![]() ![]() High-definition televisions range from 450 to about 1500 cd/m 2. ![]() Most consumer desktop liquid crystal displays have luminances of 200 to 300 cd/m 2. Typically, calibrated monitors should have a brightness of 120 cd/m 2. From Wikipedia "The sRGB spec for monitors targets 80 cd/m 2. It is used to characterize the amount of visible light from a light-emitting surface, such as the light provided by a display device. The candela per square meter (cd/m 2) is the derived SI unit of luminance for a surface. It is approximately equal to the old unit "candlepower" and is generally taken to be equivalent.įor an isotropic source, the relationship between the candela and lumens is 1 cd = 4π lm and the unit relationships is 1 cd = lm/sr. The candela is then used to define the lumen and other quantities used in the measurement of visible light. The candela is abbreviated cd and its standard symbol is I v. of a source that emits monochromatic radiation of frequency 540 x 10 12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. The candela is the luminous intensity, in a given direction. It is one of the seven foundation SI units. The candela is the foundation unit for the measurement of visible light. An interesting aspect of lambertian reflection is that although the luminous intensity will be half as much at that angle, the visual area that your eye sees is also half as much, so the surface appears equally "bright" from that angle. The ray showing 50% is at the angle 30° from the surface where cos 60° = 0.5. For an ideal lambertian surface the reflection will follow the cosine law. Reflection from a surface can be complicated by surface roughness. The intensity or pointance from such a surface in any direction is proportional to the cosine of the reflected angle. For example, an evenly illuminated flat surface such as a sheet of paper is approximately lambertian in that the luminance that you see from any direction is essentially the same. It behaves according to the inverse square law.Ī flat surface that reflects or emits equal luminance in every direction from the surface is said to be a lambertian surface. Unit illustrationįor a point source, the emitted light intensity or pointance is the same in all directions, or isotropic. In the luminous case it is measured in lumens/m 2 steradian which is equivalent to candela/m 2 = nit. In the radiant case it is measured in watts/m 2 steradian and is also called radiance. The power per unit area per unit solid angle is sometimes called sterance. Otherwise, for a flat radiating surface, known as a lambertian, the intensity falls off as the cosine of the observation angle with respect to the surface normal. If the intensity ( I = dΦ/dω ) of a source is the same in all directions, the source is called isotropic. For visible light it is expressed in lumens per steradian = candela. In the case of radiant power, it is expressed in watts per steradian. It expresses the directionality of the radiated energy and is appropriate for the description of point sources. (a) What is the intensity in of a laser beam used to burn away cancerous tissue that, when 90.0 absorbed, puts 500 J of energy into a circular spot 2.00 mm in diameter in 4.00 s (b) Discuss how this intensity compares to the average intensity of sunlight (about ) and the implications that would have if the laser beam entered your eye. 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 power (flux) per unit solid angle (sometimes called pointance) is the nearest precise terminology to the common term intensity. 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. This approximation reduces the temperature by a factor of 0.7 1/4, giving 255 K (−18 ☌ −1 ☏). ![]() 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. 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. 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. Not to be confused with Stefan's equation or Stefan's formula. ![]()
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