Optical filtering

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Optical filtering

The cover glass has to allow through the IR light at 850nm emitted and received by the

proximity sensor. If the ALS functionality of the VL6180X is to be used, the cover glass also

has to allow through enough visible light to allow the lux level to be calculated.

2.1

Optical transmission

450 and 650 nm: 5 to 15% transmission (only required if ALS functionality is required)

Above 800nm:

>80% transmission

2.2

Potential issues

2.2.1

Cross talk

When using the VL6180X in ranging mode, the cross talk is defined as the signal from the

emitter reflecting off the cover glass and being sensed by the receiver. The VL6180X can

tolerate and compensate for a certain amount of cross talk, but this need to be minimised as

much as possible. The type of IR ink coating used on the cover glass can have a significant

effect on the amount of cross talk.

The glass surface should be as smooth as possible, with ideally a roughness value (Ra) of <

0.04.

Figure 2. Effect on ink used on cover glass

Example of rough ink - high cross talk

Example of smooth ink - low cross talk