Turnoff the screen backlight without letting the device to go into sleep in Windows 8

go ro control panel > power options > Choose what closing the lid does (on the left side) and set Nothing for closing the lid
screenshot

it doesn't work. then  display turns off, it goes to sleep. I need, that backlight have to be turned off but I could work with device via RDP.

if you set to do Nothing when close a lid then display backlight does black when you close the lid but computer is still running. Set that action for laptop running on battery and plugged in as well.
I have right now set like that (laptop running with closed lid connected to external display) and it works perfectly

it is tablet, it does not have lid.

sorry, I thought you have laptop - my bad.
then try in power plan to set to go to sleep after let say 1 minute but standby or hibernation set to Never.

Simply look at the power options. Turn off display after x minutes is configurable.

as you see when I change in advanced settings turn off display after x min, also changes automaticly in plan settings Put the computer to sleep: x min.  and vice versa. I tried to create the program with C#:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;

namespace OnOffMonitor
{
    class Program
    {

        [DllImport("user32.dll")]
        static extern IntPtr SendMessage(int hWnd, int Msg, int wParam, int lParam);

        enum MonitorState
        {
            ON = -1,
            OFF = 2,
            STANDBY = 1
        }
        static void Main (string[] args)
        {
           
            SetMonitorState(MonitorState.OFF);
            
        }
       static private void SetMonitorState(MonitorState state)
        {
            SendMessage(0xFFFF, 0x112, 0xF170, (int)state);
        } 
    }
}

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but it goes to sleep too. I tried with nircmd (http://www.nirsoft.net/utils/nircmd.html) and it is the same problem.

Interesting. Normal computers and laptops would NOT keep those in sync. I wonder why your tablet would. Could it be that there's some 3rd party power management software in use? If so, uninstal t for a test. That could also mean some power saving software from your tablet's maker.

maybe it is possible to write some kind of script, what turns off backlight?

Did you do as suggested and checked for power management software? Feedback is important... :)

About the script: you already mentioned nircmd - what did you do with it? If nircmd cannot do it, no one can :)