When photometric provisions, in which one has to do with the light emission by a variety of sources, it is necessary at the observation always take account of a subjective element, because the human eye for some colors (wavelengths) is more sensitive than others, so that sources which are different emit monochromatic light or white light of different spectral composition, causing different light impressions, even though they are sent in a total of from the same amount of energy. Attempts are the subjectivity of the photometric determinations to limit by making use of photoelectric cells, provided with filters that it as much as possible the sensitivity in the neighborhood of that transfer of the eye. Simpler and more objective expire these measurements, as they are carried out with monochromatic light. The observations can then be done with the eyes, but also with photoelectric cells. Usually one uses the latter because they provide much more accurate results.
Below, the various examples of graphs showing the intensity of the light in the different frequency ranges of the fluorescent lamps, such as 33 to 640 and the TL-D 827, D-TL 830, TL 840 and TL-D-D 865. At the bottom of a SOX lamp with monochromatic light (see clearly the differences)
In order to simplify the photometric determinations considers one in many cases, the source as a point-shaped, although one has to take into account in the measurement of weather other quantities with the dimensions of the source. The main photometric quantities are as follows: the amount of light emitted by a light source, the light flux, the luminous intensity, the illuminance, the brightness (physiological), and the physical brightness or luminance.
Since in photometry the measurements relate to the average sensitivity of the human eye (both if one evaluates to the eye as when use is made of appropriate calibrated light-sensitive elements) it is of little practical utility, the total amount of energy, which is emitted by a light source, know. However, However, it is important to know the amount of light, which is emitted by the source means a quantity which is defined as the product of the amount of energy and the visibility of the radiation of the considered wavelength.