UPDATE 9 December 2020: Included updated specs from Hedeco (Lime One) website.
Given the recent interest in the Lime One Kickstarter project, which is aiming to launch a new compact light meter for your hot (or cold) shoe into the market, I thought it would be interesting to provide a basic comparison of this vs. the slightly older Reveni light meter with a similar mission, as well as the Taiwanese competitor KEKS.
The use case, of course, is that these enable you to correctly use an older film camera where the light meter is broken, inaccurate or simply missing (many budget models came without a built-in meter).
There are other light meters like these on the market, but these are three recent takes that seem to offer reasonable quality, with the caveat that the Lime One is not released yet, so less is known about it, and usual Kickstarter disclaimers apply.
Note: The 35° metering angle corresponds approximately to a 50mm focal length, so the KEKS will be slightly better for wide angle, while the Reveni will better meter classic portrait focal lengths like 85mm. However, this should only have a big impact if you’re using colour positive, i.e. slide film (the rarer case these days). Also keep in mind that you can take several readings from across the scene to figure out the settings for a wider angle shot. With negative film, over-expose if in doubt!