1.8 Reflectance

Reflectance is defined as the ratio of incident flux on a sample surface to reflected flux from the surface as shown in Figure 1.8.1. Reflectance ranges from 0 to 1. Reflectance was originally defined as a ratio of incident flux of white light to reflected flux in a hemisphere direction. Equipment to measure reflectance are called spectrometers (see 2.6).

Albedo is defined as the reflectance using the incident light source from the sun. Reflectance factor is sometime used as the ratio of reflected flux from a sample surface to reflected flux from a perfectly diffuse surface. Reflectance with respect to wavelength is called spectral reflectance as shown for a vegetation example in Figure 1.8.2. A basic assumption in remote sensing is that spectral reflectance is unique and different from one object to an unlike object.

Reflectance with a specified incident and reflected direction of electro-magnetic radiation or light is called directional reflectance. The two directions of incident and reflection have can be directional, conical or hemispherical making nine possible combinations.

For example, if incident and reflection are both directional, such reflectance is called bidirectional reflectance as shown in Figure 1.8.3. The concept of bidirectional reflectance is used in the design of sensors.

Remarks; A perfectly diffuse surface is defined as a uniformly diffuse surface with a reflectance of 1, while the uniformly diffused surface, called a Lambertian surface, reflects a constant radiance regardless of look angle.

The Lambert cosine law which defines a Lambertian surface is as follows:

I ( ) = In .cos where I( ): luminous intensity at an angle of from the normal to the surface.
In : luminous intensity at the normal angle   