Dear Thanos,

What you see visually in reflection - it can be optical gap at some angle of incidence or/and of some higher order. At the normal incidence, at which I suppose you are measuring the transmittance, the gap may be red shifted or even may disappear if the observing gap is not omnidirectional (i.e. full).

If you give me the average radius of voids in your unversed opal and the refractive index (or the dielectric constant) of your material, I can simulate the spectrum for you.

Thank you, Gazi.

We have a refractive index of n=1.54 and a void diameter of d=300 nm.

My simulation shows an optical gap centered at about 562 nm at 0 degree of the incidence angle. Another my guess is that the stray (or scattered) light may penetrate at the part of your sample which is not ideal or at the edges. In this case you could use a pinhole to chose the exact position on your specimen for the measurement.

(Edited) My advise may be not relevant in the case of UV-VIS spectrometer.

How thick (and big) is your specimen?

NaCl inverse opals, spread fragments of fiew square mm each, on a 1 square cm.

thickness of 20 micrometers each fragment.

No bandgap at 562 nm in UV VIs transmitance

Perfect inverse opal structure to SEM investigation

The dimensions are given by my colleague above ^

Thickness 20 \mu m is quite thick and simulation gives the gap at about 560 nm, with gap width about 50 nm and the center gap transmittance of about 3.6e-9 , which is quite dip (~ - 85 dB); My only explanation is - may be sensitivity of UV-VIS spectrometer is not good enough because too much light is going to the detector not through opal structure but from elsewhere.

Try a system with a pinhole anyway: you calibrate spectrometer without the sample but with a system where the light goes through a small (~1 mm^2) pinhole , then you put the opal sample at the pinhole and measure.

Thank you, Gazi, we will try with this setup and see.