To use the app, simply select the azimuth or altitude nodes to interactively drag the direction of the energy beam. You can also select from a range of different beam shapes, however you will notice that this does not affect the relative intensity or the incidence area. This is because the Cosine Law is the same for all beam shapes and/or incidence locations on a planar surface. On such surfaces, the Lambert cosine law is solely governed by incidence angle.

## Fundamental Concepts

Lambert’s cosine law (also known as the cosine emission law) states that the measure of radiant energy from a surface that exhibits Lambertian reflection is directly proportional to the cosine of the angle formed by the measurement point and the surface normal. It follows that irradiance or illuminance falling on a surface varies similarly with the cosine of the incidence angle.

The perceived measurement area orthogonal to the incident flux is significantly reduced at oblique angles, causing energy to be spread out over a wider area than it would if it was falling perpendicular to the surface. Thus, if you consider a fixed surface area, the amount of energy to which it is exposed will reduce significantly the closer the source is to grazing incidence.

### Angle of Incidence

The angle of incidence is the angle between a ray that hits a surface and a line that is perpendicular in all directions to that surface at the point of incidence. This perpendicular line is usually called the surface normal.

### Grazing Incidence

Grazing incidence is the term used to describe situations where the irradiance or illuminance is travelling almost parallel to the incident surface, meaning that the incidence angle is very close to 90 degrees. As the cosine of 90 degrees is zero, this means that the resulting relative intensity will be very low as the distribution area is very large.