A high precision method for measuring the specular reflectivity of mirrors at 10.6 μ with a CO2 laser source when the reflectivity of the mirror approaches close to unity is described. The results of measurements on a range of evaporated thin film coatings are reported. Measurements at other wavelengths using a helium–neon laser have also been made, and this is discussed.
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Measured Reflectivities on Commercially Available Reflectors at 10.6 μa
Reflectivity
Comments
Reflectivity
Comments
Gold reflector
“Enhanced” metal reflector
0.9849
0.9908
0.9867
0.9894
0.9863
}
Same source
0.9921
}
Same source
0.9860
0.9917
0.9881
}
Same source
0.9891
Glass substrate
}
Same source
0.9875
0.9904
Silicon substrate
0.9888
0.9910
Germanium substrate
0.9705
0.9933
}
Same source
0.9876
With thin quartz overcoat
0.9922
0.9709
With thin germanium overcoat
0.9792
0.9915
Solid copper substrate
0.9789
0.9866
Solid germanium substrate
0.9791
Aluminum reflector
All-dielectric reflector
0.9831
}
Same source—aged and fast evaporation in H. V.
0.9907
}
Same source
0.9835
0.9867
0.9710
Aged
0.9782
0.9771
Aged
0.9778
0.9734
With SiO overcoated (aged) 2.5 cm × 7.6 cm
0.9579
0.9840
Fresh coating
Silver reflector
0.9589
0.9693
}
Same source
0.9692
0.9661
0.9888
Fresh coating
All substrates, unless otherwise stated, were optically flat (λ/10) glass, 5-cm diam, with a laser quality smooth polish. Those labeled same source were nominally exactly the same evaporated coating although not necessarily made at the same time.
Table II
Comparison of Gold and Aluminum Measurements with Published Data
Coating prepared at Lincoln Laboratory.
Coating obtained commercially.
Coating obtained from Hass, Fort Belvoir.
The Bennett (published) data were obtained by graphical interpolation of his data as given in Refs. 6–8.
Tables (2)
Table I
Measured Reflectivities on Commercially Available Reflectors at 10.6 μa
Reflectivity
Comments
Reflectivity
Comments
Gold reflector
“Enhanced” metal reflector
0.9849
0.9908
0.9867
0.9894
0.9863
}
Same source
0.9921
}
Same source
0.9860
0.9917
0.9881
}
Same source
0.9891
Glass substrate
}
Same source
0.9875
0.9904
Silicon substrate
0.9888
0.9910
Germanium substrate
0.9705
0.9933
}
Same source
0.9876
With thin quartz overcoat
0.9922
0.9709
With thin germanium overcoat
0.9792
0.9915
Solid copper substrate
0.9789
0.9866
Solid germanium substrate
0.9791
Aluminum reflector
All-dielectric reflector
0.9831
}
Same source—aged and fast evaporation in H. V.
0.9907
}
Same source
0.9835
0.9867
0.9710
Aged
0.9782
0.9771
Aged
0.9778
0.9734
With SiO overcoated (aged) 2.5 cm × 7.6 cm
0.9579
0.9840
Fresh coating
Silver reflector
0.9589
0.9693
}
Same source
0.9692
0.9661
0.9888
Fresh coating
All substrates, unless otherwise stated, were optically flat (λ/10) glass, 5-cm diam, with a laser quality smooth polish. Those labeled same source were nominally exactly the same evaporated coating although not necessarily made at the same time.
Table II
Comparison of Gold and Aluminum Measurements with Published Data
Coating prepared at Lincoln Laboratory.
Coating obtained commercially.
Coating obtained from Hass, Fort Belvoir.
The Bennett (published) data were obtained by graphical interpolation of his data as given in Refs. 6–8.