Because of the greenhouse effect, the Earth's actual average surface temperature is about 288 K (15 ☌ 59 ☏), which is higher than the 255 K (−18 ☌ −1 ☏) effective temperature, and even higher than the 279 K (6 ☌ 43 ☏) temperature that a black body would have. The photoelectric effect has three important characteristics that cannot be explained by classical physics: (1) the absence of a lag time, (2) the independence of the kinetic energy of photoelectrons on the intensity of incident radiation, and (3) the presence of a cut-off frequency. The above temperature is Earth's as seen from space, not ground temperature but an average over all emitting bodies of Earth from surface to high altitude. Luminous intensity is defined as dId / d, where d is the luminous flux (light energy flux in watts per m2) emitted within a solid angle d. For example, if the point at which you want to calculate the light intensity is 81 cm away from the light source, report your answer as 0. Convert the distance that you measured into meters. Measure the distance between the light source and your point of interest. This approximation reduces the temperature by a factor of 0.7 1/4, giving 255 K (−18 ☌ −1 ☏). The wattage is usually printed on the bulb. The effect of albedo on temperature can be approximated by assuming that the energy absorbed is multiplied by 0.7, but that the planet still radiates as a black body (the latter by definition of effective temperature, which is what we are calculating). The Earth has an albedo of 0.3, meaning that 30% of the solar radiation that hits the planet gets scattered back into space without absorption. Since amplituds of electric and magnetic waves are related by speed of light, we can write intensity also as. This gives an effective temperature of 6 ☌ on the surface of the Earth, assuming that it perfectly absorbs all emission falling on it and has no atmosphere. For an ideal absorber/emitter or black body, the StefanBoltzmann law states that the total energy radiated per unit surface area per unit time (also known as the radiant exitance) is directly proportional to the fourth power of the black body's temperature, T : The constant of proportionality,, is called the StefanBoltzmann constant.

Where T ⊙ is the temperature of the Sun, R ⊙ the radius of the Sun, and a 0 is the distance between the Earth and the Sun. If the intensity ( I d/d ) of a source is the same in all. For visible light it is expressed in lumens per steradian candela. In the case of radiant power, it is expressed in watts per steradian. Not to be confused with Stefan's equation or Stefan's formula. It expresses the directionality of the radiated energy and is appropriate for the description of point sources.

Power intensity of light equation