Calculator
Example data
Sample conversions shown for standard dry air. Your custom conditions may differ.
| Air wavelength (nm) | Vacuum wavelength (nm) | Typical n | Difference (pm) |
|---|---|---|---|
| 300 | 300.087471 | 1.000291569 | 87.47 |
| 500 | 500.139487 | 1.000278974 | 139.49 |
| 656.28 | 656.461299 | 1.000276252 | 181.3 |
| 1000 | 1000.274166 | 1.000274166 | 274.17 |
Formula used
The basic relationship is:
λvac = n · λair and
λair = λvac / n
We estimate the refractive index n as a function of wavelength using a common dispersion equation (with wavelength in micrometers, µm):
n = 1 + 10⁻⁸ · [ 5792105/(238.0185 − σ²) + 167917/(57.362 − σ²) ]
where σ = 1/λ (in µm⁻¹).
For User Conditions, we scale refractivity by dry-air density:
(n − 1) ∝ Pdry/T, using Pdry = P − e,
and e is estimated from relative humidity and saturation vapor pressure.
How to use this calculator
- Pick a direction: Air → Vacuum or Vacuum → Air.
- Enter the wavelength and choose your unit.
- Select Standard Dry Air for quick, common conversions.
- Choose User Conditions if you know temperature, pressure, and humidity.
- Press Calculate, then download CSV or PDF if needed.
Tip: If you are converting many lines (spectra), keep the same model and conditions so your dataset stays consistent.
Air to Vacuum Wavelength Conversion for Precise Spectra
When a spectrometer reports a line position, the number may be referenced to air or to vacuum. This calculator converts between those conventions so your data matches catalogs, databases, and calibration lamps. It also reports the refractive index n that links the two scales and shows the tiny but important wavelength offset clearly.
Why air and vacuum values differ
Light travels slightly slower in air than in vacuum. That change is described by the refractive index n, where λvac = n·λair. Because n is a little larger than 1, the vacuum wavelength is slightly longer than the air wavelength. The difference is small, but it can shift line centers.
Refractive index trends across wavelength
Air is dispersive, meaning n depends on wavelength. In the visible range, n is typically around 1.00027, but it varies with λ. Shorter wavelengths generally experience a slightly larger refractivity than longer wavelengths. That is why a single constant correction is not ideal for wide spectral ranges.
Temperature and pressure effects
Air density changes with temperature and pressure, and refractivity scales roughly with density. Higher pressure increases n and makes the air→vacuum correction bigger. Higher temperature lowers density, reducing n and shrinking the correction. If you are comparing measurements from different labs, matching P and T prevents systematic offsets.
Humidity and real laboratory air
Water vapor changes the composition of air and reduces the partial pressure of dry air. This calculator models humidity by lowering dry-air pressure using relative humidity and saturation vapor pressure. For many routine spectroscopy tasks, this improves realism. For high-precision metrology, a full physical model is recommended.
Units, resolution, and practical magnitude
You can enter nm, Å, µm, or meters and get the same unit back. A typical 500 nm line converts to roughly 500.14 nm in vacuum, a difference of about 140 pm. Even picometer-level shifts matter for resolving closely spaced lines or tracking Doppler shifts in astronomy.
Where this calculator is used
Common uses include lamp calibration, emission/absorption line matching, laser wavelength reporting, astronomy radial-velocity work, and comparing instrument software outputs that use different conventions. Exporting CSV or PDF helps you document assumptions alongside your results.
FAQs
Q1: What is the difference between air and vacuum wavelength?
Air wavelength is measured as light propagates through air; vacuum wavelength is the equivalent value in empty space. Vacuum values are slightly larger because the refractive index of air is just above 1.
Q2: When should I use Standard Dry Air versus User Conditions?
Use Standard Dry Air for quick comparisons with common tables. Use User Conditions when your lab temperature, pressure, or humidity is far from standard, or when you need consistency across multiple measurements.
Q3: How big is the correction around 500 nm?
At about 500 nm, the vacuum wavelength is typically ~0.14 nm higher than the air wavelength (around 140 pm), depending on the refractive index model and atmospheric conditions you choose.
Q4: Is the formula valid for deep UV or far infrared?
The dispersion equation is most reliable in roughly 0.2–1.7 µm. Outside that range, errors can grow. If you work in extreme UV or long-wave IR, use a specialized model or reference data for your band.
Q5: Can I convert frequency the same way?
Frequency does not change between air and vacuum for the same light, but the wavelength does because speed changes. Convert wavelengths with this tool, and compute frequency separately using f = c/λvac.
Q6: Why does humidity change my result?
Higher humidity lowers the dry-air partial pressure, reducing air density and slightly reducing refractivity. That makes n smaller and the air-to-vacuum correction slightly smaller, especially at warm temperatures.