How to detect dark photos in Android

I have an Android app where user takes a photo of himself with the front camera and then the photo is being uploaded to my server. I notice that many photos comes to my server too dark (sometimes almost impossible to cleary see the user face).

I would like to filter out such photos and show notification (eg. "Photo is too dark. Take one more picture") to the user in the app side. How I could accomplish such task in Android?

EDIT:

I have found out how to calculate brightness for one single pixel (thank's to this answer: https://stackoverflow.com/a/16313099/2999943):

private boolean isPixelColorBright(int color) {
    if (android.R.color.transparent == color)
        return true;

    boolean rtnValue = false;

    int[] rgb = {Color.red(color), Color.green(color), Color.blue(color)};

    int brightness = (int) Math.sqrt(rgb[0] * rgb[0] * .299 + rgb[1]
            * rgb[1] * .587 + rgb[2] * rgb[2] * .114);

    if (brightness >= 200) {    // light color
        rtnValue = true;
    }

    return rtnValue;
}

But still I don't have clear idea how to determine whole image brightness "status". Any suggestions?