Luminance factor

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The luminance factor ( English luminance factor ) is used to evaluate the visual dispersion characteristics ( reflection behavior) of surfaces ( reflective vests , warning triangles , warning signs , road signs or video walls ). It is defined as: ${\displaystyle \beta }$

${\displaystyle \beta ={\frac {L_{M}}{L_{S}}}}$

With

• the luminance of the surface to be assessed (sample) ${\displaystyle L_{M}}$
• the luminance of an ideally scattering and at the same time ideally reflective surface (standard); Such an idealized surface that exists only in theory is called a white standard or white normal . ${\displaystyle L_{S}}$

In order to describe the full reflection behavior of a surface, luminance factors are required depending on the type and direction of illumination, the type and direction of observation and the spectral distribution , e.g. B.

• for diffuse illumination with observation below 8 ° (d / 8 °) and a spectral distribution according to standard illuminant A or
• for circumferential lighting below 45 ° with perpendicular observation (45 ° c / 0 °) and a spectral distribution with daylight .

This assignment is graphically illustrated in the form of a luminance factor indicatrix.

Fluorescent materials

Surfaces that serve to protect against hazards (e.g. safety vests) must be treated with fluorescent materials so that when illuminated with daylight (D65) the luminance factor is considerably increased and visibility is therefore considerably improved . The above general formula is diversified with the help of a fluorescent ( ) and a reflective component ( ) for the total luminance factor ( ): ${\displaystyle f}$ ${\displaystyle r}$ ${\displaystyle \beta _{t}}$

${\displaystyle {\begin{alignedat}{2}\beta _{t}={\frac {L_{M}}{L_{S}}}&={\frac {L_{\mathrm {M,r} }+L_{\mathrm {M,f} }}{L_{\mathrm {S,r} }}}\\&={\frac {L_{\mathrm {M,r} }}{L_{\mathrm {S,r} }}}&&+{\frac {L_{\mathrm {M,f} }}{L_{\mathrm {S,r} }}}\\&=\beta _{r}&&+\beta _{f}\end{alignedat}}}$

White standards fluoresce by definition not : . ${\displaystyle L_{\mathrm {S,f} }\mapsto 0}$

In the case of the above (e.g. orange) safety vest, the UV radiation contained in the lighting spectrum and the blue / turquoise / green part of the light are converted into the yellow / orange / red range by the fluorescent materials and emit more light in this spectral range ( ) ; When illuminated with standard illuminant A, this effect does not / hardly occurs, so that the energy conversion does not take place ( ). This is why it is prescribed in the relevant regulations and standards for safety vests ${\displaystyle \beta _{f}>>0}$ ${\displaystyle \beta _{f}\mapsto 0}$

• for illumination with daylight a luminance factor of ${\displaystyle \beta \geq 30\%}$
• when illuminated with standard illuminant A (in this case so-called night light) a luminance factor of . ${\displaystyle \beta \geq 3\%}$

The energy conversion can lead to a shift in the color location and thus to a change in the color impression .

Surface structure

Surfaces that are not allowed to change the color point of the lighting spectrum, but should nevertheless achieve an increase in the luminance factor, cannot be brightened with fluorescent materials. However, it is possible to process the surface by means of microstructuring and thus to achieve preferred directions ( e.g. velvet ). A luminance factor of = 1.5 means that the evaluated surface emits 50% more light than a white standard with a specific illumination and a certain observation geometry as well as a defined reference spectrum. The light is preferentially emitted / "amplified" in this direction. Luminance factors above 100% are not uncommon. In the most natural case, a preferred direction of the surface for light amplification can result from total reflection . Any other species is conceivable ( cat eyes etc.). ${\displaystyle \beta }$

Examples

• Warning triangles according to UN / ECE  R 27
• Warning signs
  • according to UN / ECE R 69
  • according to UN / ECE R 70
• Contour marking according to UN / ECE R 104
• Safety vests according to ISO  EN  20471
• Projection screens.

Web links

• Patrick Wagner: The luminance factor (gain factor). In: filmscanner.info. 18th January 2019 .