Regarding Pepperl+Fuchs, Inc. retroreflective sensors, please see below:
RLK31-54/31/115 is “Dark ON” input
RLK31-54/25/31/115 is “Light ON” input, and is currently a special-order part (non-catalog request)
Datasheets:
Summary
2046-RLK31-54/31/115-ND
183064
2046-RLK31-54/31/115
Retroreflective