---
title: Is W/m2 the same unit as W/m2/nm?
url: "https://www.biospherical.com/support/is-w-m2-the-same-unit-as-w-m2-nm/"
updated: "2024-02-22"
category: General FAQs
---

# Is W/m2 the same unit as W/m2/nm?

**Full Question:** In many scientific papers we’ve seen the units of the readings in W/m2. Is this the same unit as the one the software uses (i.e., W/m2/nm)? Do we need to do some calculations to obtain W/m2?

**Answer:** The units of W/m2 and W/(m2 nm) differ because the first measure does not specify the wavelength, meaning simply that a certain amount of radiative power per surface area was measured. When “nm” is included, the measurement is the “radiative power per surface area and per wavelength interval.”

In a more formal sense, “irradiance,”  “spectral irradiance,” and the associated units are defined as follows:

**Irradiance** *E,* is the radiant energy *dQ,* arriving per time interval *dt*, and per area dA, on a surface. The associated SI unit is W/m2.

**Spectral irradiance** *E*λ, is the radiant energy *dQ,* arriving per time interval *dt*, per area dA, and per wavelength interval *d*λ, on a surface. The associated unit is W/(m2 nm).

Irradiance can be calculated from spectral irradiance by integrating over wavelength.

A full discussion of irradiance and its units can be found in BSI’s Application Note “[Light Quantities, Units, and Conversion Factors](https://www.biospherical.com/wp-content/uploads/2024/01/lightunits_006409ka_an2012-0001r1-s.pdf).”

 [Application Note: Light Quantities, Units, and Conversion Factors – #AN-2012-0001-Rev1, BSI DCN: 006409KA](https://www.biospherical.com/wp-content/uploads/2024/01/lightunits_006409ka_an2012-0001r1-s.pdf)[Download](https://www.biospherical.com/wp-content/uploads/2024/01/lightunits_006409ka_an2012-0001r1-s.pdf)