I've bought a (cheap) polytunnel with a covering, with a thread that forms roughly 1cm squares cross hatched to presumably give it some strength. Also the plastic is coloured green. I installed it in March and so far it appears that there just isn't enough light coming through. Tomatoes, in particular, are very leggy and forming flowers very high up the stem. So I'm thinking of replacing the plastic with regular plain polytunnel covering. I'm not 100% certain that this type of covering is the problem as the polytunnel is rather sheltered. The door faces south but there's a fence on one long side and the house on the other side. It's common to see sellers claiming that plain polytunnel coveting lets in 90% of the light. Does anyone know what that figure is for the cross hatched covering?
There's nothing like direct local observation. The reasons for diminished light intensity might be thickness of hatching material, presence of shading objects, reflected light and so on which means that every location can be different. Light meters are available reasonably easily and cheaply, and there are apps that install in smart phones that use the camera sensors built into the camera to give similar readings. Just take the light reading outside the greenhouse, then the same inside the greenhouse and you have your answer. Take multiple observations in different places and you soon have a map of where the light is best and at what time of day.