chopper window adjusting?

From: Tao
email: twei@mail.chem.tamu.edu
Model: Model 651-1
Date: 12/08/03

Comments

have a light chopper(old Model 650), i believe it's the former model of 651-1. i use the frequency of 2500Hz, so each period should be 0.4 ms. since i saw on the chopper plate, the size of hole is about same with the distance between two holes, i think in each period, the time light can pass chopper and the time light is blocked by the chopper should be about same,(say 0.2 ms for each) but if i check this by oscilloscope, i found out the "open" time is much smaller than the "close" time.(like 20% to 80%) i just want to ask is this normal, or there is something wrong with my chopper? how can i change it to, like half and half?

the situation i have now;

|<--------------->| __ 0.4 ms __ | | | | ____| |______________| |__________

i thought it should be like this;

________ ________ | | | | ____| |________| |___