The Casio F-91W

A digital classic

When I was first starting this blog, I had begun to write a post detailing the history and engineering behind the Vostok Amphibia, a Russian automatic dive watch. Unfortunately, that post didn’t see the light of day as I felt that it didn’t suit the theme of the blog. However, in this post I’ll be returning back to that theme.

It seems fitting, then, that the watch in question is one as ubiquitous and iconic as the legendary Casio F-91W. Launched in 1991, its simplicity and durability, coupled with its extremely low price, has allowed it to prevail as a best-seller.

I was drawn to its iconic status and low price, and decided to buy my own F-91W recently.

The watch’s case is refreshingly slim compared to my Vostok (only 8.5mm), and is very comfortable on the wrist. The three button control is intuitive, with each press reinforced by a short beep of a piezoelectric buzzer. Turning it over reveals a stainless steel caseback held in place by four Phillips head screws. The plastic buckle is signed with the word CASIO.

While I enjoyed wearing the watch stock, the real reason I bought the watch was to take it apart and modify it. Luckily, this is incredibly easy to do.

First, I removed the four screws holding down the caseback. This revealed the rear of the watch’s module. The rubber gasket around the edge of the housing is enough to give the watch a 30m water rating. The two metal prongs protruding from the shielding are, I assume, to contact the piezoelectric buzzer which is permanently stuck to the inside of the caseback. I’m not sure what the square plastic sticker could be for other than to insulate the caseback from the shielding, so I kept it in place.

I then used a razor blade to carefully pry the module out of the case. It’s not held in by adhesive, so it’s pretty easy to get out.

Flipping it over revealed the LCD. Again using the razor blade, I began gently peeling off the polarizing filter attached to the display. Once removed, the display appeared to be switched off, even though it was still running just fine. Rotating the filter 90° over the display reveals the digits once more, but inverted. This is the effect I will be utilising in the mod.

I’ll take a moment to explain what a polarising filter is. Light is an electromagnetic wave, which can be visualised as oscillating particles moving along a single plane. Visible light is unpolarised. This is because it is made up of many waves travelling along the same plane, but rotated around different angles. The polarising filter blocks all angles of light apart from one through multiple tiny slits. This creates plane-polarised light, which is used in LCDs.

Since the old polarising filter is clearly the wrong shape to cover the screen at a 90° angle, I had to find another which I could resize to fit. Luckily, modern 3D glass lenses are polarising filters themselves, so after some experimentation and cutting, I ended up with an inverse filter for my F91W. It was then a case of placing it onto the underside of the crystal and popping the module back in.

This could be done better, using an adhesive-backed filter, as it would be perfectly applied to the display and provide better viewing angles, but I’m happy with the results I achieved.

By chance, the inverted display ended up with digits that are similar to the gold colour of the “F-91W” and “ALARM CHRONOGRAPH” when viewed in bright light, which actually looks pretty good.

Something else I tried was to add a NFC tag into the watch, but since I didn’t have one small enough to fit under the glass, I put it on the caseback. I think the metal from the caseback created enough interference that it couldn’t be read, so I don’t recommend trying that out.

From a practical point of view, the inverted display is harder to read in all conditions and certainly not as legible as stock, but I think it gives it some character and separates itself from the millions of other F91Ws out there.

Overall, I’m happy with how the display invert mod went and encourage you to try it out!

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