Neutral grey filters
Neutral grey filters are something like sunglasses for the camera system. The intensity of all wavelengths is more or less evenly dampened. A uniform reduction of the image brightness avoids the overexposure of the scene. From a particular light quantity on, the camera system is no longer capable to respond properly by reducing the exposure time. This applies mainly to CCD sensors which may produce overexposure effects on the entire image, blooming and smear effects due to a short exposure time. Extreme stopping down of the optics is often not sufficient for the required brightness reduction and in case of extremely high focal ratios and dot-shaped bright sources of light, very strong visible diffraction effects (diffraction at the slit in the form of radial effects) appear. All these effects would significantly impair the evaluation.
Extremely bright image situations occur, for instance, when:
- monitoring welding processes
- evaluating melting processes (furnaces)
- monitoring combustion processes (in engines, etc.)
- inspecting sources of illumination
Filter effect of the neutral grey filter
Neutral grey filters with absorption of up to 20 f-stops are available in trade, corresponding to a filter effect of maximally 1 : 1,000,000. Instead of the indication of the values in f-stops (each f-stop on the internal aperture scale results in a duplication of the exposure time), you can also find the indication of the optical density. The optical density (extinction) is the negative decadal logarithm of the transmission rate.
A filter effect of:
- 50% corresponds to one f-stop or an optical density of 0.3
- 10% corresponds to an optical density of 1
- 1% results in an optical density of 2
- 0.1% corresponds to an optical density of 3
This filter effect can be achieved by means of absorbing glasses or reflection layers. Neutral grey filters are therefore suitable for monochrome and colour cameras, as all wavelength ranges are (similarly) absorbed.
A progressively working "grey filter" can also be achieved by mounting two polarising filters onto the lens which are turned in opposite directions.