Looking at the light pulses produced from that B&D driver on an oscilloscope shows that there is no light emitted between the individual pulses.
Those are not actually sawtooth waves. They are an aliasing artifact of how the sample rate of the light sensor interacts with the “PWM” frequency. They mix together and produce what amounts to a beat note that shows up as a low frequency signal.
The light sensor is sampling at around 20 Hz (120 msecs * 31 / 69). 60 Hz PWM or AC ripple in the light shows up quite well in the resulting signal. The plot in Any interest in a LED/Battery analyzer device? shows the effect with an AC line powered LED bulb.