You triggered a distant memory. It appears to have been a genuine one :-) -

TTL (through the lens) metering was invented by Olympus in 1975. My memory says that they measured light reflected from the film surface to determine the light level and that they obtained a large number of film samples from many countries to arrive at a typical reflectance value to assign to the film. Olympus's history page here says that I recall correctly :-). They say -

• OM-2 This camera went on sale in 1975. It features an automatic exposure (AE) system with an aperture-preferred shutter. The OM-2 also had the world's first TTL direct metering system, which measures light reflected off the surface of the film. This allowed exposure control during shooting, and automatic TTL strobe adjustment using a specially designed strobe. To develop the TTL direct metering technology, Olympus collected 35mm film from throughout the world and measured the reflection ratios for each roll. The results were used to determine the density of printing on the shutter curtain.

That's a little concentrated: they used the derived value of film reflectance to set a value to use when the shutter was open but they also printed the shutter with a pattern with the same reflectance level so that when the shutter was closed they could use "preflash" to reflect light onto the sensor via the shutter surface. Very clever.