Mirrors play a crucial role in various optical systems. Their primary function is to redirect light, which often helps to compact an optical system. This article will explore the different kinds of thin-film coatings that can be applied to mirrors. The selection of a particular coating is influenced by the application, including the range of light spectra of interest, the desired quality of the optical wavefront, and budget restrictions.
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There is a fundamental distinction between household mirrors and optical mirrors; the latter is coated on the front surface instead of the back. This requirement means that for optical applications, a front-surface mirror is essential. Although typically housed within enclosures, the reflective surface is still susceptible to environmental degradation. Creating a robust front-surface mirror capable of withstanding wear and being easily cleaned is vital in mirror technology.
The substrate surface of a mirror should have a flat and smooth profile. Flatness is generally specified in terms of how many wavelengths of light the surface diverges from perfect flatness. For many cases, glass can be flattened to a few wavelengths of visible light. However, the most demanding applications require that the surface be flat to less than a quarter of a wavelength. Surface quality, or smoothness, is assessed based on the presence of scratches and digs post-polishing. A scratch/dig specification of 80/50 is common, while a specification of 20/10 offers higher transparency but at a greater cost.
Certain applications require a mirror's thermal conductance to be factored in. Metal substrates are often used in these cases, as they are generally more conductive than glass. Optical-quality metal surfaces can be crafted through polishing or single-point diamond turning. Common options include copper and aluminum, while beryllium is occasionally selected for its lightweight and rigidity—though it is notably toxic. In scenarios utilizing metal substrates, coatings serve to enhance reflectance while providing durability and scratch resistance.
Metal Mirror Coatings
The simplest and most widespread mirror coating involves a thin layer of metal. A 100-nm layer of aluminum or silver produces an excellent reflector in the visible spectrum. Aluminum reflects approximately 90% of light in this range, while silver achieves about 95%. The reflectivity of a metal mirror can be determined via the metal's index of refraction (n) and extinction coefficient (k). Reflectance is defined as follows:
An extensive compilation of n and k values across various wavelengths and metals is readily accessible. Table 1 presents an abridged selection, detailing data for ultraviolet (0.2 and 0.3 µm), visible (0.4 to 0.7 µm), and infrared wavelengths (1 to 10 µm). Metals characterized by k >> n tend to exhibit shininess, whereas those with k ≤ n ≤ 3 appear gray. For example, silver has n = 0.13 and k = 2.92 at 0.5 µm, making it shiny, while tungsten has n = 3.4 and k = 2.69, rendering it less shiny. As the wavelength trends into the IR region, both n and k increase, which leads to heightened reflectivity in that spectral region.
TABLE 1.
n AND k FOR SELECTED METALS
Wavelength (µm):
0.2
0.3
0.4
0.5
0.6
0.7
1.0
2.0
4.0
10.0
Aluminum* n:
k:
0.12
2.30
0.28
3.61
0.49
4.86
0.77
6.08
1.20
7.26
1.83
8.31
1.35
9.58
2.15
20.7
6.43
39.8
25.3
89.8
Beryllium n:
k:
0.84
2.52
2.42
3.09
2.89
3.13
3.25
3.17
3.43
3.18
3.47
3.25
3.28
3.87
2.44
7.61
2.38
16.7
8.3
41.0
Chromium n:
k:
0.89
1.69
0.98
2.67
1.50
3.59
2.61
4.45
3.43
4.37
3.84
4.37
4.50
4.28
4.01
6.31
3.08
13.7
14.2
27.5
For applications requiring higher durability, less shiny metals can suffice. For instance, dental mirrors utilize rhodium and rearview mirrors in vehicles use chromium.
All-Dielectric Mirror Coatings
Brighter mirrors are produced by layering high and low-index dielectric layers atop a glass substrate. For those designing a mirror for a specific wavelength of light, commonly symbolized as λ0, each layer's thickness is selected so that the product of the thickness and the layer's index of refraction is λ0/4. This is termed a λ/4 stack reflector. Increasing the layer count can enhance reflectivity at λ0, although the spectral breadth of the high-reflectance region remains limited.
A λ/4 stack can also function as a dichroic filter, adeptly isolating parts of the spectrum from an optical system. The conclusion is a durable, non-conductive broadband all-dielectric mirror with over 99% reflectance attainable across the visible spectrum. However, these higher-performing solutions often carry a premium.
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