optical airmass. The atmospheric extinction brings the number of lux down to around 100 000 lux. The total amount of energy received at ground level from the Sun at the zenith depends on the distance to the Sun and thus on the time of year. It is about 3.3% higher than average in January and 3.3% lower in July (see below). If the extraterrestrial solar radiation is 1367 watts per square meter (the value when the Earth–Sun distance is 1 astronomical unit), then the direct sunlight at Earth's surface when the Sun is at the zenith is about 1050 W/m2, but the total amount (direct and indirect from the atmosphere) hitting the ground is around 1120 W/m2. In terms of energy, sunlight at Earth's surface is around 52 to 55 percent infrared (above 700 nm), 42 to 43 percent visible (400 to 700 nm), and 3 to 5 percent ultraviolet (below 400 nm). At the top of the atmosphere, sunlight is about 30% more intense, having about 8% ultraviolet (UV), with most of the extra UV consisting of biologically damaging short-wave ultraviolet. has a luminous efficacy of about 93 lumens per watt of radiant flux. This is higher than the efficacy (of source) of artificial lighting other than LEDs, which means using sunlight for illumination heats up a room less than fluorescent or incandescent lighting. Multiplying the figure of 1050 watts per square meter by 93 lumens per watt indicates that bright sunlight provides an illuminance of approximately 98 000 lux (lumens per square meter) on a perpendicular surface at sea level. The illumination of a horizontal surface will be considerably less than this if the Sun is not very high in the sky. Averaged over a day, the highest amount of sunlight on a horizontal surface occurs in January at the South Pole (see insolation). Dividing the irradiance of 1050 W/m2 by the size of the Sun's disk in steradians gives an average radiance of 15.4 MW per square metre per steradian. (However, the radiance at the center of the sun's disk is somewhat higher than the average over the whole disk due to limb darkening.) Multiplying this by π gives an upper limit to the irradiance which can be focused on a surface using mirrors: 48.5 MW/m2. Composition and power The spectrum of the Sun's solar radiation can be compared to that of a black body with a