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diff --git a/2020/04/hyperphysics/light_and_vision/power_solid_angle.md b/2020/04/hyperphysics/light_and_vision/power_solid_angle.md
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+
+; "lux" is lumens / m^2
+
+; (var 'symbol 'name 'units)
+
+(var E "illuminance" 'lux)
+(var Theta "luminous power" 'lumens)
+(var I "pointance" 'candela)
+(var Omega "solid angle" 'sterradians)
+
+(eq Theta
+    (* 4 pi I))
+
+(eq E
+    (/ Theta A)
+    (/ (* I Omega) A)
+    (/ (* I A) (* A (exp r 2)))
+    (/ I (exp r 2)))
+
+(eq Omega
+    (/ A (exp r 2)))
+
+(eq I
+    (partial-derivative Theta omega))