It appears that brightness cannot be adjusted on ThinkVision Monitors.

ThinkVision L150 (6636-Axx), L150p (6636-Hxx), L170 (6734-Axx), and L170p (6734-Hxx) TFT monitors

In order to achieve a more noticeable change in the brightness on these monitors, a combination of both brightness and contrast buttons can be used.

The panels used in some of the ThinkVision L150, L150p, L170, and L170p monitors measure close to 300 candelas per square metre (cd/m2), so they are very bright. The adjustment range for the TFTs used in these monitors is approximately 35% of the allowable range which a cold cathode fluorescent tube (CCFL) backlight can be reduced before it shuts down rather than getting dimmer. Generally, the human eye has an inability to detect changes in brightness above 200 candelas per square metre (cd/m2). Therefore, you may be unable to perceive a change in screen brightness if you use only the brightness control alone. However, by adjusting both the monitor brightness and contrast together, you can achieve a more noticeable effect on perceived screen brightness.