Color Glass Filters

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Color Glass Filters	BACK


SPECIFICATIONS
Material:		Azure colored glass
Size tolerance:		±0.15 mm
Thickness:		3 mm(nominal)
Surface quality:		80 - 50
parallelism:		2 arc min (0.6 mrad)
Max .temperature		250 °C

TRANSMITTANCE OF COLORED GLASS FILTERS
As with any transparent optical material,colored filter glass has surface reflectance is given by:
		R = (n0-n1/n0+n1)*(n0-n1/n0+n1)……（1）
	Where:
		R = Reflectance no = index of refraction of air n1 = index of refraction of the colored glass, typically 1.51-1.56.
Therefore,R ≈ 0.04 per surface for a typical colored glass in air.The total reflection loss for two surfaces is approximately 0.08 or 8%.

The transmittance curves on pages 10-6 to 10-15 do not include the surface reflection losses. They show the internal transmittance of the filter glass.To find the total transmittance through the filter (including the reflection losses), multiply the internal transmittance at any wavelength by the reflection correction factor. Values for these are given in the table listing the individual glasses.

Internal transmittance can also be used to determine the transmission at other glass thickness by:
		(Tt2)=(Tt1)t2/t1……(2)
	Where:
		Tt1 = Internal transmittance at λ for thickness t1 Tt2 = Internal transmittance at λ for thickness t2

COMBINING COLORED GLASS FILTERS
You can combine colored glass filters in series to change the band pass or increase the attenuation.The internal transmission at any given wavelength is the product of the internal transmission of each filter at that wavelength.(Note the term internal transmittance is used; surface reflections must be accounted for separately.) If two colored glass filters are used in series,the total transmission(TT)through the two filters must be corrected for reflection loss by:
		TT=(T1)(R1)(T2)(R2)
	Where:
		T1 = Internal transmission of the first filter R1 = Reflection loss factor for the first filter T2= Internal transmission for the second filter R2=Reflection loss factor for the second filter

If the bandpass filter transmits the signal and the radiation transmitted outside the bandpass is "noise,"then the signal to noise ratio can often be improved by using several filters of same type.

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