Place a Spotlight in the interior of the lamp

Create a spotlight, position it inside the lamp and parent it to the group's hierarchy. Create a simple scene to test-render the features of the spotlight.

Adjust the color of the light if you like, the cone angle, the dropoff and the penumbra; leave the intensity and decay alone for now. Also, keep shadows off for the time being.

In this example, I've placed the light behind the lamp head model in order to accurately depict the size of the cone of the light.

Select the spotlight and open the Attribute Editor. Select the spotlightShape node, and click on the texture icon next to Light Fog in the Light Effects section.

Notice that when you create the light fog, a cone appears with the spotlight. This describes the volume of fog within the spotlight. Although the edge of the fog lines up nicely with the rim of our lamp body, the fog begins behind the light and doesn't extend far enough. We'll adjust this using decay regions and intensity curves.

Turn Shadows On

At this point, you can turn Depth Map shadows on for this light. However, if you have positioned the spotlight behind the lamp body as I have in this example, the lamp will cast a shadow as well. Select the lamp surfaces, click on Windows-General Editors->Attribute Spreadsheet and turn Casts Shadows to Off in the render section of the spreadsheet.