Arduino controlled light dimmer software I found this software:

/*

AC Voltage dimmer with Zero cross detection
Author: Charith Fernanado &lt;a href="http://www.inmojo.com"&gt;  http://www.inmojo.com

&lt;/a&gt;
Adapted by DIY_bloke
License: Creative Commons Attribution Share-Alike 3.0 License.
Attach the Zero cross pin of the module to Arduino External Interrupt pin
Select the correct Interrupt # from the below table 
(the Pin numbers are digital pins, NOT physical pins: 
digital pin 2 [INT0]=physical pin 4 and digital pin 3 [INT1]= physical pin 5)
check: &lt;a href="http://arduino.cc/en/Reference/attachInterrupt"&gt;  http://www.inmojo.com

&lt;/a&gt;

Pin    |  Interrrupt # | Arduino Platform
---------------------------------------
2      |  0            |  All -But it is INT1 on the Leonardo
3      |  1            |  All -But it is INT0 on the Leonardo
18     |  5            |  Arduino Mega Only
19     |  4            |  Arduino Mega Only
20     |  3            |  Arduino Mega Only
21     |  2            |  Arduino Mega Only
0      |  0            |  Leonardo
1      |  3            |  Leonardo
7      |  4            |  Leonardo
The Arduino Due has no standard interrupt pins as an iterrupt can be attached to almosty any pin. 

In the program pin 2 is chosen
*/
int AC_LOAD = 3;    // Output to Opto Triac pin
int dimming = 128;  // Dimming level (0-128)  0 = ON, 128 = OFF

void setup()
{
  pinMode(AC_LOAD, OUTPUT);// Set AC Load pin as output
  attachInterrupt(0, zero_crosss_int, RISING);  // Choose the zero cross interrupt # from the table above
}

//the interrupt function must take no parameters and return nothing
void zero_crosss_int()  //function to be fired at the zero crossing to dim the light
{
  // Firing angle calculation : 1 full 50Hz wave =1/50=20ms 
  // Every zerocrossing thus: (50Hz)-&gt; 10ms (1/2 Cycle) 
  // For 60Hz =&gt; 8.33ms (10.000/120)
  // 10ms=10000us
  // (10000us - 10us) / 128 = 75 (Approx) For 60Hz =&gt;65

  int dimtime = (75*dimming);    // For 60Hz =&gt;65    
  delayMicroseconds(dimtime);    // Wait till firing the TRIAC      
  digitalWrite(AC_LOAD, HIGH);   // Fire the TRIAC
  delayMicroseconds(10);         // triac On propogation delay 
                 // (for 60Hz use 8.33) Some Triacs need a longer period
  digitalWrite(AC_LOAD, LOW);    // No longer trigger the TRIAC (the next zero crossing will swith it off) TRIAC
}

void loop()  {
  for (int i=5; i &lt;= 128; i++){
    dimming=i;
    delay(10);
   }
}


But don't uderstand this part:


  (10000us - 10us) / 128 = 75 (Approx) For 60Hz =>65


I know 10000us is 10ms, and we need to wait less then 10ms to dim light.

But don't understand why here we has -10us and /128.

Anyone know why is that?
-10uS because of the 10uS delay at the end of the loop, and /128 because he has decided there will be 128 possible dimming levels.