If the arctube is not in the center of the circular cross-section of the shield, you are OK.
The rule saying "the reflector should not reflect back to the arctube" mean in fact, then the image projected by the reflector should not be focused back on the arctube. And the only single reflection shape able to focus the image of an object back to the object itself is the circle (or a sphere) with it's center on the exact place as the object. If it's off-centered (by the distance many times the object's dimension, so in the case of 8mm thick HPS arctube few cm off-center is enough), the image is off-focus, so only negligible amount of the energy return back onto the arctube. For his are sensitive only lamps with vacuum outer, where the dominant arctube cooling mechanism is thermal radiation (so practically only lamps with ceramic arctubes), so the reflected thermal radiation is able to significantly alter the thermal balance. Those with gas filled outer use mainly the convection to cool down the arctube, so the reflected radiation does not alter the thermal balance in any significant manner.
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