What this phototransistor does in a design
The ROHM RPI-0451E is a transmissive phototransistor in a surface-mount package — the emitter and detector face each other across a 0.177 mm gap, so an opaque object passing through the slot breaks the beam and switches the output. The phototransistor output type means the collector current rises with incident IR; at the rated switching current of 35 mA, the output can drive a logic input or a small relay through a pull-up resistor. Response time is 10 µs — fast enough to catch edges on a spinning encoder disc or a part moving past a sensor at moderate line speed. The -25°C to 85°C operating range covers most indoor industrial and consumer enclosures; the lower limit is the main constraint for outdoor cold-soak applications.
ROHM lists the RPI-0451E as Active.