Unobstructed, midday sunlight provides about 100,000 lumens per square meter on the Earth's surface. Illuminance varies according to the geometric relationship of the Sun to the Earth's surface.
A lumen is the measurement of light available at a given distance, and illuminance is the measure of the intensity by which something is illuminated by a light source. When these measurements are taken according to the metric system, it is referred to as lux. The intensity of light at a given spot on Earth is not a measurement of how much light is being produced by the Sun, but how much the Sun illuminates the Earth at a given time and place.