# 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. Jun 25, 2018 at 15:37
• Thank you. I would probably get a fine of < 10^3 thousand dollars if I released the specs. Jun 29, 2018 at 2:34