POV-Ray

I have installed POV-Ray 3.7 beta 34 on Windows 7
The setting ‘Output_Alpha=on’ doesn’t work as documented. With this setting the Background appear black and the scene is transparent.

Check with this Code:

camera {
   location  <3, 3, -3>
   direction <0, 0, 2.9>
   look_at   <0, 0, 0>
   right 1.0*x
}
light_source { < 3, 3, -3> color red 1 green 1 blue 1 }
sphere 
{
   <0,0,0> 0.8
   pigment {color rgb<1,1,0>}
   finish
   {
      ambient 0.2
      diffuse 0.8
   }
}

and with this ini file:

Input_File_Name=C:\Users\dfv_rei1\AppData\Local\Temp\Cuadrigula\PreviewObject.pov
Output_File_Name=C:\Users\dfv_rei1\AppData\Local\Temp\Cuadrigula\PreviewObject.tga
Output_File_Type=t
Output_Alpha=on
Bits_Per_Color=24
+W121 +H121
+a0.3
+q11
+a