YS110 Calibration - Part 2
The theory behind calibration of my Quench controller was fairly simple: all I needed was to establish a quench delay interval for each setting on the strobe power control. A small table of 13 values was all I needed. Refer back to this post for a refresher of quench controller theory.
When I first started testing with an old YS50 I was not worried about having a correct calibration. As long as the strobe produced more light as I cranked the power setting up I was happy. At that time I was primarily focused on creating a stable control system. I had no concern that the power settings were correct Exposure Value steps, so I ignored proper calibration. I did want to be able to fire a minimum and a maximum flash. Through experimentation I was able to establish a minimum and maximum quench delay. For the minimum I slowly increased the delay between asserting X and asserting Q until I got the strobe to fire reliably. Too small of a delay and the strobe did not fire. For the maximum I kept increasing the delay until the strobe failed to turn on the green "TTL Okay" light. In reality the TTL Okay light simply indicates that the strobe did not perform a full dump. The strobe has no idea if the camera achieved proper TTL.
I then naively set up a simple linear power setting map from the minimum to the maximum quench delay interval. This seemed to work fine. The lowest power settings gave minimal light. The highest power setting caused a full dump of the strobe. These 2 extremes allowed me to test the edge case timing behaviours while I was creating the control logic. I was happy. And oblivious. Mostly because I did not know any better. My Linear Interpolation guess was wrong. Very wrong.
Now I had finally progressed to the point where I was ready to make my YS110 strobes work. It was finally time to worry about calibration. Based on my prior work I knew I would have to establish minimum and maximum quench delay intervals. Then I would worry about the stuff in between. By this time I was aware that my original linear interpolation scheme was incorrect. I had already come across some information regarding the discharge power curve of speedlights.
This discussion of Studio Flash versus Speedlights provides some insight into the behaviour of speedlights (underwater strobes are Speedlights). This discussion of Flash Discharge Curves provides an excellent set of discharge graphs at various power settings. A strobe quickly reaches maximum output and then decays over a very long time period. Two important concepts popped out for me:
- a full discharge is very long compared to just a few steps lower
- lowest power output is achieved before the strobe has reached the peak of its possible power curve
The next step was to figure out how to establish a valid calibration map for my YS110...