A downlight’s function is to direct the light emitted by the bulb in a specific direction . The reflector within the downlight redirects the omni-directional light out the bottom of the fixture. But, even the most sophiticated reflector models do not exceed 75% efficiency. When the reflector is a simple flat piece of galvanized metal, like in some older marine fixtures, the efficiency does not exceed 60%.

In this case, 1 halogen lumen ( the light unit quantity) is equivalent to 0,7 led lumen , because led light is correctly directed from the source.

A 10W G4 halogen producing 140 lumens can be replaced by a monoface Led bulb generating 100 lumens.

In a shade type lamp fixture the light is isotropic, so it is spread in all directions. In this case, 1 halogen lumen is equal to 1 led lumen. Here you must choose a 360° multiface led bulb to have the the same omni-directional lighting output as an incandescent bulb would provide. A BAY15D 25W incandescent delivering 200 lumens must be replaced by a G4 multiface 360° 2,8W led bulb delivering the same 200 lumens.

In some gooseneck reading fixtures, the bulb is placed very deeply in the reflector. If the reflector is too narrow to accept a monoface G4 with vertical connecting pins (calculated with 70% of original lighting power), you have to use a G4 360° led bulb (calculated on 100% of original lighting power). As the usual halogen bulb used in these reading lights is a 5W, it can be replaced by a G4 omnidirectional type delivering 60 lumens.