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Mesopic Photometry

What is photometry?
Why do we need mesopic photometry?
Where do we need mesopic photometry?
What is performance based mesopic photometry?

 

What is photometry?

Photometry is the measurement of radiant power weighted according to the spectral sensitivity of the human eye. The spectral sensitivity is described by spectral luminous efficiency functions, separately defined for different light levels. At photopic light levels the spectral luminous efficiency function, namely the V(λ) function, is generally used. At scotopic conditions, the human spectral sensitivity is described by the V'(λ) function. However, for mesopic light levels, no official luminous efficiency function or family of functions is available.

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Why do we need mesopic photometry?

The photopic luminous efficiency function V(λ) was established by CIE (International Commission on Illumination) in 1924 and at present, it still forms the basis of all lighting dimensioning and photometry. However, it is generally known that in the mesopic region (light levels between approximately 0.001 and 10 cd/m2 ) the use of V(λ) alone as the basis of photometry results in sizeable errors in the assessment of light. This is due to the fact that human spectral sensitivity changes with decreasing light levels (Purkinje shift). The use of an inappropriate spectral luminous efficiency function in the mesopic region may lead to large economic as well as energy losses, not to mention safety issues for example in night time driving conditions.

It is acknowledged in CIE publication N° 41 (Light as a true visual quantity: principles of measurement, 1978) that: Since the luminous efficiency function of the human eye is known to vary with a wide variety of viewing conditions, the assessment of radiant power can give accurate values only when the measured light corresponds to conditions under which V(λ) was obtained .

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Where do we need mesopic photometry?

The most relevant mesopic lighting applications are street and road lighting and other outdoor lighting. But also, for example, emergency lighting, air traffic, and military applications are of great importance.

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What is performance based mesopic photometry?

The human spectral luminous efficiency functions are derived from psychophysical experiments, which measure spectral sensitivity with certain visual criteria and under defined set of conditions. The majority of the existing mesopic models are based on brightness matching, for example brightness comparison of two fixed lights in a bipartite photometric field. However, a link to visual tasks and the prediction of visual performance with the brightness based models is still deficient. It has recently been internationally acknowledged, that there should be more understanding of the factors that affect visual performance in the mesopic range and that these factors should be considered in new approaches of developing mesopic photometry. For example, in practical mesopic lighting applications, the detection and recognition of visual objects at or near visual threshold, and the reaction time needed to perceive objects in the visual field, are more relevant than the visual assessment of brightness. Mesopic photometry, based on experiments assessing these more relevant visual tasks, is called performance based mesopic photometry.

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