Because of the way you usually look at lighting in computer graphics. These directions are written in the matter of solid angles, or to make the theory even easier, in differential solid angles. Another point is that your surface may reflect light differently depending on the where it comes from, which also makes important the direction part. I hope it is clear, that radiance is irradiance from a certain direction. If not, please try to specify which part still bothers you.
Yes they do. I see how this is confusing. I hope I could clear up why you do this. We don't associate direction other than considerung only surfaces perpendicular to the light direction with irradiance. We do however with radiance. Also, remember that these two physical quantities don't have the same units and should not really be compared this way. To be honest, terms like these are very confusing as they aren't clear cut and on one side of the border. They are more grayish.
I'm gonna tell you how I convinced myself, as I too had this confusion as soon as I read your question. But I managed to convince myself through this argument. First of all we are gonna clear up 4 terms, Radiance, Irradiance, Differential radiance and Differential Irradiance. To be more formal and according to wikipedia,.
Next is differential radiance. Next is Irradiance. Irradiance isn't normally associated with a direction. According to Wikipedia it's. But more commonly and what makes more sense to me, and to the answer of your question, think of irradiance as the integration of radiances over a set of directions. So if we integrate the radiances from every direction that leads us to the original definition of irradiance where direction isn't of concern.
However usually we are concerned with only a subset of all the directions such as the Upper hemisphere or the lower hemisphere. This means for example,.
As we can see here, we have limited the irradiance to a set of directions, the upper hemisphere. This doesn't necessarily change it into radiance which is associated by direction. Instead what this means is, when calculating irradiance we are concerned with the light coming only from these directions, although we haven't incorporated the directional quantity into the formula like with radiance.
This is the difference between irradiance from a certain direction and radiance. Think of it like this. You are holding a paper and there are 2 light bulbs in front of you. You want to measure the irradiance. Normally it would just be the radiant flux received by both bulbs per unit area.
But now let's say I limit the direction so I am only concerned with the first bulb. Note that I am still calculating the "irradiance". If I move farther away the flux will decrease thus the irradiance even tho I am concerned with a specific direction. However this isn't the case with radiance where moving farther away won't change it since we divide by the solid angle too balancing the change.
Spectral radiance is a key measure when selecting a source for an application. In general, most radiation sources exhibit variations in spectral radiance across their spectrum of emission. In the design of optical instruments, scientists and engineers choosing light sources will be exposed to a variety of source specifications and radiometric terms. It is important to understand the nature of the specifications and to couch them in radiometric terms that will enable appropriate design decisions.
In general, for typical optical instrument applications, such as spectroscopy and imaging, it is the radiance and spectral radiance of the light source that most needs to be understood. For an instrument with limiting apertures and solid angles, it is the radiance of the source that determines how much radiation passes through the instrument. By carefully matching the instrument with a source of appropriate radiance, an optimum system can be designed. It describes angular spans in three-dimensional space, analogous to the way in which the radian [rad] describes angles in a two-dimensional plane.
The radiance of a source is increased by increasing its emitted power, by making the emitting area of the source smaller or by emitting the radiation into a smaller solid angle. Strictly speaking, radiance is defined at every point on the emitting surface, as a function of position, and as a function of the angle of observation. Often, as in the example above we use radiance of a source to mean the radiance averaged over a finite sized aperture and over some solid angle of interest.
Radiance is a conserved quantity in an optical system so that radiance measured as watts per unit area per unit solid angle incident on a detector will not exceed the radiance at the emitter.
In practice, for any bundle of rays mapping an emitter to a detector, the radiance seen at the detector will be diminished by the light which is absorbed along the way or scattered out of the solid angle of the bundle of rays reaching the detector. Let us consider an example. Basically radiance is a measure of the light as energy per unit area on a surface whereas irradiance measures the light per unit volume falling in a conic section in the hemisphere above a point this volume is called a solid angle.
Irradiance is often calcuated using ray tracing where you send off several rays in a solid angle into the hemisphere above your shading point to calulate the light arriving at that point. This is often more useful than knowing the Radiance for a surface. This often happens when talking about rendering. Thank you that helps alot.
So when the terms radiance and irradiance are used in CG it is only to imply a meaning to what we are trying to simulate.
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