UV umbrellas should be woven tighter to block out more sunlight, but even a non-UV umbrella will block a significant amount of sunlight.
I bought one of those UV-C detection cards that also detect UV-A and UV-B. Both a UV umbrella and a non-UV umbrella blunted the midday sun down from very high levels to low/medium levels.
I also bought a Japanese umbrella (brand Wpc) that is rated for UV and it blocks the UV levels down to zero.
A lot of plastics aren’t great at blocking/absorbing UV. And normal umbrellas, not being designed for UV, may lose a lot of their strength with heavy UV exposure from what they do absorb.
I bought one of those UV-C detection cards that also detect UV-A and UV-B. Both a UV umbrella and a non-UV umbrella blunted the midday sun down from very high levels to low/medium levels.
I also bought a Japanese umbrella (brand Wpc) that is rated for UV and it blocks the UV levels down to zero.