High reflectivity is important to maximize the amount of light in the image.
The broad spectral range limits the choice of reflective coatings to metal based designs. Multilayer dielectric reflectors are not feasible for this application. In addition, gold and copper coatings are soft and subject to mechanical damage. Rhodium and platinum both have inadequate reflectance over the entire spectral range but particularly below 1 micron. The reflectance of aluminum is intermediate between rhodium and gold for most of the spectral range, but has higher reflectance below 600 nm than all other metals except silver.