What Is Photometer ?Photometer is a device which is used to measure the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Such devices are generally transducers that convert an electric current into a mechanical indication—e.g., a pointer moving across a dial. The source of the current may be a selenium cell, which generates a current when light falls on it, or it may be a permanent source, such as a battery, in which case the current passes through a cadmium sulfide cell whose resistance varies with the amount of light falling on it.
Principal Of Photometer :-Most photometers detect the light with photoresistors, photodiodes or photomultipliers. To analyze the light, the photometer may measure the light after it has passed through a filter or through a monochromator for determination at defined wavelengths or for analysis of the spectral distribution of the light.
Working Of Photometer :-The system includes a non-moving, truncated paraboloid of revolution, having a paraboloid axis, a small entrance aperture, a larger exit aperture and a light-reflecting inner surface that receives and reflects light in a direction substantially parallel to the paraboloid axis. A system of wide field of view lenses can be substituted for this optical arrangement depending on the application. The system also includes a light processing filter to receive and process the redirected light, and to issue the processed, redirected light as processed light, and an array of light receiving elements, at least one of which receives and measures an associated intensity of a portion of the processed light. The system tracks a light source moving along a path and produces a corresponding curvilinear image of the light source path on the array of light receiving elements. Undesired light wavelengths from the light source may be removed by coating a selected surface with a coating that absorbs incident light in the undesired wavelength range .
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Different Types Of Photometer :-
1. Built-In PhotometerIn the early days of light meter reading, the photometer was always a separate piece of equipment from the camera. However, almost all cameras now come equipped with an internal light meter. In most cases, as with taking a casual family photo or a quick snapshot, the built-in photometer, which comes standard on cameras made in the last 50 years or so, is sufficient for reading light intensity and luminescence. While many older SLR cameras require the photographer to use the built-in photometer to set an aperture for exposure, digital cameras usually take photometer readings and set apertures automatically.
2. Hand-Held PhotometerThe hand-held photometer is standard equipment for most professionals. The hand-held meter can be used much like the built-in camera photometer, but it may also be used to concentrate on the light readings of particular areas of the potential photograph.
3. Spot PhotometerThe photographer may take a variety of light readings from different areas ("spots") of the photographic subject and use these readings to set the camera's exposure to highlight these chosen areas. Some spot photometers are sophisticated to the degree that they offer multi-segment metering systems that can take multiple simultaneous readings of very specific areas of the subject.
4. Incident-Light PhotometerAnother common type of meter for photographers is the incident-light meter. This meter can often give more truthful measurements of light by calculating not the amount of reflected light, but rather the amount of light falling on the subject, thus using "incident" light measurements. Incident-light meters can avoid potentially incorrect exposures based on subjects with unusual reflected-light readings. These photometers also can be set to take light meter readings only when set-up lights flash, allowing the photographer to gather correct light readings for the potential flash photograph, and not just the ambient light of the subject.
5. Color PhotometerColor photometers not only gauge the amount of light reflected or falling upon the subject, but also the intensity of color radiating from the subject. These meters can be helpful for photographers looking to balance color saturation and hue in their photographs by adjusting exposures according to color readings.
Applications Of Photometer :-Photometers are used in the following applications whic are provided below :-
- Photon counting
- Visible light reflectance photometry
- UV and visible light transmission photometry
- Infrared light transmission photometry
- Atomic absorption photometry