You can also takes a look at mxnet augmentation code. Their illumination argumentation produce some magic for me:

https://github.com/dmlc/mxnet/blob/master/python/mxnet/image.py
#brightness, contrast, saturation-------------
#from mxnet code
if 1:  #brightness
    alpha = 1.0 + illumin_limit*random.uniform(-1, 1)
    perturb *= alpha
    perturb = np.clip(perturb,0.,255.)
    pass

if 1:  #contrast
    coef = np.array([[[0.299, 0.587, 0.114]]]) #rgb to gray (YCbCr) :  Y = 0.299R + 0.587G + 0.114B

    alpha = 1.0 + illumin_limit*random.uniform(-1, 1)
    gray = perturb * coef
    gray = (3.0 * (1.0 - alpha) / gray.size) * np.sum(gray)
    perturb *= alpha
    perturb += gray
    perturb = np.clip(perturb,0.,255.)
    pass

if 1:  #saturation
    coef = np.array([[[0.299, 0.587, 0.114]]]) #rgb to gray (YCbCr) :  Y = 0.299R + 0.587G + 0.114B

    alpha = 1.0 + illumin_limit*random.uniform(-1, 1)
    gray = perturb * coef
    gray = np.sum(gray, axis=2, keepdims=True)
    gray *= (1.0 - alpha)
    perturb *= alpha
    perturb += gray
    perturb = np.clip(perturb,0.,255.)
    pass