Generated by SassMeister.com.

SassMeister-input.scss

// ----
// Sass (v3.4.7)
// Compass (v1.0.1)
// ----

// sRGB GAMMA CORRECTION, per spec: https://en.wikipedia.org/wiki/SRGB
@function re-gamma($n) { @if $n <= 0.0031308 { @return $n * 12.92; } @else { @return 1.055 * pow($n,1/2.4) - 0.055; } }
@function de-gamma($n) { @if $n <= 0.04045 { @return $n / 12.92; } @else { @return pow((($n + 0.055)/1.055),2.4); } }

// sRGB BT-709 BRIGHTNESS
@function luma-ln($c) {
	$rlin: de-gamma(red($c)/255);
	$glin: de-gamma(green($c)/255);
	$blin: de-gamma(blue($c)/255);
	@return re-gamma(0.2126 * $rlin + 0.7152 * $glin + 0.0722 * $blin) * 100;
}

@function luminance($color: #000) {

  // Formula:
  // http://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef

  $rgb: red($color) green($color) blue($color);

  // Speed up and increase compatability with precompiled rgb values
  $l-precompiled: 0 0.0003 0.00061 0.00091 0.00121 0.00152 0.00182 0.00212 0.00243 0.00273 0.00303 0.00335 0.00368 0.00402 0.00439 0.00478 0.00518 0.00561 0.00605 0.00651 0.007 0.0075 0.00802 0.00857 0.00913 0.00972 0.01033 0.01096 0.01161 0.01229 0.01298 0.0137 0.01444 0.01521 0.016 0.01681 0.01764 0.0185 0.01938 0.02029 0.02122 0.02217 0.02315 0.02416 0.02519 0.02624 0.02732 0.02843 0.02956 0.03071 0.0319 0.0331 0.03434 0.0356 0.03689 0.0382 0.03955 0.04092 0.04231 0.04374 0.04519 0.04667 0.04817 0.04971 0.05127 0.05286 0.05448 0.05613 0.05781 0.05951 0.06125 0.06301 0.0648 0.06663 0.06848 0.07036 0.07227 0.07421 0.07619 0.07819 0.08022 0.08228 0.08438 0.0865 0.08866 0.09084 0.09306 0.09531 0.09759 0.0999 0.10224 0.10462 0.10702 0.10946 0.11193 0.11444 0.11697 0.11954 0.12214 0.12477 0.12744 0.13014 0.13287 0.13563 0.13843 0.14126 0.14413 0.14703 0.14996 0.15293 0.15593 0.15896 0.16203 0.16513 0.16827 0.17144 0.17465 0.17789 0.18116 0.18447 0.18782 0.1912 0.19462 0.19807 0.20156 0.20508 0.20864 0.21223 0.21586 0.21953 0.22323 0.22697 0.23074 0.23455 0.2384 0.24228 0.2462 0.25016 0.25415 0.25818 0.26225 0.26636 0.2705 0.27468 0.27889 0.28315 0.28744 0.29177 0.29614 0.30054 0.30499 0.30947 0.31399 0.31855 0.32314 0.32778 0.33245 0.33716 0.34191 0.3467 0.35153 0.3564 0.36131 0.36625 0.37124 0.37626 0.38133 0.38643 0.39157 0.39676 0.40198 0.40724 0.41254 0.41789 0.42327 0.42869 0.43415 0.43966 0.4452 0.45079 0.45641 0.46208 0.46778 0.47353 0.47932 0.48515 0.49102 0.49693 0.50289 0.50888 0.51492 0.521 0.52712 0.53328 0.53948 0.54572 0.55201 0.55834 0.56471 0.57112 0.57758 0.58408 0.59062 0.5972 0.60383 0.6105 0.61721 0.62396 0.63076 0.6376 0.64448 0.65141 0.65837 0.66539 0.67244 0.67954 0.68669 0.69387 0.7011 0.70838 0.71569 0.72306 0.73046 0.73791 0.7454 0.75294 0.76052 0.76815 0.77582 0.78354 0.7913 0.7991 0.80695 0.81485 0.82279 0.83077 0.8388 0.84687 0.85499 0.86316 0.87137 0.87962 0.88792 0.89627 0.90466 0.9131 0.92158 0.93011 0.93869 0.94731 0.95597 0.96469 0.97345 0.98225 0.9911 1;

  $adjusted-rgb: ();
  @each $value in $rgb {
    $adjusted-rgb:
      append(
        $adjusted-rgb,
        nth($l-precompiled, $value + 1)
      );
  } $rgb: $adjusted-rgb;

  $l: 0.2126 * nth($rgb,1) + 0.7152 * nth($rgb,2) + 0.0722 * nth($rgb,3);

  @return $l * 100;
}

.test {
  out: luminance(hsl(0,0,50));
  out: luminance(rgb(128,128,128));
  out: luma-ln(hsl(0,0,50));
  out: luma-ln(rgb(128,128,128));
}

SassMeister-output.css

.test {
  out: 21.586;
  out: 21.586;
  out: 50.19608;
  out: 50.19608;
}