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Instruments that measure electromagnetic radiation have several concepts and components in common. One of the techniques used most frequently in the clinical laboratory is photometry or specifically absorbance or reflectance spectrophotometry. Photometry employs color and color variation to determine the concentrations of various substances. A photometric component is employed in many of the automated analyzers currently in use in the clinical laboratory and any person doing clinical laboratory techniques should understand the principles of photometry. 


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Photometry is the measurement of the luminous intensity of light or the amount of luminous light falling on a surface from a light source. Photometric instruments measure this intensity of light without consideration of wavelength. In contrast, spectrophotometry is the measurement of the intensity of light at selected wavelengths.


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Photometry is a science in which the visible light measured according to the sensitive capacity of the human eye. Since it is based on the visual response of the human eye, it is comes under the category of quantitative science. This eye sensitivity is changes according to the wavelength of the light. In this method, instead of directly measuring the radiant energy, we measure the subjective impression by radiant energy.


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Conceptually, aperture photometry is analogous to photoelectric photometry. After the image has been obtained, a computer program is used to reconstruct the signal which would have been obtained from an object in the field of view if the light had passed through a physical aperture (usually circle) of a certain diameter in arcseconds. The imaginary aperture is called a "software aperture". Faint objects are best measured with a small aperture, but bright standard stars usually need a larger aperture.


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Reflectance photometry is a branch of photometry in which the reflected light is used for the studies. Reflectance photometer is used to detect the reflected light. Reflectance photometers measure the light that is reflected by a colored product. The reflected light is detected by a photocell and the information is converted into the appropriate units. Reflectance photometers use solid phase chemistry technology, meaning that reagents are present in dried form in the test unit.


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Flame photometry is quite a simple technique, which is widely used in clinical biochemistry laboratories for determining electrolytes. It is based on the principle that when the atoms of certain elements absorb radiant energy, their outermost electrons jump to higher orbits. When the displaced electrons return to their original orbit the absorbed energy is released in the form of radiation characteristic of that element. The released radiation is generally in the visible range. It is possible to measure this radiant light as done in calorimetry or spectrophotometry. The emitted light is proportional to the amount of the analyzed element.

Flame Photometry


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The concept of stellar photometry or the measurement of starlight, is much more recent than that of the estimation of stellar magnitudes. The earliest photometry was still visual in that the human retina was the only light receptor. However, the technique relied on diminishing the intensity of a light beam by a measurable amount, either so as to extinguish the rays just to the point of invisibility or so as to change the intensity so that the brightness of two sources to be compared is made equal. This is the basis of visual photometry, whether it be by extinction or by comparison.