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Out-of-Band Transmission (Blocking) A passband filter is designed to allow light within a specified region to pass while effectively blocking unwanted wavelengths both longer and shorter than the central wavelength. The ratio of in-band transmission (desired) to the out-of-band transmission (undesired) is called the signal to noise ratio. For sophisticated instruments, it is the single most important parameter to be considered when specifying a filter and is considered extremely important by us at SpectroFilm. A variety of terms are used to specify out-of-band transmission (blocking) and they compare as follows: Because there is a certain amount of fluctuation in the transmission out-of-band, the terms above are generally specified as an average within a range of wavelengths or as a maximum allowable level. This range of blocking is generally a combined consideration of the light source (sun, lamp, laser, spark, to name a few) and the spectral response region of the detector being used (Film, Eye, PMT, photovoltaic, etc). All sources vary in intensity versus wavelength. Some like the tungsten filament lamp emit in broad ranges of wavelengths whereas others emit discreet lines at specific wavelengths. Variation in intensity can also change as a function of the temperature of the filament. Detectors also vary greatly from one to another and can have certain variances from unit to unit depending on type. Sensitivity and range vary with detector. Angle Of Incidence A filter is always a part of an optical system. As light is manipulated, turned, and focused, the angle at which the light impinges upon the filter may affect how the filter performs. Most filters are used in systems where the light is not collimated. The angle may be as small as a few milliradians to as much as 45 degrees. The CWL of the filter will shift toward shorter (blue) wavelengths as the angle of incidence increases and is a function of the effective index of refraction of the materials used in the construction of the interfering layers. The lower the numerical value of the index, the faster the shift. Figure 9. Half Bandwidth (HBW) vs. maximum diameter (of one piece in the visible) 1 \| 2 \| 3 \| 4 \| 5 \| 6 \| 7 \| Next page – 8 \| 9 \| 10
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Out-of-Band Transmission (Blocking)

A passband filter is designed to allow light within a specified region to pass while effectively blocking unwanted wavelengths both longer and shorter than the central wavelength. The ratio of in-band transmission (desired) to the out-of-band transmission (undesired) is called the signal to noise ratio. For sophisticated instruments, it is the single most important parameter to be considered when specifying a filter and is considered extremely important by us at SpectroFilm. A variety of terms are used to specify out-of-band transmission (blocking) and they compare as follows:

out-of-band transmission table

Because there is a certain amount of fluctuation in the transmission out-of-band, the terms above are generally specified as an average within a range of wavelengths or as a maximum allowable level.

This range of blocking is generally a combined consideration of the light source (sun, lamp, laser, spark, to name a few) and the spectral response region of the detector being used (Film, Eye, PMT, photovoltaic, etc).

All sources vary in intensity versus wavelength. Some like the tungsten filament lamp emit in broad ranges of wavelengths whereas others emit discreet lines at specific wavelengths. Variation in intensity can also change as a function of the temperature of the filament.

Detectors also vary greatly from one to another and can have certain variances from unit to unit depending on type. Sensitivity and range vary with detector.

Angle Of Incidence

A filter is always a part of an optical system. As light is manipulated, turned, and focused, the angle at which the light impinges upon the filter may affect how the filter performs. Most filters are used in systems where the light is not collimated. The angle may be as small as a few milliradians to as much as 45 degrees.

The CWL of the filter will shift toward shorter (blue) wavelengths as the angle of incidence increases and is a function of the effective index of refraction of the materials used in the construction of the interfering layers. The lower the numerical value of the index, the faster the shift.

Figure 9. Half Bandwidth (HBW) vs. maximum diameter (of one piece in the visible)

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SpectroFilm is the leading supplier of standard and custom OEM laser diode and optical passband filters.