and the meter provided for "measurement of the effective illumination obtaining at the position of the subject." A uniform perfect diffuser (one following Lambert's cosine law) of luminance L {\displaystyle L} emits a flux density of π {\displaystyle \pi } L {\displaystyle L} ; reflectance then is

Lohse, Bernhard; Stille, Ulrich [in German] (January 1948) [1947-08-19]. Written at Braunschweig, Germany. Deutsche Physikalische Gesellschaft (ed.). "Einführung und Bestimmung des Lichtäquivalents". Zeitschrift für Physik (in German). Berlin / Göttingen / Heidelberg, Germany: Springer-Verlag. 125 (1–3): 133–158. Bibcode: 1948ZPhy..125..133L. doi: 10.1007/BF01337623. ISSN 0044-3328. S2CID 125512557 . Retrieved 2023-03-19. The peak of the luminosity function is at 555 nm (green); the eye's image-forming visual system is more sensitive to light of this wavelength than any other. For monochromatic light of this wavelength, the amount of illuminance for a given amount of irradiance is maximum: 683.002lx per 1W/m 2; the irradiance needed to make 1lx at this wavelength is about 1.464 mW/m 2. Other wavelengths of visible light produce fewer lux per watt-per-meter-squared. The luminosity function falls to zero for wavelengths outside the visible spectrum. The illuminance provided by a light source on a surface perpendicular to the direction to the source is a measure of the strength of that source as perceived from that location. For instance, a star of apparent magnitude 0 provides 2.08 microlux (μlx) at the Earth's surface. [16] A barely perceptible magnitude 6 star provides 8 nanolux (nlx). [17] The unobscured Sun provides an illumination of up to 100kilolux (klx) on the Earth's surface, the exact value depending on time of year and atmospheric conditions. This direct normal illuminance is related to the solar illuminance constant E sc, equal to 128 000lux (see Sunlight and Solar constant). Reflected-light meters measure the light reflected by the scene to be photographed. All in-camera meters are reflected-light meters. Reflected-light meters are calibrated to show the appropriate exposure for "average" scenes. An unusual scene with a preponderance of light colors or specular highlights would have a higher reflectance; a reflected-light meter taking a reading would incorrectly compensate for the difference in reflectance and lead to underexposure. Badly underexposed sunset photos are common exactly because of this effect: the brightness of the setting sun fools the camera's light meter and, unless the in-camera logic or the photographer take care to compensate, the picture will be grossly underexposed and dull. One nox(nx) equals 1 millilux (1 mlx) at light color 2042K or 2046K (formerly 2360K). [19] [20] [21] [22]A foot-candle is equal to the total intensity of light that falls on a one square foot surface that is one foot away from the point source of light. Lux Meter Working Principle We conducted the reference measurements with an Illuminance meter from PRC Krochmann (Model 106e, special model, class A). And of course with valid retraceable calibration.

Extinction meters tended to provide inconsistent results because they depended on subjective interpretation and the light sensitivity of the human eye, which can vary from person to person. [7] Photoelectric types [ edit ] Analog handheld light meter - Gossen Lunasix 3 (in US: Luna Pro S); available from 1961 to 1977 The next exposure meters, developed at about the same time but not displacing actinometers in popularity until the 1920s and 1930s, are known as extinction meters, evaluating the correct exposure settings by variable attenuation. [4] One type of extinction meter contained a numbered or lettered row of neutral density filters of increasing density. The photographer would position the meter in front of their subject and note the filter with the greatest density that still allowed incident light to pass through. In another example, sold as Heyde's Aktino-Photometer starting from the early 1900s, the photographer views the scene through an eyepiece and turns the meter to vary the effective density until the scene can no longer be seen. [6] The letter or number corresponding to the filter strength causing the "extinction" of the scene was used as an index into a chart of appropriate aperture and shutter speed combinations for a given film speed. [3] :72 The lux is one lumen per square metre (lm/m 2), and the corresponding radiometric unit, which measures irradiance, is the watt per square metre (W/m 2). There is no single conversion factor between lux and W/m 2; there is a different conversion factor for every wavelength, and it is not possible to make a conversion unless one knows the spectral composition of the light. Hi friends, in this article, I am going to discuss about the lux meter working principle and its application. Lux meters are used to measure levels of light in schools, hospitals, production areas, laboratories. They are also used to monitor light-sensitive displays in museums, art galleries etc. Illumination (symbol E)Since light usually contains different colors different wavelengths, the reading represents the combined effects of all the wavelengths. Typically, standard colors or color temperatures are expressed in degrees Kelvin (K). The standard color temperature for the calibration of most lux meters is 2856°K, an amount that is more yellow than pure white. difference algorithm, where lights are switched on at a lower light level than they switch off, thus taking ensuring the difference between the light level of the 'on' state and 'off' state is not too big

With a K {\displaystyle K} of 14, the reflectance would be 17.6%, close to that of a standard 18% neutral test card. In theory, an incident-light measurement should agree with a reflected-light measurement of a test card of suitable reflectance that is perpendicular to the direction to the meter. However, a test card seldom is a uniform diffuser, so incident- and reflected-light measurements might differ slightly. In practice, additional complications may arise. Many neutral test cards are far from perfectly diffuse reflectors, and specular reflections can cause increased reflected-light meter readings that, if followed, would result in underexposure. It is possible that the neutral test card instructions include a correction for specular reflections. Find sources: "Light meter"– news · newspapers · books · scholar · JSTOR ( September 2012) ( Learn how and when to remove this template message)Illuminance is measured with a flat receptor. It is straightforward to compare an incident-light measurement using a flat receptor with a reflected-light measurement of a uniformly illuminated flat surface of constant reflectance. Using values of 12.5 for K {\displaystyle K} and 250 for C {\displaystyle C} gives

a b Pears, Alan (June 1998). "Chapter 7: Appliance technologies and scope for emission reduction". Strategic Study of Household Energy and Greenhouse Issues (PDF). Department of Industry and Science, Commonwealth of Australia. p.61. Archived from the original (PDF) on 2011-03-02 . Retrieved 2008-06-26. {{ cite book}}: |work= ignored ( help)

Product overview: Fluke 941 Light Meter

A lux is equal to the total intensity of light that falls on a one square meter surface that is one foot away from the point of source of light. Foot Candle or Lumens / Sq. Foot: It is defined as the illumination produced by a uniform source of 1 C.P. on the inner surface of a sphere of 1-meter radius. How to use a lux meter (Australian recommendation)" (PDF). Sustainability Victoria. April 2010. Archived from the original (PDF) on 2011-07-07.