Tower Controller described an operational error discovered after the fact during a tape review that alleged that proper phraseology was not used when visual separation clearances were issued.

Narrative

I coordinated with Local 2 for the release of Air Carrier X on Departure X (straight-out) off of Runway XXR. Local 2 released Air Carrier X in reference to Air Carrier Y on Departure Y (straight-out) off of Runway XYL. Visual separation was to be applied between the aircraft. I pointed out Air Carrier Y to Air Carrier X when airborne and advised him to report the MD80 in sight. He reported it in sight but did not use his call sign. I then instructed Air Carrier X to maintain visual separation from the MD80 and to contact Departure. Air Carrier X read back; 'Thank you; good day' and did not use his call sign. This event was discovered as a result of a random tape monitor evaluation during a site visit. It was classified as an operational error because I did not ensure a call sign was included in each of the read-backs involving visual separation. I have used visual separation throughout my many years of experience without the need for a call sign with every read-back. The terms 'in sight;' 'tally-ho;' 'got him;' and etc. have worked stupendously along with voice recognition. We are trained to use voice recognition and to call this event an operational error is wrong; especially when the pilot is not mandated by an FAR to use his call sign with every transmission. It's also wrong to place the burden on the Controller to obtain a call sign with every read-back. It's added workload that could lead to an actual catastrophic error elsewhere; especially in the Tower environment. I am aware of no change to the JO7110.65(T) in reference to visual separation; it still states to 'obtain acknowledgment from the pilot that the aircraft is in sight.' Only recently has the interpretation changed to require a call sign with that acknowledgment. I have experienced and witnessed controllers having to make excessive transmissions in order to get a call sign with every read-back involving visual separation; meanwhile; a potential conflict could have developed on the runways. It's an unnecessary task and an overwhelming distraction. I recommend that pilots be mandated by an FAR to use a call sign with every transmission. In addition; I recommend an example be added to the visual separation section of JO7110.65(T). It should state acceptable pilot acknowledgments that an aircraft is in sight; such as 'affirmative;' 'in sight;' 'tally-ho;' 'got him;' and etc. without the use of a call sign.