def Brightness(Lumens, b1, l1):
	l2 = 10200.0
	b2 = 255.0
	a2 = (b1 * l2 - b2 * l1) / (l1 * l1 * l2 - l2 * l2 * l1)
	a1 = (b1 - a2 * l1 * l1) / l1
	return a1 * Lumens + a2 * Lumens * Lumens

from pylab import plot, show, xlabel, ylabel, title

b1 = 100.0
l1 = 5000.0

xx = range(0, 10200)
yy = [Brightness(L, b1, l1) for L in xx]

plot(xx, yy)
plot([0, l1, 10200], [0, b1, 255], 'o')
show()