What causes a transmissive diffraction grating to be more effective than a reflective one? I was curious as to what would make a transmissive grating more effective than a reflective one. I have been searching but am having trouble finding an answer.
In literature, some semitransparent biological materials are iridescent under (oblique) transmitted light. However, the same materials, under epi illumination with a black background, no matter the angle, are non-iridescent.
I was wondering why this might be the case. In literature, biological iridescence is explained as being a result of periodic nanostructures in the material that act as diffraction gratings (e.g. peacock feathers).
But shouldn't diffraction gratings act the same way in reflection and transmission (if they're semitransparent rather than opaque)?
Some relevant information:

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*Many of these materials are not thin, so it can't be thin film interference causing the iridescence.

*Is it possible that diffraction gratings at interfaces act differently (only transmissive) from those that are not at an interface?

 A: The efficiency of a diffraction grating, whether transmissive or reflective, depends on several factors. One of them is the shape and depth of a grating.  The depth of the grating controls how much energy gets sent to each diffracted order.  The ideal depth for a transmissive grating depends on the refractive index of the material, and is in general different than the ideal depth for a reflective grating. The technical term used to describe the depth is blaze height.
The other factor that affects how a grating appears is what coating is on it. For example, gratings used in laboratory instruments such as spectrometers, are typically reflective gratings. The grooves in the grating are made by a molding process from a master grating, and then the gratings are overcoated with gold or aluminum. (The molding process is actually called replication; it's a bit more complicated than regular plastic molding).  This overcoating makes them much higher efficiency than they would be without the coating. So for your example of the biological material that is iridescent in transmission but not reflection, it may be that in reflection the diffraction efficiency is just so low as to not be easily visible.
====== New note added in response to comments below. ======
There are different processes that can produce these visual effects. I don't know what particular things you are looking at, but there may be a combination of effects. In the first part of my answer, I was talking about diffraction by a regular repeating structure, with the physical variation of the structure occuring along an axis perpendicular to the primary light direction.
However, multilayer thin films may also produce colorful effects. For example, gasoline spread on water produces a colorful effect. This is caused by the light reflections reinforcing themselves for only certain angles and wavelengths. The would generally be called "thin-film interference". The transmission or reflection of multilayer thin films can be predicted by theory. You can see more about iridescence in nature here:  Iridescence
Of course, one can have structures that combine both of these effects. A diffractive grating, used in transmission or reflection, can be used to direct light of certain wavelengths. The coating on the diffractive surface would be designed to help achieve the desired optical function.
