What is being measured when companies quote solar performance of fabrics?

This varies enormously and should not be relied upon if comparing performance statistics.

Some fabric manufacturers measure the optical (or visible) light, whilst others measure the heat energy (some measure both). Despite both types of energy coming from the same source they are two completely different things and so should not be compared. In both cases they are always measured using artificial light and/or heat sources, rather than the sun.

The output of the light and/or heat source as well as its angle and distance from the test fabric will wildly affect the figures achieved. Manufacturers do not always state whether the test is conducted through any particular form of glazing.

Some general blind retailers may well also quote SPF (Sun Protection Factor) which is a relative measure of how much UV is being blocked (not heat) and as almost all conservatory blind fabrics significantly reduce UV this is quite misleading.

Never make a decision based on statistics or claims of performance. There is no single fabric that outperforms all others without also significantly reducing natural light to the point where it darkens your conservatory and adjoining rooms.

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