LUT and invisible marks

Application

The customer wants to detect an invisible UV mark by using one of our LUTxU models (LUT with UV LED)
invisible means: You cannot see it under daylight condition, only visible under UV illumination

Which invisible colors can be used?

Only invisible blue or green can be used unconsidered. 

We must take care in case of invisible red for all of our LUTs. 

Visible red fluorescence is also no problem. 

Why do we have a problem with invisible red?

invisible red has a longer decay time compared with invisible blue or green.

Technical background

all of our LUT sensors are safe against ambient light in the same way as other optical sensors.
the sensor is operating with pulse modulation, the receiver is synchronized with the sender.

the receiver opens for a short time frame during the sender is flashing.
when there is a signal detection from the fluorescent target, the Q output reacts after 2 cycles of detection.
That is called "2-Bit evaluation" which makes the sensor safe against ambient light and more robust in terms of EMC.

Physical background of fluorescent targets

The receiving signal from fluorescent target after flashing with UV has a short slope delay as you can see in the below curve.

example for fluorescent target as we have it with invisible red

the target can light up very bright but the received signal amplitude could be too low for reliable detection!
That is because the falling slope of signal is cut.

targets with longer decay time are called "phosphorescent"

example for fluorescent target as we have it usually

The full amplitude is effective in the reading window, sensor is detecting reliably.