Radiometric units table: add spectral (per nm) SI units

chapters/lightfields-02-measurement.qmd

@@ -155,14 +155,14 @@ The geometric relationship between surfaces and relatively small light sources i
 
 @tbl-SI-units is a guide to help you remember the four fundamental spectral radiometric measurements. Please note that people will sometimes report a measure of any of these radiometric quantities by summing the energy across all wavelengths. This would be called, for example, the *total radiance* or *total irradiance,* rather than the spectral radiance.
 
-| Term | Application | SI unit |
-|------------------------|------------------------|------------------------|
-| Radiant flux | Light emitted in all directions from a point | Watt (W) |
-| Radiant intensity | Light emitted from a point in a direction per solid angle | W sr⁻¹ |
-| Radiance | Light from an extended source in a direction from an oriented surface | W sr⁻¹ m⁻² |
-| Irradiance | Light incident on a plane from all directions | W m⁻² |
-
-: The four fundamental radiometric measurement quantities. The table lists the name of the measurement, how it is used, and the International System of Units (SI) definition for each {#tbl-SI-units}
+| Term | Application | SI unit (total) | SI unit (spectral) |
+|------------------|------------------------|------------------|------------------|
+| Radiant flux | Light emitted in all directions from a point | Watt (W) | W nm⁻¹ |
+| Radiant intensity | Light emitted from a point in a direction per solid angle | W sr⁻¹ | W sr⁻¹ nm⁻¹ |
+| Radiance | Light from an extended source in a direction from an oriented surface | W sr⁻¹ m⁻² | W sr⁻¹ m⁻² nm⁻¹ |
+| Irradiance | Light incident on a plane from all directions | W m⁻² | W m⁻² nm⁻¹ |
+
+: The four fundamental radiometric measurement quantities. The table lists the name of the measurement, how it is used, and its International System of Units (SI) unit. The *total* unit applies when the energy is summed across all wavelengths; the *spectral* unit, used throughout this book, adds a factor of nm⁻¹ (per nanometer). Photon-based units replace watts with photons per second (q/s), as in @sec-lightfield-energy. {#tbl-SI-units}
 
 I list the corresponding **photometric** measurements in @sec-human-wavelength-encoding. The photometric units corresponding to these radiometric terms have the same geometry, but they sum across wavelengths, weighted by the relative visual significance of each wavelengths. The term 'significance' is doing a lot of work there, which I explain later.