Find the illuminance on a surface from a point source using the inverse-square law.
Worked example: 1000 cd at 2 m → 1000 ÷ 2² = 250 lux. Double the distance, quarter the light.
The inverse square law says that the light from a small source falls off with the square of the distance. The illuminance in lux is the intensity in candela divided by the distance in meters squared. A source of 1,000 candela at 2 meters gives 250 lux, and doubling the distance to 4 meters cuts the lux to a quarter, or 62.5.
The law works well when the distance is at least five times the size of the source, so it is accurate for a spotlight on a far wall but not for a large panel close to the surface. It also assumes the light hits the surface straight on. If the surface is tilted, multiply the result by the cosine of the angle between the light and the surface normal.
Several sources add together, so to find the total level at a point, calculate each one and add them up. Reflections from walls and ceilings add more light in a real room, which the simple formula does not include. A light meter gives the real result, and comparing it with the calculation shows how much the room contributes.