# Filter arrangement LP and bypass

I am currently looking at an optics systems that channels fluorescence and collects the photons into a PMT. Before, photons can enter a PMT, they pass through a $500\, \mathrm{nm}$ Long Pass (LP) filter and then a $525/50\, \mathrm{nm}$ Band Pass (BP) filter (filters are right next to each other).

So the LP filter transmits a wavelength greater than $500\, \mathrm{nm}$ and the BP filter transmits a narrow range $475$ to $575\, \mathrm{nm}$. My question is how can the BP filter transmit light towards the shorter range ($\sim475$) if the LP filter absorbs light less than $520$?

Just to be clear, is it true a photon will pass through the LP filter if it has an energy level $E=hf=hc/\lambda$ where $\lambda>500\, \text{nm}$ and will not pass if $\lambda < 500\, \text{nm}$?

• You would have to look at the filter specs and see if that stack will in fact do what you want it to. Note that filters do not have hard edges, but just how broad they are is given by the filter manufacturer. – Jon Custer Jun 25 '18 at 15:37
• Thank you. I would probably get a fine of < 10^3 thousand dollars if I released the specs. – hwhorf Jun 29 '18 at 2:34

## 2 Answers

Your question is: My question is how can the BP filter transmit light towards the shorter range (∼475) if the LP filter absorbs light less than 520?

The BP filter can not transmit the light you might be interested in, because it does not get through the LP filter. Stated another way, with the LP filter in place, there is no 475 nm light that could be transmitted by the BP filter.

Filters stacked like this are multiplicative in transmission. If, at a given wavelength, the transmission of the two filters is T1 and T2, then the total transmission will be T1*T2.

• Yes great question. The bypass (seen as) 525/50 allows light to pass in the range of 500-550. This means /50 represents total width. – hwhorf Jun 29 '18 at 2:32

Looking at the parameters provided, I get the impression that inherited the setup from an existing system. It is obvious that, if the LP filter passes everything that is passed by the BP filter, then the LP filter would not be necessary. So the reason for having both these filters seem to be that they combine to give a narrower BP filter; one that only allows wavelength between 500 nm and 575 nm. Could that be the reason?

• It's cause the laser is blue. Sorry for the late reply. – hwhorf Jun 29 '18 at 2:27