code: public static Vector3d createDirectionRad(double theta, double phi) {
          final double f = TrigMath.sin(phi);
          return new Vector3d(f * TrigMath.cos(theta), f * TrigMath.sin(theta), TrigMath.cos(phi));
      }
docs: Gets the direction vector of a certain theta and phi in radians. This uses the standard math spherical coordinate system.
  
  @param theta in radians
  @param phi in radians
  @return the direction vector