I am going to assume you are talking about the Advantech Touch Panel PC that is a Z500 based (Menlow) system.
I am aslo going to assume that you are talking about the LCD backlight brightness, not other color control methods for adjusting brightness.
There are a couple of problems that we find with backlight brightness control:
1. The Z500 has a couple of standard ways the backlight brightness can be controlled built in to it. Some manufacturers decide to implement their own way of controlling the backlight (based on unique LCD requirements or by not understanding the capabilities of the chipset). When they do that, there is no way for the driver to control the backlight beyond off and on. Sometimes the manufacturer includes a way for you to set or control that function in that case, but usually not. Worse, sometimes they just do not wire up the control at all. I am not intimatly familiar with that particular Advantech platform (but I would like to be because it sounds really cool). You would need to contact Advantch to ask them how they expect backlight brightness to be controlled.
2. When the manufacturer DOES implement the hardware following our recommendations, sometimes there is confusion on the graphics driver settings on the particular method they use. If you play with the IEGD CED utility for the configuration of the driver, you will see in the LVDS section a number of configuration options for the backlight. If you have installed a different version of the driver from what Advantech originally provided, you may be using our driver defaults which may be different from the way Advantech implemented their backlight control. In this case the driver can turn on the backlight full bright, but the brightness control will have no affect. Again, this needs to be asked of Advantech of how they configure the driver for their platform.
Once that is all set up properly, the driver should be able to control backlight brightness through the standard Windows interfaces, or directly with the IEGD User Interface as shown with the example code we provide with the driver for the IEGD Gui interface to the driver (our special ESCAPE interface into the driver).
Hope this helps...