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Transmittance is characterized by both its internal transmittance and color code.

Internal transmittance refers to transmittance measured by excluding reflection losses at the entrance and exit surfaces of a glass for a given thickness such as 5mm or 10mm with a wavelength range from 280nm to 2550nm. Optical glasses have an excellent transmittance from the visible region to the near IR region.

Figure 1 shows the transmittance vs. wavelength of N-BK7.




Figure 1: N-BK7 internal transmittance curve


To determined color code which lists the wavelength λ80 and λ5, the internal transmittance curve of a sample with thickness 10±0.1mm is measured from UV to visible range. λ80 is the wavelength for which the glass exhibits 80% transmittance while λ5 is the wavelength at which the glass exhibits 5% transmittance. The two wavelengths are rounded to the nearest 10nm. For example, N-BK7 has a color code 33/29, which means λ80=330nm and λ5=290nm.


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