Oil End Sensor - Ricoh FT5540 Service Manual

12.5 OIL END SENSOR

Oil end detection is based on the difference between the index of refraction of air and the
index of refraction of silicone oil.
The index of refraction of silicone oil (1.40) is very near that of the sensor casing (1.49); so,
the oil-casing interface has a very large critical angle. As a result, when oil covers the sensor,
light from the LED is only refracted slightly at the boundary between the casing and the oil.
The phototransistor stays off.
However, the index of refraction of air is much lower (1.00). So, the critical angle for the
air-casing interface is much less than that of the oil-casing interface. Light from the LED strikes
the air-casing interface at an angle greater than the critical angle. So, when oil runs out, light
from the LED reflects instead of refracting and activates the phototransistor. This sends a
LOW signal to the main board, which lights the Add Oil indicator and stops copier operation.
The Start key turns red and displays Open Front Door and Add Oll on the guidance display.
2-83
31 August 1989