ALS sensitivity/performance can depend on multiple factors, ensuring an optimized optical window is essential. If the sensor is placed under plastic or cover glass then the opaque/darkness of the glass/plastic must not interfere with the light reaching the sensor. If there is material covering the aperture it should still transmit enough visible or IR light to the sensor. Common material used for optical glass is acrylic/plexiglass or polycarbonate. Typically around 7-10% transmission in visible light is an acceptable level.