If we start by dissecting the term, which begins with a "Pick", it means that we are moving into the picking moment in the warehouse. Instead of pen, paper and more manual acknowledgement, lights, digital displays and TV screens are used to both facilitate and speed up the picking process.
When the operator stands at/walks towards the designated shelf, he will be met by a luminous light together with a specified number on a display for how many he should pick. By clicking on the illuminated button, the pick is acknowledged and the communication with the overlying system (WMS, ERP) updates the status to "picked".
The really big advantage of a pick-to-light is that the products are always available. This is a solution that is kept out of all types of cubicle systems, ASRS, elevator machines and the like.
Since replenishment always takes place from the back, it is also a great way to keep track if you handle date-stamped products. In a regular shelf system, the operator is required to constantly move the "old" goods forward and replenish the new ones behind.
It's hard to find a faster pick than in a pick-to-light. Here an FTE (Full-time-equilivant) can pick up to 10 times as fast as in a regular warehouse pick. Even when comparing to robotic warehouses, it is difficult to find a system with more box presentations/minute than the operator can pick with 84 available SKUs within 1 square meter from where he stands.
A pick-to-light is mainly suitable for those who have a steep curve with a long tail when talking about the "pareto principle" aka the 80-20 rule. That is to say: If you have a small number of SKUs that account for the majority of your turnover, you want to be able to turnover these items as quickly as possible. They should hardly enter and turn in the warehouse. If you recognize this pattern, an investment in a pick-to-light system may be something for you. However, if you do not have this, the pick-to-light can be perceived as bulky/SKU. Which means that if the inventory transactions are roughly the same across all active items, there is little to suggest that a pick-to-light would be the solution.
The staffing issue for pick-to-light is also becoming increasingly relevant. Because the turnover of warehouse staff, combined with the extremely variable seasonal sales pattern, ensures that efficiency/FTE is not person-dependent. In a classic warehouse, you always have staff that "outperforms" over time and it can be very time consuming to train external staff if the process is too complicated. When it comes to picking numbers on a display from a brightly lit lamp, the efficiency is the same regardless of gender, age or work ethic.