VLT(Visible Light Transmission)
The attenuation of light by a filter is defined in the visible spectrum by the so-called VLT (visible light transmission), the daylight transmission or the luminous transmittance. The VLT is determined in relation to the standard illuminant D65 (see ISO/CIE 10526:1991 and ISO/CIE 10527:1991) and evaluated according to the spectral sensitivity of the eye to daylight.
Should the measured VLT-value be less than 20 %, the user should ensure that their working environment receives additional illumination (EN 207). With a low VLT and bad illumination the colour vision is restricted and the spectral sensitivity of the eyes VN(λ) moves towards the shorter wavelengths.
Color Vision
The eyes can adapt to different light situations and the total amount of light can be balanced by additional illumination. Therefore another important aspect for the selection of a laser safety filter is colour vision. If colour vision is impaired or restricted, some colours may not be recognized. This effect may also apply to warning lights or displays, or the ability to distinguish between instruments or vessels marked by colour such as those found in medical surroundings.
Glass and Plastic Absorption Filters
Absorbing materials are the common standard in order to block light of a defined wavelength area from the spectrum. As basic materials special optic filter glass or amorphous polymers with special absorber dyes are mostly used. Absorption means that the light energy of the specified wavelength for which the filter is designed for is transformed to heat, when the beam hits the filter. Therefore it is necessary to select the filter material carefully for thermal stability in order to protect for a direct hit.
Plastic filters will carbonize at high power densities and can be quickly penetrated, whereas Glass filters will break due to the thermal distortion in case of exceeded damage threshold. That’s why LASERVISION glass filters delivered by ASCLEPION are enhanced by splinter protection (lamination with neutral glass) by default to avoid injuries in case of a hazardous situation. In addition, even a cracked filter will keep the required Optical Density as long as the pieces remain together. Glass filters are not only of higher value but are clearly superior to plastic filters in terms of thermal stability of the filter material itself as well. Therefore they are especially suitable for continuous lasers of medium to high power.
Reflection filters (interference layers on glass or platic substrates)
The much more advanced but technically more complicated technology is the coating of a substrate with dielectric interference layers. For the so called blocking laser wavelength, a constructive multiple reflection is achieved and the filter reflects nearly all of the laser light. Due to the long standing experience and research in the field of coating technology LASERVISION is able to offer these coatings on glass as well as on acrylic substrate and now it is possible to achieve with this technology even with plastic filters high protection levels, which have been available in the past only with glass filter technology.
Comparison
Compared to absorbing filters reflective coatings have many advantages. With exception of the blocked wavelength nearly all light passes the filter without attenuation. Therefore, compared to any absorbing filter in the visible, coated filters have a much better colour vision and higher visible light transmission as it is especially important in medical applications. Additionally coated filters are clearly of lower weight due to the absence of thick absorbing glass filters, which gives a much better wearing comfort. |
