I'm designing a small heart rate sensor using green smd LEDs and a photodiode. The photodiode needs to measure the reflected light, and not direct light from the LEDs. The LEDs are near the perimeter of the photodiode, so I'm trying to figure out some sort of thin wall to block their light from hitting the photodiode. The LEDs have a relatively wide viewing angle (110˚) and are ~1mm tall, so ideally something that is about 2mm tall.
I'm wondering if anyone has suggestions for some type of wall or component I could place between each LED and photodiode to block light?