The SU800 does not emit visible light, just long wave length infrared (IR). It is actually very close to IR, as a small, dull red glow can sometimes be seen when it fires. The SU800 emits all the same preflash and image capture pulses that an on-camera SB900 or SB800 would, only it is all in the infrared portion of the spectrum.
The pulses emitted from on-camera SB900/800 and popup flash units are in visible light, as the same head and flash tube is used to illuminate the subject. That said, these pulses do contain an IR component which is used for the preflash and image capture signalling.
Some believe that on-camera SB900/800 masters have more range than the SU800. In my experience, individual conditions will vary considerably as a function of distance, time of day, angles, etc. I have fired remote speedlights at distances beyond sixty feet under carefully controlled conditions.
The small, round sensors just ahead of the battery doors on SB900/800/600 speedlghts are infrared filters. When these units are used as remotes, the filters absorb the visible component of the pulses and pass the IR component from the on-camera SB900, SB800 and popup units, which is then decoded and applied.
Hope this clarifies things for you.
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