A really short history about the evolution of photosynthesis instruments:
In the middle of the last century scientist used first a closed system approach where the air circulates between the infrared gas analyzer (IRGA) and the leaf chamber (cuvette).
Minimal instrument set-up, only one absolute IRGA for CO2 and H2O is needed.
The microclimate (T, RH, CO2, H2O) in the cuvette changes within minutes due to the leaf activity. Therefore it is not possible to properly perform longer measurements like light- or CO2- curves.
Surprisingly even nowadays some manufacturers try to build instruments with only one absolute CO2 IRGA and capacitive humidity sensors. We would call this a "hybrid open system" configuration.
Cheap to manufacture and lightweight.
Need to constantly switch the IRGA between the incoming and outgoing air stream and therefore the true differential CO2 concentration is never known.
Capacitive humidity sensor do not have enough accuracy and resolution for photosynthesis measurements. Also they respond very slow (∼ minutes) and are prone to drift.
Can not deliver research grade results.
With the availability of differential IRGAs and electronics flow meters open system where introduced where the air only flows once through the system.
Allows for full CO2 and humidity control of the cuvette. The microclima stays stable.
Needs more instrumental effort: a differential IRGA and a flowmeter.
The response time of the measurement depends on the flow rate which is a problem when measuring low photosynthesis rates.
To overcome all the above mentioned disadvantages LI-COR came up with a unique system where the IRGA is part of cuvette.
The measurement is flow rate independent.
No tubes or filters between cuvette and IRGA are needed.
Needs an IRGA insensitive to dust (which is solved by LI-COR gas analyzers).