadband sources such as incandescent lamps, white LEDs, and the Sun are different because they emit across a wide spectrum. In that case, there is no single exact photon energy. You usually have three options:
- Use a representative center wavelength for a fast estimate.
- Use a spectrally weighted average photon energy.
- Integrate the spectral power distribution across wavelength for high accuracy.
In laboratories, the third method is preferred when detector calibration, radiometry, or photochemistry is involved. However, for many engineering estimates, using the dominant wavelength or center wavelength is acceptable.
| Source Type | Typical Wavelength | Example Optical Power | Approximate Photon Rate |
|---|---|---|---|
| Red laser pointer | 650 nm | 5 mW | 1.64 × 1016 photons/s |
| Green DPSS laser | 532 nm | 5 mW | 1.34 × 1016 photons/s |
| Blu ray diode class source | 405 nm | 5 mW | 1.02 × 1016 photons/s |
| IR remote emitter | 940 nm | 10 mW | 4.73 × 1016 photons/s |
| Telecom laser | 1550 nm | 1 mW | 7.80 × 1015 photons/s |
Applications where photon rate matters
Knowing photons per second is not just an academic exercise. It is a critical design quantity in many technical fields:
- Photodetectors: estimating photocurrent from quantum efficiency.
- Fluorescence and microscopy: predicting signal level and bleaching risk.
- Solar research: converting irradiance into photon flux for bandgap matching.
- Fiber optics: analyzing received optical power and detector sensitivity.
- Photochemistry: comparing reaction rates to incident photon count.
- Quantum optics: distinguishing classical high flux beams from low flux single photon regimes.
How photon rate connects to detector current
One of the most useful follow on calculations is detector current. If a detector has quantum efficiency QE, then only a fraction of the incident photons produce charge carriers. The number of electrons per second is approximately:
electrons/s = photons/s × QE
Then current is:
I = electrons/s × e
where e = 1.602176634 × 10-19 C. This is why converting optical power to photons per second is often the first step in sensor design and noise analysis.
Fast mental estimate method
If you need a quick estimate in the visible spectrum, remember that visible photon energies are usually on the order of 3 × 10-19 to 5 × 10-19 J. That means 1 mW of visible light often corresponds to roughly 2 × 1015 to 3 × 1015 photons per second. It is not exact, but it gives you the right order of magnitude almost immediately.
Authoritative references for constants and spectral data
When doing precise calculations, especially for publication, regulatory work, or instrument calibration, use constants from primary sources. The following references are especially useful:
- NIST Fundamental Physical Constants
- NREL Solar Spectral Irradiance Resources
- NASA electromagnetic spectrum overview
Bottom line
To calculate photons per second, start with the optical power and determine the energy of one photon from either wavelength, frequency, or direct photon energy. Then divide power by photon energy. That is the entire method. The reason it is so powerful is that it lets you translate ordinary engineering quantities such as watts and milliwatts into the particle level language used in spectroscopy, detector science, and photonics. Once you are comfortable with the unit conversions, the calculation becomes fast, reliable, and widely applicable.
If you want a practical answer in one sentence, here it is: convert your power to watts, convert your wavelength or frequency into photon energy in joules, and divide power by photon energy to get photons per second.