Expo­sure lev­el indi­ca­tor. In Man­u­al mode, the expo­sure lev­el indi­ca­tor shows how much your set expo­sure val­ues devi­ate from the lev­els the cam­era believes are opti­mal, which always reside at the stan­dard expo­sure index. (See Man­u­al Expo­sure).

This pitfall (but not in the setting-sun case) is avoided by incident-light meters which measure the amount of light falling on the subject using a diffuser with a flat or (more commonly) hemispherical field of view placed on top of the light sensor. Because the incident-light reading is independent of the subject's reflectance, it is less likely to lead to incorrect exposures for subjects with unusual average reflectance. Taking an incident-light reading requires placing the meter at the subject's position and pointing it in the general direction of the camera, something not always achievable in practice, e.g., in landscape photography where the subject distance approaches infinity.

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Polarization is the attribute that a wave's oscillations have a definite direction relative to the direction of propagation of the wave.

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Despite the neu­tral­i­ty of inci­dent-light mea­sure­ments, there are sev­er­al inher­ent dis­ad­van­tages you should know about. First, mea­sur­ing the inten­si­ty of light falling upon a dis­tant sub­ject may be dif­fi­cult, imprac­ti­cal, or impos­si­ble. For instance, an inci­dent read­ing of the night sky is impos­si­ble, of a sun­lit moun­tain from an out­look cov­ered by the canopy of trees is imprac­ti­cal, and of a sub­ject under com­plex or mot­tled light­ing is impre­cise. Sec­ond­ly, since inci­dent-light meters mea­sure sub­ject illu­mi­na­tion, the expo­sure val­ues they pro­vide for the aper­ture are in the­o­ret­i­cal f‑stops. If the trans­mis­sion val­ue of your lens is sig­nif­i­cant­ly dif­fer­ent from the the­o­ret­i­cal f‑number it’s set to, the expo­sures won’t be accu­rate (see F‑Stops and T‑Stops).

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

Every reflect­ed-light meter is cal­i­brat­ed to pro­vide an expo­sure read­ing that ren­ders the sub­ject it’s point­ed at as mid­dle grey. If you take a reflect­ed-light read­ing off a black square of paper, the meter will pro­pose expo­sure val­ues that will ren­der the paper mid­dle grey in the pho­to­graph, there­by over­ex­pos­ing it. If you take a reflect­ed-light read­ing off a white square of paper, the meter will pro­pose expo­sure val­ues that will ren­der the paper mid­dle grey in the pho­to­graph, there­by under­ex­pos­ing it. The only tone for which reflect­ed-light meters pro­vide objec­tive­ly accu­rate expo­sure val­ues is mid­dle grey.

Inci­dent-light meters. Inci­dent-light meters mea­sure the amount of light falling on the sub­ject and are only avail­able as off-cam­era, hand­held devices. The most promi­nent fea­ture of inci­dent-light meters is the translu­cent white hemi­spher­i­cal dome (the “lumi­sphere”) that both enclos­es and pro­vides even illu­mi­na­tion to the pho­to­cell with­in, which mea­sures the light’s inten­si­ty. To obtain an expo­sure read­ing, you hold the inci­dent-light meter at the posi­tion of the sub­ject, ensur­ing that the dome is in the same light, point the lumi­sphere towards the cam­era, and take a mea­sure­ment.

There are other types of specialized photographic light meters. Flash meters are used in flash photography to verify correct exposure. Color meters are used where high fidelity in color reproduction is required. Densitometers are used in photographic reproduction.

Some light meters also have the ability to provide a readout in many different units. Lux and footcandles are the common units for visible light, but so are Candelas, Lumens, and Candela per square meter. In the realm of disinfection, UVC is typically measured in watts per square centimeter, or watts for a given individual lamp assembly, whereas systems used in the context of the curing of coatings often provide readouts in Joules per Square centimeter. Regular measurements of UVC light intensity thus can serve to provide assurance of proper disinfection of water and food-preparation surfaces, or reliable coating hardness in painted products.

Meter calibration establishes the relationship between subject lighting and recommended camera settings. The calibration of photographic light meters is covered by ISO 2720:1974.

At best, a flat card is an approximation to a three-dimensional scene, and measurement of a test card may lead to underexposure unless adjustment is made. The instructions for a Kodak neutral test card recommend that the indicated exposure be increased by 1⁄2 step for a frontlighted scene in sunlight. The instructions also recommend that the test card be held vertically and faced in a direction midway between the Sun and the camera; similar directions are also given in the Kodak Professional Photoguide. The combination of exposure increase and the card orientation gives recommended exposures that are reasonably close to those given by an incident-light meter with a hemispherical receptor when metering with an off-axis light source.

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Reflect­ed-light meter­ing is how cam­eras mea­sure sub­ject bright­ness to deter­mine opti­mal expo­sure. Most dig­i­tal cam­eras offer sev­er­al meter­ing modes that fea­ture vary­ing degrees of sophis­ti­ca­tion. Regard­less of the meter­ing mode, it’s impor­tant to remem­ber that every reflect­ed-light meter, includ­ing your camera’s, is cal­i­brat­ed to pro­vide an expo­sure read­ing that ren­ders the sub­ject as a mid­dle grey tone. The most sig­nif­i­cant dif­fer­ence between the meter­ing modes is in their method of inter­pret­ing the dis­tri­b­u­tion and vari­a­tion of the scene’s tonal range.

The 18-per­cent, or mid­dle grey, stan­dard is the math­e­mat­i­cal aver­age of all tones even­ly dis­trib­uted across a scale from absolute black to absolute white. In dig­i­tal pho­tog­ra­phy, the mid­dle grey tone lies at the pre­cise mid­dle of the lumi­nance his­togram, and there are cor­re­spond­ing mid­dle tones for the red, green, and blue colour chan­nels. (All future ref­er­ences to 18-per­cent grey or mid­dle grey refer to the tone inde­pen­dent of its colour). When print­ed, a mid­dle grey tone reflects 18 per­cent of inci­dent light (i.e., light falling upon it). Pho­to­graph­ic expo­sures derived from meter­ing the aver­age of all tones in an aver­age scene are remark­able at obtain­ing aver­age results.

The incident-light calibration constant depends on the type of light receptor. Two receptor types are common: flat (cosine-responding) and hemispherical (cardioid-responding). With a flat receptor, ISO 2720:1974 recommends a range for C {\displaystyle C} of 240 to 400 with illuminance in lux; a value of 250 is commonly used. A flat receptor typically is used for measurement of lighting ratios, for measurement of illuminance, and occasionally, for determining exposure for a flat subject.

With a slightly revised definition of reflectance, this result can be taken as indicating that the average scene reflectance is approximately 12%. A typical scene includes shaded areas as well as areas that receive direct illumination, and a wide-angle averaging reflected-light meter responds to these differences in illumination as well as differing reflectances of various scene elements. Average scene reflectance then would be

In most cases, an incident-light meter will cause a medium tone to be recorded as a medium tone, and a reflected-light meter will cause whatever is metered to be recorded as a medium tone. What constitutes a "medium tone" depends on meter calibration and several other factors, including film processing or digital image conversion.

For determining practical photographic exposure, a hemispherical receptor has proven more effective. Don Norwood, inventor of incident-light exposure meter with a hemispherical receptor, thought that a sphere was a reasonable representation of a photographic subject. According to his patent (Norwood 1938), the objective was

A light meter (or illuminometer) is a device used to measure the amount of light. In photography, an exposure meter is a light meter coupled to either a digital or analog calculator which displays the correct shutter speed and f-number for optimum exposure, given a certain lighting situation and film speed. Similarly, exposure meters are also used in the fields of cinematography and scenic design, in order to determine the optimum light level for a scene.

Light meters also are used in the general field of architectural lighting design to verify proper installation and performance of a building lighting system, and in assessing the light levels for growing plants.

Cen­tre-weight­ed meter­ing. This meter­ing mode is designed pri­mar­i­ly for por­trait pho­tog­ra­phy. It con­sid­ers the aver­age bright­ness of the entire frame but gives the great­est promi­nence to the tones in the cen­tral region. Cen­tre-weight­ed meter­ing isn’t rec­om­mend­ed for sce­nar­ios in which mul­ti­ple shots of var­i­ous sub­jects are antic­i­pat­ed, as the expo­sures will fluc­tu­ate.

Later[when?] meters removed the human element and relied on technologies incorporating selenium, CdS, and silicon photodetectors.

Light measurementunit K

Range: for systems that are not linear and auto ranging this function allows the user to select the portion of the meter electronics that best handles the signal level in use.

When the word light meter or photometer is used in place of radiometer or optometer, or it is often assumed the system was configured to see only visible light. Visible light sensors are often called illuminance or photometric sensors because they have been filtered to be sensitive only to 400-700 nanometers (nm) mimicking the human eyes' sensitivity to light. How accurately the meter measures often depends on how well the filtration matches the human eyes' response.

In a typical scene, many elements are not flat and are at various orientations to the camera, so that for practical photography, a hemispherical receptor usually has proven more effective for determining exposure. Using values of 12.5 for K {\displaystyle K} and 330 for C {\displaystyle C} gives

The only thing a light meter can do is mea­sure the actu­al bright­ness of an object; it can make no assess­ment of whether the object is light or dark, in deep shade or in bright sun. It’s up to you to eval­u­ate and inter­pret the nature of your sub­ject accord­ing to the infor­ma­tion pro­vid­ed by the meter. By under­stand­ing how light meters work and are cal­i­brat­ed, you’ll be bet­ter able to use them to deter­mine the appro­pri­ate expo­sure for a scene.

Along with having a variety of features, a light meter may also be usable for a variety of applications. These may include the measurement of other bands of light such UVA, UVB, UVC and Near IR. For example, UVA and UVB light meters are used for phototherapy or treatment of skin conditions, germicidal radiometers are used for measuring the UVC level from lamps used for disinfection and sterilization, luminance meters are used to measure the brightness of a sign, display or exit sign, PAR quantum sensors are used to measure how much of a given light source's emission will help plants grow, and UV-curing radiometers test how much of the lights emission is effective for hardening a glue, plastic, or protective coating.

Units: For illuminance the units are typically only lux and foot-candles but many light meters can also be used for UV, VIS and IR applications so the readout could change to W/cm^2, candela, Watts etc.

Calibration of cameras with internal meters is covered by ISO 2721:1982; nonetheless, many manufacturers specify (though seldom state) exposure calibration in terms of K {\displaystyle K} , and many calibration instruments (e.g., Kyoritsu-Arrowin multi-function camera testers[11] ) use the specified K {\displaystyle K} to set the test parameters.

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Hand­held reflect­ed-light meters come in two broad vari­eties. Wide-angle reflect­ed-light meters take an aver­age read­ing from across a rel­a­tive­ly large area. Spot meters are reflect­ed-light meters that read the bright­ness from a rel­a­tive­ly small por­tion of the scene—typically one degree of your field of view. All hand­held spot meters fea­ture a mag­ni­fied viewfind­er with a clear­ly marked cir­cle that out­lines the meter­ing zone. Spot meters tend to be more expen­sive than both inci­dent- and wide-angle reflect­ed-light meters.

Exposure meters generally are sorted into reflected-light or incident-light types, depending on the method used to measure the scene.

Lightintensitymeasurementunit

Although a light meter can take the form of a very simple handheld tool with one-button operation, there are also many advanced light-measurement systems available for use in numerous different applications. These can be incorporated into automated systems that can, for example, wipe lamps clean when a certain reduction in output is detected, or that can trigger an alarm when lamp-failure occurs.

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.

Many modern cameras include sophisticated multi-segment metering systems that measure the luminance of different parts of the scene to determine the optimal exposure. When using a film whose spectral sensitivity is not a good match to that of the light meter, for example orthochromatic black-and-white or infrared film, the meter may require special filters and re-calibration to match the sensitivity of the film.

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Light measurementunit lux

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 constants K {\displaystyle K} and C {\displaystyle C} shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a large number of observers, of a number of photographs, for which the exposure was known, obtained under various conditions of subject manner and over a range of luminances.

Reflect­ed-light meters. Reflect­ed-light meters work by mea­sur­ing the light reflect­ed off the sub­ject, there­by mea­sur­ing the subject’s bright­ness. Reflect­ed-light mea­sure­ments are tak­en from the intend­ed posi­tion of the cam­era regard­less of whether the meter is on- or off-cam­era. Vir­tu­al­ly every dig­i­tal cam­era meters expo­sure using reflect­ed-light enter­ing the lens, which is known as through-the-lens (TTL) meter­ing.

Light meters or light detectors are also used in illumination. Their purpose is to measure the illumination level in the interior and to switch off or reduce the output level of luminaires. This can greatly reduce the energy burden of the building by significantly increasing the efficiency of its lighting system. It is therefore recommended to use light meters in lighting systems, especially in rooms where one cannot expect users to pay attention to manually switching off the lights. Examples include hallways, stairs, and big halls.

Expo­sure com­pen­sa­tion amount. Expo­sure com­pen­sa­tion allows you to increase or decrease image bright­ness beyond what the cam­era deter­mines is opti­mal expo­sure. (See Expo­sure Com­pen­sa­tion). When the cam­era is set to the Pro­gram, Shut­ter Pri­or­i­ty, or Aper­ture Pri­or­i­ty modes, the expo­sure meter shows the amount of com­pen­sa­tion applied in units of EV. When the expo­sure lev­el indi­ca­tor is set to zero, there’s no expo­sure com­pen­sa­tion, and the cam­era reverts to its stan­dard expo­sure pro­gram­ming.

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.

With a hemispherical receptor, ISO 2720:1974 recommends a range for C {\displaystyle C} of 320 to 540 with illuminance in lux; in practice, values typically are between 320 (Minolta) and 340 (Sekonic). The relative responses of flat and hemispherical receptors depend upon the number and type of light sources; when each receptor is pointed at a small light source, a hemispherical receptor with C {\displaystyle C} = 330 will indicate an exposure approximately 0.40 step greater than that indicated by a flat receptor with C {\displaystyle C} = 250. With a slightly revised definition of illuminance, measurements with a hemispherical receptor indicate "effective scene illuminance."

The sensor will send a signal to the meter that is proportional to the amount of light that reaches the sensor after being collected by the optics and passing through the filter. The meter then converts the incoming signal (typically current or voltage) from the sensor into a reading of calibrated units such as Foot-Candles (fc) or Lux (lm/m^2). Calibration in fc or lux, is the second most important feature of a light meter. It not only converts the signal from V or mA, but it also provides accuracy and unit to unit repeatability. National Institute of Standards and Technology (NIST) traceability and ISO/IEC 17025 accreditation are two well known terms that verify the system includes a valid calibration.

Auto-expo­sure brack­et­ing range. Expo­sure brack­et­ing is the tech­nique of tak­ing mul­ti­ple expo­sures of the same scene while slight­ly vary­ing the amount of expo­sure between the indi­vid­ual pho­tos. This tech­nique is incred­i­bly use­ful when cap­tur­ing sub­jects with com­plex light­ing, or when you intend to com­bine mul­ti­ple expo­sures into one high dynam­ic range (HDR) image. Auto-expo­sure brack­et­ing (AEB) auto­mates the process with­in a range defined by the user. When AEB is acti­vat­ed, the camera’s expo­sure meter will show mul­ti­ple expo­sure lev­el indi­ca­tors spec­i­fy­ing the AEB range of the con­sec­u­tive shots.

In Scientific Research & Development uses, a light meter consists of a radiometer (the electronics/readout), a photo-diode or sensor (generates an output when exposed to electromagnetic radiation/light) a filter (used to modify the incoming light so only the desired portion of incoming radiation reaches the sensor) and a cosine correcting input optic (assures the sensor can see the light coming in from all directions accurately).

A light meter is a tool that mea­sures light inten­si­ty and aids pho­tog­ra­phers in set­ting the appro­pri­ate expo­sure. Pho­to­graph­ic light meters are avail­able as off-cam­era hand­held devices, and they’re built into vir­tu­al­ly every dig­i­tal cam­era.

Light measurementfor plants

Another way to avoid under- or over-exposure for subjects with unusual reflectance is to use a spot meter: a specialized reflected-light meter that measures light in a very tight cone, typically with a one degree circular angle of view. An experienced photographer can take multiple readings over the shadows, midrange, and highlights of the scene to determine optimal exposure, using systems like the Zone System.

Every dig­i­tal cam­era that fea­tures either expo­sure com­pen­sa­tion or a man­u­al expo­sure mode will fea­ture an expo­sure meter either in the viewfind­er, the top LCD pan­el, or both. The expo­sure meter is a sim­ple lin­ear scale of rel­a­tive expo­sure val­ues marked by notch­es, dots, or num­bers, and on some cam­eras, all of the above. The stan­dard expo­sure index, char­ac­ter­ized by the big cen­tral notch, or a zero, always rep­re­sents the opti­mal expo­sure as deter­mined by the camera’s pro­gram­ming. Flank­ing the stan­dard expo­sure index are marks that rep­re­sent incre­ments of 1/3 EV and 1 EV. The marks on the right side rep­re­sent added (or pos­i­tive) expo­sure, and the marks on the left rep­re­sent sub­tract­ed (or neg­a­tive) expo­sure. The expo­sure lev­el indi­ca­tor is the nee­dle or mark­er beneath the notched scale.

lightunits of measure w/m2

There are, however, significant obstacles to overcome in order to achieve a successful implementation of light meters in lighting systems, of which user acceptance is by far the most formidable. Unexpected or too frequent switching and too bright or too dark rooms are very annoying and disturbing for users of the rooms. Therefore, different switching algorithms have been developed:

If a scene differs considerably from a statistically average scene, a wide-angle averaging reflected-light measurement may not indicate the correct exposure. To simulate an average scene, a substitute measurement sometimes is made of a neutral test card, or gray card.

to provide an exposure meter which is substantially uniformly responsive to light incident upon the photographic subject from practically all directions which would result in the reflection of light to the camera or other photographic register.

Many modern consumer still and video cameras include a built-in meter that measures a scene-wide light level and are able to make an approximate measure of appropriate exposure based on that. Photographers working with controlled lighting and cinematographers use handheld light meters to precisely measure the light falling on various parts of their subjects and use suitable lighting to produce the desired exposure levels.

Aver­age meter­ing. This mode mea­sures the aver­age bright­ness of the entire frame and sets expo­sure to yield a mid­dle grey tone ren­der­ing of that aver­age. This mode pro­duces par­tic­u­lar­ly accu­rate results for land­scapes with the sun out­side the frame. It pro­vides rel­a­tive­ly con­sis­tent expo­sures across mul­ti­ple shots of dif­fer­ent sub­jects under the same light­ing.

The earliest exposure meters were called actinometers (not to be confused with the scientific instrument with the same name), first developed in the late 1800s after commercial photographic plates became available with consistent sensitivity. These photographic actinometers used light-sensitive paper; the photographer would measure the time required for the paper to darken to a control value, providing an input to a mechanical calculation of shutter speed and aperture for a given plate number.[3]: 69  They were popular between approximately 1890 and 1920.[4]

Evaluative/Matrix/Multi meter­ing. This is the default and most sophis­ti­cat­ed meter­ing mode on most DSLRs and mir­ror­less cam­eras because it works in a vari­ety of sit­u­a­tions and pro­vides accu­rate results most of the time. It works by divid­ing the frame into mul­ti­ple meter­ing seg­ments and ana­lyzes their bright­ness and, some­times, colour. The result­ing matrix of mul­ti­ple seg­ments is eval­u­at­ed based on com­po­si­tion, colour, and dis­tri­b­u­tion of tones. Nikon cam­eras that fea­ture 3D matrix meter­ing also fac­tor the dis­tance infor­ma­tion pro­vid­ed by the aut­o­fo­cus mod­ules. Some mir­ror­less cam­eras eval­u­ate the scene for the pres­ence of promi­nent faces.

ISO 2720:1974 recommends a range for K {\displaystyle K} of 10.6 to 13.4 with luminance in cd/m2. Two values for K {\displaystyle K} are in common use: 12.5 (Canon, Nikon, and Sekonic[8]) and 14 (Minolta,[9] Kenko,[9] and Pentax); the difference between the two values is approximately 1⁄6 EV.

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

When a light wave possesses many different polarisation axes, it is unpolarised · When a light wave possesses only one polarisation axis, it is polarised.

The earliest calibration standards were developed for use with wide-angle averaging reflected-light meters (Jones and Condit 1941). Although wide-angle average metering has largely given way to other metering sensitivity patterns (e.g., spot, center-weighted, and multi-segment), the values for K {\displaystyle K} determined for wide-angle averaging meters have remained.

Togeth­er, the expo­sure meter scale and expo­sure lev­el indi­ca­tor serve one of three func­tions depend­ing on the expo­sure mode you’re using.

ISO 2720:1974 calls for reflected-light calibration to be measured by aiming the receptor at a transilluminated diffuse surface, and for incident-light calibration to be measured by aiming the receptor at a point source in a darkened room. For a perfectly diffusing test card and perfectly diffusing flat receptor, the comparison between a reflected-light measurement and an incident-light measurement is valid for any position of the light source. However, the response of a hemispherical receptor to an off-axis light source is approximately that of a cardioid rather than a cosine, so the 12% "reflectance" determined for an incident-light meter with a hemispherical receptor is valid only when the light source is on the receptor axis.

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]

Selenium and silicon light meters use sensors that are photovoltaic: they generate a voltage proportional to light exposure. Selenium sensors generate enough voltage for direct connection to a meter; they need no battery to operate and this made them very convenient in completely mechanical cameras. Selenium sensors however cannot measure low light accurately (ordinary lightbulbs can take them close to their limits) and are altogether unable to measure very low light, such as candlelight, moonlight, starlight etc. Silicon sensors need an amplification circuit and require a power source such as batteries to operate. CdS light meters use a photoresistor sensor whose electrical resistance changes proportionately to light exposure. These also require a battery to operate. Most modern light meters use silicon or CdS sensors. They indicate the exposure either with a needle galvanometer or on an LCD screen.

Islightmeasured in wavelengths

Light measurementdevice

For incident-light meters, camera settings are related to ISO speed and subject illuminance by the incident-light exposure equation:

Spot meter­ing. The spot meters found in cam­eras are sim­i­lar to those in hand­held light meters. They read light from a small cen­tral area that typ­i­cal­ly com­pris­es one to five per­cent of the frame. On Nikon and most mir­ror­less cam­eras, the spot meter over­laps with the active focus point. On Canon cam­eras, the spot meter is always found in the cen­tre of the frame. Spot meter­ing is both incred­i­bly empow­er­ing and finicky. When using any of the auto-expo­sure modes with spot meter­ing, the cam­era will set its expo­sure val­ues to pro­duce a mid­dle grey ren­di­tion of what­ev­er sub­ject the spot is point­ed towards, and all oth­er tones present in the scene will align accord­ing to their rel­a­tive bright­ness. For this rea­son, using spot meter­ing in any of the auto-expo­sure modes will pro­duce wild­ly incon­sis­tent expo­sures of diverse sub­jects under the same light­ing. For exam­ple, when using auto-expo­sure, spot meter­ing a black dog will pro­duce an expo­sure where the dog appears grey and the scene is over­ex­posed; spot meter­ing a white dog will pro­duce an expo­sure where the dog appears grey, and the pic­ture is under­ex­posed; and, spot meter­ing a grey dog will yield an expo­sure where the dog appears grey, and the scene is cor­rect­ly exposed. Pon­der this point.

Light measurementapp

Ultra violet radiation [UVR] is electromagnetic radiation in the range of wavelength 100 –. 400nm. UV light sources used in laboratories include hand held UV ...

In practice, the variation of the calibration constants among manufacturers is considerably less than this statement might imply, and values have changed little since the early 1970s.

Since inci­dent-light meters mea­sure the inten­si­ty of light strik­ing the sub­ject, they pro­vide accu­rate expo­sure infor­ma­tion regard­less of your subject’s inher­ent bright­ness; so long as the sub­jects in your scene are even­ly illu­mi­nat­ed by the same source of light, dark tones will be ren­dered as dark, grey tones as grey, and light tones as light. Fur­ther­more, inci­dent-light meters are incred­i­bly use­ful for mak­ing accu­rate mea­sure­ments in a con­trolled-light­ing envi­ron­ment, such as a stu­dio, and are espe­cial­ly prac­ti­cal when used to cal­cu­late pre­cise con­trast ratios between dif­fer­ent lights.

It is commonly stated that reflected-light meters are calibrated to an 18% reflectance,[10] but the calibration has nothing to do with reflectance, as should be evident from the exposure formulas. However, some notion of reflectance is implied by a comparison of incident- and reflected-light meter calibration.

For reflected-light meters, camera settings are related to ISO speed and subject luminance by the reflected-light exposure equation:

The tungsten halogen lamp is similar to an inert gas-filled lamp, except it contains a small quantity of an active halogen gas such as Bromine. The inert gas ...

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.