Add Photon type

src/colour/mod.rs

@@ -1,2 +1,5 @@
 pub mod colour_rgb;
 pub use colour_rgb::{ColourRgbF, ColourRgbU8, NamedColour};
+
+pub mod photon;
+pub use photon::Photon;

src/colour/photon.rs

@@ -0,0 +1,10 @@
+/// A quantum of light with a given wavelength and intensity
+pub struct Photon {
+    /// The wavelength in nanometres
+    pub wavelength: f64,
+    /// The intensity of the light
+    ///
+    /// Depending on context, this might represent actual intensity in W/sr,
+    /// radiant flux in W, irradiance in W/m^2, or radiance in W/(m^2sr).
+    pub intensity: f64,
+}