Vista - SpotLight in WPFcann't form a circle light effect ,Why?

Hi! Guys,
I have a problem. When I use a spotlight in the 3-D scene to light a side
plane of a cube ,the result is not a circle light effect.
<Viewport3D ClipToBounds="True" Width="150" Height="150" Canvas.Left="0"
Canvas.Top="10">

<!-- Defines the camera used to view the 3D object. -->
<Viewport3D.Camera>
<PerspectiveCamera Position="0,0,2" LookDirection="0,0,-1"
FieldOfView="60" />
</Viewport3D.Camera>

<!-- The ModelVisual3D children contain the 3D models -->
<Viewport3D.Children>
<!-- A SpotLight is used to light the scene. The InnerConeAngle
and OuterConeAngle are used
to control the size of the light cone created by the
SpotLight. The Direction and Position
properties determine where the SpotLight is pointing in the
scene. In this example, the Position
of the SpotLight is set so that the SpotLight is only
illuminating the upper right-hand corner
of the 3D object. -->
<ModelVisual3D>
<ModelVisual3D.Content>
<SpotLight x:Name="mySpotLight" InnerConeAngle="15.0"
OuterConeAngle="22.0" Color="#FFFFFF" Direction="0,0,-1"
Position="0,0,5" Range="30"/>
</ModelVisual3D.Content>
</ModelVisual3D>
<ModelVisual3D>
<ModelVisual3D.Content>
<GeometryModel3D>

<!-- The geometry specifes the shape of the 3D plane. In
this sample, a flat sheet is created. -->
<GeometryModel3D.Geometry>
<MeshGeometry3D
TriangleIndices="0,1,2 3,4,5 "
Normals="0,0,1 0,0,1 0,0,1 0,0,1 0,0,1 0,0,1 "
TextureCoordinates="0,0 1,0 1,1 1,1 0,1 0,0 "
Positions="-0.5,-0.5,0.5 0.5,-0.5,0.5 0.5,0.5,0.5
0.5,0.5,0.5 -0.5,0.5,0.5 -0.5,-0.5,0.5 " />
</GeometryModel3D.Geometry>
<GeometryModel3D.Material>
<MaterialGroup>
<DiffuseMaterial>
<DiffuseMaterial.Brush> <SolidColorBrush
Color="Blue" />
</DiffuseMaterial.Brush>
</DiffuseMaterial>
</MaterialGroup>
</GeometryModel3D.Material>
</GeometryModel3D>
</ModelVisual3D.Content>
</ModelVisual3D>
</Viewport3D.Children>

</Viewport3D>