After Effects Expression to control flare brightness from spotlight orientation

Spot_Flare_Brightness.js

// AE expression to modulate a spotlight's flare brightness
// depending on whether the spot is facing into or away
// from the camera
// h 6/12/2016

// change these to reference your spotlight
// and camera as appropriate

L = thisComp.layer("Light 1"); 
C = thisComp.activeCamera;

// maximum and minimum brightness
// these are multiplied by the existing brightness
// parameter, so you can still tweak the overall
// flare brightness in the usual way

maxBrightness = 1;
minBrightness = 0.2;

// our calculations are based on 0 --> 180
// light pointing AT camera --> AWAY from cam
// so halve the light cone angle for simplicity

coneAngle = L.lightOption.coneAngle / 2;
coneFeatherAngle = coneAngle - (coneAngle * L.lightOption.coneFeather * 0.01);

// work out the angle between the light's direction
// and the vector between camera and light
// (all credit Dan Ebberts)

v1 = normalize(C.toWorld([0,0,0]) - L.toWorld([0,0,0]));
v2 = L.toWorldVec([0,0,1]);
relativeAngle = radiansToDegrees( Math.acos (dot(v1,v2)))

// map the angle to the brightness multipliers

if (relativeAngle <= coneFeatherAngle) {
	output = maxBrightness;
} else {
	if (relativeAngle >= coneAngle) {
		output = minBrightness;
	} else {
		output = linear (relativeAngle, coneFeatherAngle, coneAngle, maxBrightness, minBrightness);
	}
}

// multiply our final output multplier by
// the existing brightness parameter

output * value