Light meter
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.
[1] The earliest exposure meters were called actinometers (not to be confused with the scientific instrument with the same name), described as early as 1840[3]: 415 but developed in the late 1800s after commercial photographic plates became available with consistent sensitivity.
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.
[8] Starting in 1932,[9]: 20 electronic light meters removed the human element and relied on technologies incorporating (in chronological order) selenium, CdS (1960s), and silicon (semiconductor, 1970s) photodetectors.
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.
[3]: 417 However, CdS sensors fell out of favor due to their slower response and extended sensitivity to red and infrared wavelengths.
[3]: 417 [12]: 58 Semiconductor sensors are also photovoltaic, but the voltage generated is much weaker than selenium cells and semiconductor-based light meters need an amplification circuit and therefore require a power source such as batteries to operate.
These are usually named after the materials and filtration used to ensure the spectral response is similar to the human eye or photographic film, such as 'Silicon Blue Cell' (SBC) or 'GaAs'.
Exposure meters generally are sorted into reflected-light or incident-light types, depending on the method used to measure the scene.
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.
[11]: 102, 126 Many modern cameras include sophisticated multi-segment metering systems that measure the luminance of different parts of the scene to determine the optimal exposure.
What constitutes a "medium tone" depends on meter calibration and several other factors, including film processing or digital image conversion.
Meter calibration establishes the relationship between subject lighting and recommended camera settings.
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.
are in common use: 12.5 (Canon, Nikon, and Sekonic[15]) and 14 (Minolta,[16] Kenko,[16] and Pentax); the difference between the two values is approximately 1⁄6 EV.
The earliest calibration standards were developed for use with wide-angle averaging reflected-light meters (Jones and Condit 1941).
Don Norwood, inventor of incident-light exposure meter with a hemispherical receptor, thought that a sphere was a reasonable representation of a photographic subject.
However, a test card seldom is a uniform diffuser, so incident- and reflected-light measurements might differ slightly.
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.
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.
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.
This can greatly reduce the energy burden of the building by significantly increasing the efficiency of its lighting system.
Therefore, different switching algorithms have been developed: 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).
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).
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.
The meter/radiometer/photometer portion may have many features including: Zero: subtracts ambient/background light levels, or stabilize the meter to the working environment Hold: freezes the value on the display.
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.
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.
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.
