The Adhan is certain words said loudly at the time of obligatory Prayers to inform busy people that the Prayer is due, and that they have to come to the mosque. It is purely an act of worship, and nothing cannot take its place, and if it happens, it is regarded as an unacceptable bid`ah (matter innovated in religion).
The case at hand which is using rays to call for prayers is slightly different, because such rays do not replace the Adhan, but they are an additional means to announce the Prayer. Undoubtedly, if people took it as a substitute to the Adhan, this would be totally unaccepted.
In his response to your question, Dr. Muzzamil Siddiqi, president of the Fiqh Council of North America (FCNA), stated,
I see nothing wrong with the use of some special light (green or any color) at the time of the Prayer in addition to the Adhan inside or outside the mosque. This light should not be considered a substitute of the Adhan, but it could be an additional indication that the Prayer is due.
In Islamic Shari`ah, Adhan is fard kifayah (communal obligation) for the congregational Prayer in every mosque, yet additional means can be used to inform those to whom the voice of the mu’adhdhin (the one who says Adhan) would not reach. It is reported that sometimes Bilal ibn Rabah (may Allah be pleased with him) would come to Prophet Muhammad (peace and blessings be upon him) after the Adhan to tell him that the Prayer is near.
Furthermore, Sheikh ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto,Ontario, Canada, maintains, I do not find the use of lights for this purpose objectionable. That would be another medium for notifying people of the arrival of the Prayer time.
It is analogous to the use of lights or gunshots to inform people of the arrival of Ramadan or `Eid Al-Fitr — a practice which prevailed in Muslim communities all over the world until recent times, when loudspeakers and the like were not used yet.
This means cannot be ruled as a bid`ah as long as it does not replace the Adhan. There are numerous parallels for this in Islam: building minarets to enhance the reach of the Adhan, using microphones in the mosques, etc. The creative use of such means has been accepted by the Muslim community, and as such, no one should confuse any of them as an integral of worship.
Giving more clarification on the issue at hand, Dr. Main Khalid Al-Qudah, member of the Permanent Fatwa Committee, Assembly of Muslim Jurists in America (AMJA), said,
It is not recommendable to use these virtual rays, because it is not a part of the Sunnah of the Prophet (peace and blessings be upon him) to do so as a means to inform people with the Adhan time. It should be well-understood that any act of worship must be done in the exact way applied or indicated or approved by the Prophet (peace and blessings be upon him), yet the Adhan is one of these acts of worship.
On the other hand, if Muslims are clearly instructed that these rays are not part of the Adhan, then the ruling is that it is not prohibited or even disliked to use this method, since it is regarded as a permissible way to publicize the Adhan.
to conclude, using this new method is permissible as long as it is the only allowed way to inform people about the Adhan time publicly.