Visual photometry

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The visual photometry based on the principle of comparative measurements, wherein an unknown quantity of a light source is compared by a specified size. Basically, visual (subjective) photometry is differentiated from physical (objective) photometry. The aim of photometry is to make a transition of radiometric radiation into visible light describable by means of calculable metrics. For this purpose, it makes use of suitable sensitivity functions, which represent the visual perception of the human eye through measurement parameters.

Emergence

The origins of light measurement technology are based on the interaction of different researchers and research areas. The methods and working methods are as different as the scientific areas involved. In many cases, however, it can be seen that early writings and methods build on one another or that research results were shared. In this way, an independent branch of science could develop from the individual approaches and ideas of individuals in the course of the 17th to 19th centuries.

The first basics of visual photometry were published by Johann Heinrich Lambert in his "Photometria" in 1760. As early as 1726, Pierre Bouguer described the contrast sensitivity of the human eye as a measuring instrument in his "Essai d'optique, sur la gradation de la lumière" . Bouguer and Lambert are now considered the founding fathers of visual photometry. At the end of the 18th century , the lamps and headlights used began to become more economical. The foundations for efficient lighting technology had long been in place when photometry was still in its early stages. One example is the Agrand lamp, which had a significantly lower consumption compared to contemporary oil lamps. The development of lighting technology brought with it the need for measurability and comparability of light sources. The lighting technology was in turn driven by the newly emerging branches of industry, which wanted to continue their activities even after dark.

Due to the advancing developments in optics and precision mechanics , precise photometric measurements were ultimately necessary. With the spread of the gas flame in the 19th century, there was an increased focus on the subject of photometry. The first light measuring devices (photometers) were based on suggestions from Lambert and Bouguer. As a result, a large number of photometric measuring instruments were created. Examples of historical photometers can be found in subsection 4.

Basics

The human eye was the only available measuring instrument for visual photometry. The comparative measurements carried out on this basis can certainly not be regarded as absolute. The visual photometry must define a light standard and define measurement conditions for the comparison. In Germany, the Hefner candle had functioned as a primary light unit since 1898. Fixed conditions were necessary in order to guarantee the reproducibility of the light measurement in addition to a minimum level of accuracy. Unavoidable measurement inaccuracies can be seen, for example, with heterochromic brightness methods. Significant fluctuations were unavoidable, particularly at the respective ends of the visible spectrum.

The measurements were preferably carried out as a simple comparison process (so-called direct comparison). The sizes could be determined more precisely by means of contrast comparison. Both methods were only able to provide usable results with isochromic or slightly heterochromic light sources. Contrast comparison, on the other hand, is a somewhat more precise method. However, both methods only led to usable results with isochromic or slightly heterochromic light sources. A comparison of colored light with a white reference source always showed an overestimation of the highly saturated light sources in terms of their luminance by up to 57%. A successive comparison, such as that used in the Bechstein flicker photometer, proved to be more suitable for this.

meaning

With the development of photoelectric receivers, the foundations of today's physical photometry arose. Due to the spectral distribution of the light, the light measuring instruments must be adapted to the light sensitivity of the human eye. Usually the V (λ) function published in 1924 by the International Commission on Illumination (Commission Internationale de l'Éclairage; CIE) is used.

The sensitivity curves used to calibrate current measuring devices were created using visual photometry. The eye sensitivity values ​​determined in this way currently have to be adjusted many times under changed conditions. The sensation metrics of the physical photometry commonly used today cannot compete with the high level of visual photometry for many photometric quality factors.

The comparative methods of visual photometry represent the basis of today's light measurement technology and made it possible to manufacture physical receivers around 1920. Initially, photocells , then selenium photocells or photoresistors, served as electrical receivers. The current light measurement is based exclusively on physical photometry. Semiconductor sensors usually function as optical receivers. With the help of suitable filters, the receivers can be adapted to the V (λ) curve based on visual photometry with almost any precision. The task of the physical sensors is to imitate the perception and evaluation function of the human eye.

Visual photometer

Examples of historical light measuring devices are visual photometers, which mainly come from the late 19th and early 20th centuries.

  • Bunsen grease spot photometer
  • Lummer-Brodhun photometer cube
  • Pocket photometer according to Bechstein
  • Martens polarization photometer
  • Weber milk glass photometer

supporting documents

Individual evidence

  1. ^ Gall, Dietrich: The visual photometry and its importance for current evaluation criteria, 2006, p. 1 .
  2. ^ Johnston, Sean F .: A History of Light and Color Measurement, 2003, p. 12 .
  3. ↑ Hentschel, Hans-Jürgen .: Light and lighting. Basics and applications of lighting technology, 2002, p. 73 .
  4. ^ Gall, Dietrich .: The visual photometry and its importance for current evaluation criteria, 2006, p. 2 .
  5. ↑ Hentschel, Hans-Jürgen .: Light and lighting. Basics and applications of lighting technology, 2002, p. 73 .

literature

  • JOHNSTON, SEAN F .: A History of Light and Color Measurement, IOP Publishing Ltd., London, 2003
  • GALL, DIETRICH: The visual photometry and its importance for current evaluation criteria, LICHT conference, Bern, 2006
  • HENTSCHEL, H.-J., (Ed.): Light and lighting, basics u. Application of lighting technology, Hüthig, 2002
  • HOLZINGER, ANDREAS: From wax candles to light bulbs, Harri Deutsch publishing house, Frankfurt, 1998
  • LUX, HEINRICH: Modern lighting, BG Teubner, Leipzig, 1914
  • SCHIVELBUSCH, WOLFGANG: Lichtblicke - On the history of artificial brightness in the 19th century, Fischer Verlag, Frankfurt, 2004
  • WYSZECKI G. & STILES WS: Color Science - Concepts and Methods, Qualitative Data and Formulare, 2nd edition, John Wiley and Sons, 2000