Reflectance
Markus Otto, Winzenrender, www.winzenrender.com
The reflectance of a material refers to how it reflects the light it receives at the surface, in plain words: its surface color and intensity. They are defined by the Reflectance 0º (the color at front angles) and Reflectance 90º (color at glancing angles).
Reflectance 0º and Reflectance 90º
Choose a reflectance color by clicking on the Color Picker, or specify a texture by clicking on the Texture slot 
. You can enable or disable the texture using the check button near the Texture slot.
A fully white reflectance (RGB 255) means that all the light that falls on the object is reflected back. Black reflectance (RGB 0) means that all the light is absorbed. Please note that the reflectance colors describe the amount of light the object reflects back, but not how that light is reflected back (in a diffuse way, or a specular way for shiny objects). This is instead controlled by the Roughness parameter. See the Surface Properties page which describes the Roughness parameter in detail.
Avoid setting the Reflectance 0° color too bright. Setting it to 255 for example means this material will reflect back almost all the light it receives, which does not happen in the real world.
Maxwell Render still keeps the amount of light reflected/ absorbed within physical limits but the result with such high Reflectance values means the light will keep bouncing around in your scene with very little loss in energy, which will produce noisier renders and a washed-out look with very little contrast.
A reasonable setting even for very bright materials would be around 225-235. For example, the Reflectance value of a white piece of paper, when converted to RGB, is around 225. Please note that this only matters for the Reflectance 0° color. The Reflectance 90° color can be left at fully white (RGB 255)
This rule is valid also when using textures for the reflectance 0 - if your texture contains fully white areas use the RGB Clamp setting in the Texture picker to contain the range of brightness in the texture ( set the high value of RGB clamp to lower than 255).
There are two reflectance colors: the light reflected when the object is seen at 0º degrees (frontal view) and at 90º degrees (glancing angle). The Reflectance 0° is the object’s main color. This is useful when you have a material which reflects one color when viewed straight on, but at sharper viewing angles reflects another color, such as taffeta, silk, and velvet. Think of the Reflectance 90° color as the object’s “specular” reflection color.
Reflectance & Roughness
How much influence the Reflectance 0° color vs. the Reflectance 90° color has on the look of the material depends mostly on the roughness settings and to a certain degree on the Nd. In the example below Reflectance 0 was set to red and Reflectance 90 to blue. Only the roughness was decreased from 100 on the far left to 0 on the far right.
Reflectance 0 RED, Reflectance 90 BLUE. Decreasing roughness 100, 90, 70, 50, 25, 10, 0
As you can see the reflectance 90 color does not appear when roughness is very high - it is only the reflectance 0 color that determines the overall color of the material. But as the roughness decreases, the Reflectance 90 color appears more and more, especially as the viewing angle increases towards the edges of the sphere. The second thing we ca see is that the red color from Reflectance 0 is less and less visible. This is because a very shiny surface will act as a mirror, reflecting back the environment and not its own main color.
For most materials, the Reflectance 90 color is white. However, materials like metals usually have tinted reflections. For example, if you would like to create a golden material, you should set the Reflectance 0 color to a yellowish tone and then use a brighter yellow for the Reflection 90° color so that the specular reflections become tinted as well.
Setting color using RGB or HSV (Hue Saturation Value)
It might be easier to keep Reflectance 0 colors within reasonable brightness limits by switching the Color Picker from RGB mode to HSV mode. In this mode, you control the brightness of the hue directly with the V or Value slider. This allows you to control the overall brightness with one slider instead instead of 3 for RGB, while maintaining the same hue and saturation.