Calculate Image brightness with Python Pillow

brightness.py

import sys
from PIL import Image


def calculate_brightness(image):
    greyscale_image = image.convert('L')
    histogram = greyscale_image.histogram()
    pixels = sum(histogram)
    brightness = scale = len(histogram)

    for index in range(0, scale):
        ratio = histogram[index] / pixels
        brightness += ratio * (-scale + index)

    return 1 if brightness == 255 else brightness / scale


if __name__ == '__main__':
    for file in sys.argv[1:]:
        image = Image.open(file)
        print("%s\t%s" % (file, calculate_brightness(image)))

In my case imaged below 45 threshold were considered dark..
Try to play with image sizes and Percentage of circles
In this code is 3% distance between circles try to have 1% for a better precision

so what is the number representative of... I understand brightness but what is it specifically? whats it comparing it to? and circles? what do you mean by circles? and if I use PIL and manually set the brightness of an image to .5 that's saying set it to 50% of original brightness, however, I want to match the brightness of another image, will I be able to literally set the second image to the result of this code? or is that the wrong number?

This code gives average brightness level of an image, those circles are indicating that we are picking 3x3 matrix segment of image from that part and finding the brightness and we are doing it for each segment i.e each circles and at the end we are finding average out of them.
There is no absolute value for brightness or darkness higher the value more brighter the image

Does anyone know the motivation for the implementation? why does it subtract from the length/range of the histogram?