Add and/or update power() getter/setters (#22298)

src/lights/PointLight.js

@@ -18,16 +18,15 @@ class PointLight extends Light {
 
 	get power() {
 
-		// intensity = power per solid angle.
-		// ref: equation (15) from https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf
+		// compute the light's luminous power (in lumens) from its intensity (in candela)
+		// for an isotropic light source, luminous power (lm) = 4 π luminous intensity (cd)
 		return this.intensity * 4 * Math.PI;
 
 	}
 
 	set power( power ) {
 
-		// intensity = power per solid angle.
-		// ref: equation (15) from https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf
+		// set the light's intensity (in candela) from the desired luminous power (in lumens)
 		this.intensity = power / ( 4 * Math.PI );
 
 	}

src/lights/RectAreaLight.js

@@ -13,6 +13,20 @@ class RectAreaLight extends Light {
 
 	}
 
+	get power() {
+
+		// compute the light's luminous power (in lumens) from its intensity (in nits)
+		return this.intensity * this.width * this.height * Math.PI;
+
+	}
+
+	set power( power ) {
+
+		// set the light's intensity (in nits) from the desired luminous power (in lumens)
+		this.intensity = power / ( this.width * this.height * Math.PI );
+
+	}
+
 	copy( source ) {
 
 		super.copy( source );

src/lights/SpotLight.js

@@ -26,16 +26,15 @@ class SpotLight extends Light {
 
 	get power() {
 
-		// intensity = power per solid angle.
-		// ref: equation (17) from https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf
+		// compute the light's luminous power (in lumens) from its intensity (in candela)
+		// by convention for a spotlight, luminous power (lm) = π * luminous intensity (cd)
 		return this.intensity * Math.PI;
 
 	}
 
 	set power( power ) {
 
-		// intensity = power per solid angle.
-		// ref: equation (17) from https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf
+		// set the light's intensity (in candela) from the desired luminous power (in lumens)
 		this.intensity = power / Math.PI;
 
 	}