Photometry (OPtical) – Wikipedia, free encyclopedia

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The photometry It is the science that is responsible for the measure of light, such as the brightness perceived by the human eye. [ first ] That is, study the ability of electromagnetic radiation to stimulate the visual system. It should not be confused with radiometry, in charge of the measure of light in terms of absolute power.

The human eye and photometry [ To edit ]

Fotopic (black) luminosity function and cracopic luminosity function [first] (green). Fotopica includes standard CIE 1931 [2] (solid), the modification of Judd-Vos of 1978 [3] (Script line), and Sharpe data, Stockman, Jagla & Jägy of 2005 [4] (dash line). It shows the relative sensitivity of the eye to the different wavelengths. The horizontal axis is the wavelength in nanometers.

The human eye does not have the same sensitivity for all wavelengths that form the visible spectrum. Photometry introduces this fact by pondering the different radiometric magnitudes measured for each wavelength by a factor that represents the sensitivity of the eye for that length. The function that introduces these pesos is called Luminosity Function or Relative Luminous Efficiency Function of a model eye, which is usually denoted as

Vλ{displaystyle scriptstyle {V_{lambda }}}

,

V(λ){displaystyle scriptstyle {V(lambda )}}

O

y¯(λ){displaystyle scriptstyle {{bar {y}}(lambda )}}

(This standard model or observer is very similar to those of colorimetry). This function is different depending on whether the eye is adapted to conditions of good lighting (photopic vision) or evil (clay vision). Thus, in photopic conditions, the curve reaches its peak for 555 Nm, while in clay conditions it does for 507 nm.

Relationship with radiometry [ To edit ]

Consider, for example, the radiometric magnitude of radiant energy,

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Qe{displaystyle scriptstyle {Q_{e}}}

, which describes the total energy measured by a “physical” detector in the presence of electromagnetic radiation. It is interesting to obtain a measure of the amount of light that would be perceived by a human eye, regardless of the amount of energy associated with: quantity of light,

QV{displaystyle scriptstyle {Q_{V}}}

. To do this, the radiant energy corresponding to each wavelength should be known, multiply by the corresponding values ​​of the light function and integrate all wavelengths:

Spectral radiant energy,

QE(λ){displaystyle scriptstyle {Q_{E}(lambda )}}

, is measured in the SI in J/M units. Instead,

QV{displaystyle scriptstyle {Q_{V}}}

It is measured in lm · s. The K constant takes the value of 683 lm/w in photopic conditions and 1700 lm/W for squeeze conditions.

Luminous efficacy [ To edit ]

The luminous efficacy of a radiation is defined as the quotient between a photometric magnitude, for example

QV{displaystyle scriptstyle {Q_{V}}}

, and the corresponding radiometric magnitude, in this case

QE{displaystyle scriptstyle {Q_{E}}}

. So:

For example, the luminous efficacy of an infrared laser would be equal to 0 lm/w, while that of monochromatic light at 555 nm would be 683 lm/w (for this case,

Kr=K{displaystyle scriptstyle {K_{r}=K}}

).

Main photometric magnitudes [ To edit ]

The following table collects the main photometric magnitudes and its unit of measure:

Candela is a basic unit of SI. The remaining photometric units can be derived from basic units.

References [ To edit ]

  1. ↑ Michael Bass (ed.), Handbook of Optics Volume II – Devices, Measurements and Properties, 2nd Ed. , McGraw-Hill 1995, ISBN 978-0-07-047974-6 pages 24-40 through 24-47
  • Wyszzecki, Günter. Stiles, W. S. (1982). Color Science: Concepts, Methods and Formulae . New York: John Wiley & Sons. ISBN 0-471-02106-7 .

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