# Second Radiation Constant

Use Second Radiation in Calcul.io. See what this built-in constant represents, enter it in an editable calculation, and review related values and units.

## secondRadiation

Second radiation constant

Try it yourself:

```calculio
secondRadiation
```

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## Second radiation constant in blackbody theory

**secondRadiation** is the second radiation constant used in Planck’s law for blackbody spectral radiance. It combines the Planck constant, speed of light, and Boltzmann constant.

## Use

It appears in the exponential wavelength-temperature term of thermal-radiation formulas. See [firstRadiation](https://calcul.io/constants/firstRadiation/index.md), [boltzmann](https://calcul.io/constants/boltzmann/index.md), and [planckConstant](https://calcul.io/constants/planckConstant/index.md).

## Caveats

Keep wavelength and temperature units consistent. For peak-wavelength estimates, use [wienDisplacement](https://calcul.io/constants/wienDisplacement/index.md) instead of manually optimizing Planck’s law.

[All constants](https://calcul.io/constants/index.md)