Professional lighting design is the practice of designing light as a system rather than selecting fixtures at the end of a project. The method runs in a fixed order: understand the medium (lumens and lux, colour temperature, colour rendering, beam and glare), read the existing conditions (the daylight across the day and the building's wiring and ceilings), write a brief based on how people actually use light (by task, time of day, age and mood), compose the scheme from four layers (ambient, task, accent and decorative), resolve the controls (zoning, switching and dimming), and document everything in a reflected ceiling plan, switching plan, fixture schedule and written specification. The Design Key — The Lighting teaches this complete method, from first principles to a scheme an electrician can install and a client can sign off. Lighting is the one material in an interior you specify without ever holding it, and the one that decides whether every other material in the room reads as intended. This is why it cannot be the last decision. A grid of downlights dropped onto a finished plan, every fixture wired to a single switch, no dimming and no layers — this is how most rooms are lit, and it is why most rooms feel flat after dark, glare in the wrong places, and leave the work surfaces in shadow. The Design Key — The Lighting exists to replace that default with a method. You begin with the medium itself. Before reading a room or choosing a fixture, you learn to describe light precisely: the difference between lumens (what a source produces) and lux (what actually lands on a surface); colour temperature and where each range belongs; colour rendering and why a cheap lamp can quietly ruin an expensive palette; beam angle and distribution; and the glare and contrast that make a technically bright room uncomfortable to be in. You learn to read a fixture's specification critically and predict its effect before it is switched on. Then you read what is already there. You assess the daylight — orientation, aperture, the daily and seasonal arc — and identify the hours and zones natural light cannot reach, which is exactly where the artificial scheme has to carry the room. You read the building's electrical reality — the panel, the spare capacity, the existing circuits — and the ceiling construction that decides what can be recessed and what cannot. This is the renovation-first discipline of The Design Key applied to light. You will learn to write a lighting brief built on how a household actually lives: the tasks performed in each room, the way the same space needs different light in the morning, the evening and the middle of the night, the way older eyes need substantially more light and suffer more from glare, and the atmosphere the home is meant to hold after dark. You will learn to compose the scheme from its four layers — ambient, task, accent and decorative — into a coordinated, room-by-room plan in which nothing sits on the ceiling without a job. You will learn to select fixtures from function backwards, and to keep colour temperature and rendering consistent within every sightline. You will learn to resolve the controls before the scheme is finalised — how to zone and circuit a scheme so the layers operate independently, where switches belong relative to how people move through a room, why dimming so often flickers or buzzes and how to specify dimming that works, and when scene-based or smart control earns its complexity. And you will learn to document all of it — a reflected ceiling plan, a switching and circuit plan, a fixture schedule and a written specification — to a standard an electrician can install from, finishing with the commissioning walk-through that makes the finished scheme deliver what was designed. The course is written for one audience: anyone who wants to design lighting using real professional methodology. Whether you are a homeowner planning your own renovation, a designer formalising your process, or a tradesperson who wants to think upstream of the electrician, the content does not change. The decisions are the same. The order is the same. The drawings are the same. The Lighting sits in the Systems layer of The Design Key, alongside The Ventilation, and the method runs underneath every Rooms course in the Programme. It is a stand-alone course with its own Course Certificate, counting toward the Master Certificate of the full Design Key Master Programme. The fixtures will change, the home will change, the budget will change — the order will not. That order is the method. It is the same order you will apply to the next scheme, and to every room you light after.