if ([eqR:kW/m2] > [SEP:kW/m2]) return NULL;
if (Scientific::equals([eqR:kW/m2], [SEP:kW/m2])) return [DP:m] / 2;
$d = [DP:m];
$r = [DP:m] / 2;
$L = [Hf:m] / $r;
$theta = deg2rad([fTA:o]);
$steps = 100;
$delta = M_PI_2 / $steps;
$d0 = $r;
$qR0 = [SEP:kW/m2];
while (TRUE) {
  $Ta = min(2.02 * pow([Pw:Pa] * ($d - $r), -0.09), 1);
  $X = $d / $r;
  $phi = asin(1 / $X);
  $B1 = sqrt((pow($X, 2) - 1) * ((1 + cos(2 * $theta)) / 2) + (1 - 1 / pow($X, 2)) * ((1 - cos(2 * $theta)) / 2));
  $A1 = 1 / $B1 * (atan2($L - ($X - 1 / $X) * sin($theta), $B1) + atan2(($X - 1 / $X) * sin($theta), $B1));
  $A2 = 0;
  $A3 = 0;
  $A4 = 0;
  for ($i = 0; $i <= $steps; $i++) {
    $v = $i * $delta;
    $a2 = sin($v) / ((1 + pow($L, 2) + pow($X, 2) - 2 * $X * $L * sin($theta)) + 2 * ($L * sin($theta) - $X) * sin($v));
    $a3 = ($X * sin($v) - 1) / (1 + pow($X, 2) - 2 * $X * sin($v));
    $a4 = ($X * sin($v) - 1) / ((1 + pow($L, 2) + pow($X, 2) - 2 * $L * sin($theta)) + 2 * ($L * sin($theta) - $X) * sin($v));
    if ($i) {
      $A2 += $delta * ($a2 + $b2) / 2;
      $A3 += $delta * ($a3 + $b3) / 2;
      $A4 += $delta * ($a4 + $b4) / 2;
    }
    $b2 = $a2;
    $b3 = $a3;
    $b4 = $a4;
  }
  // Horizontal view factor
  $Fh = 1 / M_PI * (sin($theta) * ($A1 * cos($phi) + $L * $A2) + $A3 - $A4);
  // Vertical view factor
  $Fv = 1 / M_PI * (cos($theta) * ($A1 * cos($phi) + $L * $A2));
  // Overall view factor
  $F = min(sqrt(pow($Fh, 2) + pow($Fv, 2)), 1);
  // Radiation intensity at the distance
  $qR = [SEP:kW/m2] * $F * $Ta;
  if (abs($qR - [eqR:kW/m2]) < 0.01) return $d;
  $d1 = $d0 + ($d - $d0) / ($qR - $qR0) * ([eqR:kW/m2] - $qR0);      
  if ($qR > [eqR:kW/m2]) {
    $d0 = $d;
    $qR0 = $qR;
  }
  $d = $d1;
}