Pulsed Illumination Synchronization for Global Reset Rolling Shutter Imagers
Global Reset rolling shutter imagers eliminate motion blur when pulsed light fires strictly within the dark global integration window under controlled ambient light.
Thermal Droop
Efficiency Reduction
The reduction in the light emission efficiency of a semiconductor light source as its junction temperature rises during operation represents a primary challenge in maintaining stable illumination levels for precision optical measurements. This phenomenon, known as thermal droop, occurs in both light-emitting diodes and laser diodes when continuous electrical current generates internal heat. It causes the optical power output to decline even when the input current is held perfectly constant.
It requires careful management in applications that depend on a stable light source.
Semiconductor Behavior
The underlying physical cause of this behavior lies in the increased probability of non-radiative recombination at higher temperatures. As thermal energy within the crystal lattice rises, charge carriers are more likely to recombine via phonon-assisted processes rather than emitting photons. This shift in recombination pathways reduces the internal quantum efficiency of the diode.
It is most pronounced in high-power devices that operate at high current densities.
Metrological Consequence
Precision radiometry is directly affected by this unstable emission profile. If an instrument uses an uncompensated LED as a reference light source, the thermal droop will introduce a time-dependent drift in the calibration baseline as the source warms up. This drift can be misinterpreted as a change in the sample being measured.
To prevent this, system designers must either stabilize the light source temperature or actively monitor the optical output.
System Compensation
Active stabilization is achieved using thermoelectric coolers and closed-loop feedback circuits. A photodiode monitors the actual light output and adjusts the drive current to maintain a constant intensity. By calibrating this feedback loop, the system can neutralize the effects of the thermal change.
This control ensures long-term consistency in automated inspection systems.