How is light transmission measured?

Hemispherically — across light arriving from the whole sky rather than straight down. That matters because daylight is largely diffuse, and a perpendicular measurement produces a higher, flattering number. Two covers quoted on different bases cannot be compared directly.

Why does transmission matter more than other structural properties?

Because within a crop's usable range roughly a percent more light returns a percent more yield, and daylight is free. Transmission is therefore a ceiling on production expressed as a percentage of the structure's own design decisions.

Where is transmission lost?

In two places. Every structural member above the crop casts a shadow, which is why the Venlo roof uses many small ridges rather than a few large spans. And every covering absorbs and reflects a share, which is what anti-reflective coatings and low-iron glass are bought to reduce.

Can transmission be improved later?

Not meaningfully. Cleaning the glass and replacing a degraded plastic cover recover what was lost to fouling or ageing, but the transmission the structure was designed to is a fixed ceiling. Supplemental lighting adds photons rather than recovering daylight, and it costs energy to do so.

Greenhouse Light Transmission

Free, finite, and decided once

Daylight is the cheapest input you will ever have

It arrives every morning and costs nothing. The only question a greenhouse answers is how much of it survives the trip to the leaf.

And it matters more than almost anything else, because inside a crop’s usable range one percent more light is about one percent more yield.

So transmission is not a material property. It is a ceiling on production, written as a percentage.

It gets spent in two places

Steel. Every bar over the crop throws a shadow. That is the whole reason the Venlo roof is built from many small ridges instead of a few big spans — less structure standing between the sky and the plant.

Cover. Every material absorbs and reflects a share. Anti-reflective coating and low-iron glass exist to claw a little of that back, and on a big project a little is a lot.

Ask how it was measured

The honest figure is hemispherical — measured across light coming from the whole sky, which is how daylight actually arrives.

Measure straight down instead and the number looks better. Two covers quoted on different bases cannot be compared, and the difference is quietly worth real money.

If a supplier will not say which basis they used, that is your answer.

Glass holds its number. Plastic does not.

Glass does not cloud, yellow or go soft. The transmission it is specified at is roughly the transmission it still delivers decades later.

Plastic starts lower and drops further, on a shorter replacement cycle. Two covers that look close on day one are not close in year ten.

And then it is finished

You can clean glass. You can replace a tired plastic cover. You cannot raise the ceiling the structure was designed to.

Lamps add photons, and they add an electricity bill with them. Nothing gives back daylight the building was never built to catch.

DutchGreenhouses® works the structure, the covering and the screen package as one light budget, so every percent is spent on purpose. Get in touch before the structure is fixed — afterwards is too late for this one.

From the knowledge base