How to adjust the brightness of VGA monitor in Col VF61

We have a Col VF61 with Iris carrier board running angstrom. We have connected a VGA monitor with carrier board using DVI-VGA cable. We need to increase/decrease the brightness of the monitor.

By using the tools like xrandr (or similar tools), we can adjust the brightness but this seems not to be working in VF61. We have also tried changing the brightness in /sys/class/backlight/backlight/brightness but this also does not have any effect. Is there any other tool/driver available to adjust the brightness.?

How exactly is the brightness of your monitor adjustable? Or asked differently by what means can the target module influence the brightness of your monitor? Are you talking about a PWM connected backlight or what exactly is it? Maybe revealing the exact brand/model of your monitor plus how exactly it is hooked up (e.g. by what cables/adapters) would help as well.

I am using HP monitor connected to Iris carrier board using DVI to VGA cable

There is no standard brightness feature when using VGA. The monitor usually provides the brightness settings directly through a menu on the monitor. So I guess there is no way of adjusting the brightness directly from the Colibri VF61 computer module.
It might be possible, that your monitor supports some special brightness settings through some communication channels. But that’s not something we are experienced with.
The brightness settings you mentioned above are usually used to control the brightness of directly attached LCD panels with backlight control through usually PWM.