opacity-photoshop-script.jsx

/*
  Script to gradualy increase opacity by layer and set blendMode in Photoshop 
  Usage:
  Save script
  In Photoshop assuming file is open with multiple layers
  File -> Scripts -> Browse... 
  Select opacity-photoshop-script.jsx from file system
*/

var numLayers = app.activeDocument.layers.length; // total number of layers
var step = Math.round(100 / numLayers); // opacity step
var currentVal = step;

for (var x = 0; x < numLayers; x++) {
  var layer = app.activeDocument.layers[x];
  layer.blendMode = BlendMode.LIGHTEN; // set blending mode to Lighten
  if (currentVal < 100) {
    layer.opacity = currentVal;
  }
  if (numLayers === x-1) {
    layer.opacity = 100; // make sure last one is always 100 opacity
  }
  currentVal += step;
}