Thank you very much for the information Booxtor.
It is clear that technology in its first generation frontligth adds fragility and need to be very careful with the screen treatment.
This is true with all models that have passed through my hands: Paperwhite, Firefly and Frontlight Cybook Odyssey HD and a single glance at Glo Kobo.
The second question is whether the filter produces some opacity diffuser intervention affecting the native contrast of the screen.
In the first three models, the sacrifice is small contrast with the light off.
The Kobo Glow, I've only seen it once and I could barely taste it loses enough contrast because of the diffusing layer. You can tell at a glance.
According to review some specialized, low to 10:1.
With the light on, something is lost in the first three, compared to the native contrast Pearl HD: 12:1.
The least, is the Cybook Odyssey HD Frontlight whose measured contrast microscopy is 11.5:1.
Undoubtedly, the Cybook is the best I've seen illuminated display.
Also the fastest.
But its format support is very limited. And the possibilities of setting very poor reading.
For that reason, I see no good idea to add a protective filter to such readers.
We speak of a first generation.
Hopefully the following improvements.
Anyway, the front light is still a good idea and a benefit for the reader.
Valuing pros and cons, my choice was decanted into the Firefly, by his superior format support and overall performance.