As it’s optical season I finally got round to testing all the filters I have gathered off of AliExpress!
I’ve tried many of thee filters, they come in two varieties, Long Pass or Band Pass
So far I’ve been using a Long Pass filter with black colour (4th down) in front of my 4x Photodiode receive head, but after the tests today I’ll be switching over to a Band Pass with silver coating (3rd down)
Here’s the lowdown based on my measurements using an 850nm led source and cheap webcam spectrometer.
| Description | Pass | Percent Transmission | Link |
| 7mm black round | LP | 90.33% | https://www.aliexpress.com/item/1005005884038415 |
| 8mm silver round | BP | 90.26% | |
| 30mm sillver round | BP | 88.32% | https://www.aliexpress.com/item/1005006133721297 |
| 24mm black round | LP | 80.63% | https://www.aliexpress.com/item/4000847847616 |
| 17mm square silver | BP | 78.42% | https://www.aliexpress.com/item/4001235176893 |
| 8mm square silver | BP | 70.06% | https://www.aliexpress.com/item/4001235176893 |
| 52mm long pass 850 | LP | 67.78% |
The difference between the original LP and the new BP filter i am using is just over 1dB in received SNR. The difference of nearly 10% calculates to +1dB too so it roughly adds up 🙂
The 7mm round ones will suit the Rx head with the small lenses, these filter will fit between the photodiode and the lens.
