Yes it was warm Ray, but all other shots that day donít exhibit the effect.
I was inaccurate in my title Bart, it should have been Processing Artifact?
It is not a PP artifact; the effect is quite visible in the RAW thumbnail in Bridge; so I was wondering about the in-camera processing. (shots of the chair from f8 through f/9.5 and 1/20 to 1/60 s exhibit the effect)
The effect is only seen in shots in which the chair is present. I have attached a similar shot taken later in the day where the effect is absent. The only significant difference I can see is that the back of the chair is no longer in direct sunlight. So hereís a thought: The angle of the lens to the openings between the slats in the chair is such that the visible opening is quite narrow; such that itís acting as a diffraction grating and when intense sunlight is striking the back on the chair a diffraction pattern shows up. However, if thatís the cause, it seems the vertical lines should then also appear on the chair as well.
I will try to reproduce the effect if the opportunity presents itself.