Optical-physical Effects in connection with photo filters, Jos. Schneider Optische Werke

Optical-physical Effects¹⁾

Fluorescence Reflection Re-radiation Absorption Polarization

Many materials light up when exposed to ultraviolet light in many different colors. The reason for this is the energy loss of ultraviolet light in the material exosed. If one wishes to shoot flourescent light, the visible light in front of the camera must be absorbed by B+W black filter 403. The fluorescent colors are then clearly visible. The exciting UV radiation must be cut out before reaching the lens otherwise high focus loss would result.

Reflection, reflected light

Reflection is the beaming back of light by mirrors, water surfaces and so on. The effect of reflected ligth - from window panes and areas of water - is of course wellknown. Its elimination by means of a polarizer is discussed in the section on Polarizers. One has to be particularly careful with mirrors. These reflect 85-95% of incident light, reflect thus 10 times brighter than a window pane. Since the metallic coating of mirrors polarizes weakly, elimination of reflections cannot be achieved with a polarizing filter. This also applies to reflecting metal surfaces. High lights behave in a similar manner to reflections, they too can cause ghosting effects and lack of focus because of uncontrolled light reaching the lens.

Re-radiation ( diffuse reflectance )

Re-radiation is the term for the beaming back of light from non-reflecting or shiny surfaces. It only affects the shot negatively if one color prevails. It is possible to determine color cast by looking at the scene without the use of any special devices. Dominant colors making up 50% of the mitif produce color cast.

Re-radiation light is for example, the green of plants, the gray of an asphalt road, the white of an expance of snow or the red of a blossom. The same applies to every dominant color in a motif, all objects have their colors falsified by the reflected light, the picture has a color cast.

Absorption

In the optical sense, the property of a material, for example, a filter, to keep out light. Almost every object absorbs light whether it be gaseous, fluid or solid. Only white, gray and black objects absorb all rays evenly. Colored objects, on the other hand, only absorb certain light colors. For example, green leaves absorb red and blue light in order to activate biochemical plant growth processes. Green light is reflected as it is not required.

The absorption of radiation is the result of electromagnetic oscillation. Radiation is not eliminated, however, but leads to chemical changes and physical phenomena such as fluorescence and phosphorescence or to a temperature rise in the absorbing material.

Polarization

In most cases, light does not conform to any law in its oscillation. A polarizer ( polarizing filter ) can be used to fade out a chosen plane of this unpolarized light. The light then only oscillates in one plane. In this way, it is possible to intensify contrast.

A Polarizer can be imagined as a device with many bars grid-like arranged and close together. The polarizing coating only works if the molecules or crystals can be brought into a uniform direction by extension or pulling from one side. If the grid is vertical, only particles in a vertical plane are allowed to pass, those particles with horizontal oscillation will be withheld due to their large dimension in this plane. So, only those photons travelling in the favored plane will be able to pass.

A polarizer differs from a color filter therefore because it absorbs light oscillation planes instead of light wave-lengths ( light colors ).

¹⁾"B+W-Filters and special effects attachments: their correct use for better photos and films" by Arnold Mellert u. Dietrich Oppitz, 1978, ISBN 3-7949-0319-6 (out-of-print), by courtesy of Fachverlags Schiele&Schön

Optical-physical Effects 1)

Optical-physical Effects¹⁾