Glossary for detailled explanation - take your pick:
Optical filters only let pass light waves in certain ranges and block other wave ranges. Electric filters only let pass signals in certain frequency bands and are blocking others.
A flush mounted sensor can be "mounted flush into a material" when it can be surrounded by this material up to the level of its active surface without affecting its key parameters.
To achieve a light spot that is as small as possible when using an optical device, the light beam of the sensor is focused through lenses. Because of focusing and the resulting spot, the sensors are suitable to detect smallest parts. Frequently, focusing is used for light sensors or reflection light barriers.
FOR OPTICAL DEVICES The functional reserve is a measure for excessive radiated power that strikes the light ingress surface and is evaluated by the light receiver. Because of pollution, change of the object's reflective factor and aging of the sensor diode, the functional reserve can decrease over the course of time so that an exact working cannot be guaranteed anymore. Because of this, a part of the devices has a second LED (green) which lights when approx. 80 % of the available range is used. Alternatively, at other devices the yellow LED alerts when there is insufficient functional reserve. At devices with functional reserve output, this signal is switched to one of the outputs. In this way, an insufficient reliable status can be detected in time.