In article ,
Nick Maclaren wrote:

I don't have a clue what the primary 'burning' effect on plants is
(i.e. ultraviolet or surface heating) and what the 'tanning' effect
is. But they assuredly exist. Can you clarify those at all?

Glass very effectively absorbs UV so it doesn't reach the plants at
all. So any damage in a closed greenhouse is likely thermal damage.
There are specialty quartz glasses that transmit some UV, but they are
very expensive. Some clear plastics absorb more UV than others, but
most that are produced for outdoor use include additives that intercept
UV in order to delay the breakdown of the plastic itself.

Having inadvertently damaged innumerable transplants over the years
when they got zapped by sun before they hardened off, to me the
damage looks quite different when fresh from that caused by thermal
burning or freezing. In the latter, the whole tissue looks "cooked".
In the former, it seems like chlorophyll is selectively destroyed, so
you get patches of paper white tissue. Of course, in both cases the
tissue later browns and dries out, but the appearance of fresh damage
is distinctive, to my eye at least.

Most of the plants I've seen injured by heat (from fluorescent light
ballasts), freezing (but it was warm when I went to bed!) and sunburn
(it was supposed to be heavily overcast today!) were young tomato,
pepper and cucurbit plants, all of which have thin leaves that are
not flat, so the distribution of the damage on the leaf surface for
sunburn might be different for the flat, thick leaves of Clivia.

Disclaimers: I haven't seen the picture of your Clivia. I'm in Toronto,
Canada, where we have a continental climate somewhat modified by Lake
Ontario, so my gardening practices are no doubt different from those
of the readers of uk.rec.gardening.