Sorry I couldn't get back to you sooner Mother, tried several times yesterday but was apparently running into server problems.

I'm pretty sure what we are seeing is UV triggering at the start of your Red LED PAD schedule, which works the same way as Blue triggerring would if i occured at that time.

The plants are just as UV sensitive as they are Blue light sensitive and may trigger at radiance levels as low as 1/5th Watt per Square Foot of monochromatic radiation if it appears between the 280 to 520nm band.

Detecting UV contamination is a little more tricky than detecting visible Blue light contamination. What I do is expose phosphors with some Blue emission directly to the LED (and other) radiation and observe the phosphor looking through a Blue filter. If UV is present the phosphor will emit some Blue light that is visible through the Blue Filter, which blocks out the Red.

I assumed you would not be getting this problem since it did not appear before as a symptom.

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I'm also assuming that your 4/9/11 schedule follows the Dark sequence of SID then Red PAD, and not Red PAD then SID. If you are using PAD then SID that would be the problem most likely and if so you should switch to SID then PAD.
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UV from Red and other LEDs -
With the demand for higher output LEDs and especially with the popularity of the Dogma that UV increases potency, LEDs producing seconday UV emissions are being manufactured and marketted for plant growth and they are appearing on the market.

Part of the situation is that the best emitting LED us "Clearlake" cases which do not filter out UV. The Red cases, used to color correct the outputs of cheaper LEDs, usually contain filter pigments that do block these UV emissions.

If this is the problem it is easily corrected by using some form of UV blocking material that doesn't block Red (or Far Red).

I use transparent Red or Yellow plastic filters (Reds are a better bet), but carbonate materails are probably the most available to the average person. Carbonate greenhouse materials are available with clearly stated UV blocking characteristics. Nylon doesn't block UV and it's fairly easy to distinguish Nylon from carbonate materials, since clear Nylon is brittle and breaks/shatters easily when you bend it, while Carbonate tends to crease/pull/distend rather than break.

Borosilicate glass blocks some UV, but not as good as Carbonate.

Camera shops can order UV filters, but they tend to be more pricy.

(I'll try to get one more related post in before risk more server issues).

Sal.