public static function lumosity_difference( $color_1 = '#ffffff', $color_2 = '#000000' ) {
		$color_1 = self::sanitize_hex( $color_1, false );
		$color_2 = self::sanitize_hex( $color_2, false );
		$color_1_rgb = self::get_rgb( $color_1 );
		$color_2_rgb = self::get_rgb( $color_2 );
		$l1 = 0.2126 * pow( $color_1_rgb[0] / 255, 2.2 ) + 0.7152 * pow( $color_1_rgb[1] / 255, 2.2 ) + 0.0722 * pow( $color_1_rgb[2] / 255, 2.2 );
		$l2 = 0.2126 * pow( $color_2_rgb[0] / 255, 2.2 ) + 0.7152 * pow( $color_2_rgb[1] / 255, 2.2 ) + 0.0722 * pow( $color_2_rgb[2] / 255, 2.2 );
		$lum_diff = ( $l1 > $l2 ) ? ( $l1 + 0.05 ) / ( $l2 + 0.05 ) : ( $l2 + 0.05 ) / ( $l1 + 0.05 );
		return round( $lum_diff, 2 );
	}