Photometry intensity measurements

Photometry The measurement of quantities associated with light, i.e., based on the average apparent intensity of a light source as viewed by a normal light-adapted human eye. Photometric units report light intensity in terms of the illuminance, e.g., candlepower (lumen/ft2) or lux (lumen/m2). Photometric units are only appropriate for visible radiation. 

The science of photometry— the measurement of the effect of light intensity whether transmitted or absorbed on various materials— provided another impetus for research on chemical effects of light during this period. These studies had broad focus, spanning research on astronomy and chemical effects of light. Very basic photometers were made with paper saturated with silver nitrate, silver chloride, or other light-sensitive solutions. ... 

Phase Shift a change in the periodicity of a waveform such as light. Photometry instrumental methods, including analytical methods, employing measurement of light intensity. See telephotometer. 

Flame photometry is the name given to the technique that measures the intensity of the light emitted by analyte atoms in a flame. It is the oldest of all the atomic techniques. It is not highly applicable because of the low temperature of the flame. Only a handful of elements can be measured with this technique, including sodium, potassium, lithium, calcium, strontium, and barium. The technique was formerly used... 

Measurements of the intensity and wavelength of radiation that is either absorbed or emitted provide the basis for sensitive methods of detection and quantitation. Absorption spectroscopy is most frequently used in the quantitation of molecules but is also an important technique in the quantitation of some atoms. Emission spectroscopy covers several techniques that involve the emission of radiation by either atoms or molecules but vary in the manner in which the emission is induced. Photometry is the measurement of the intensity of radiation and is probably the most commonly used technique in biochemistry. In order to use photometric instruments correctly and to be able to develop and modify spectroscopic techniques it is necessary to understand the principles of the interaction of radiation with matter. 

Because, at concentrations smaller than parts per million, the radiation intensities, 7o and, have to be measured with very great precision, the determination with absolute photometry requires a physical-chemistry laboratory with an experienced staff. 

The use of electrophotometry requires a sample preparation with a coloured solution. Together with an electrophotometer for alkaloid analysis, a constant light intensity and filter, as well as an electronic installation for measurement, must be used. The electrophotometry method is an application of both calorimetry and photometry in the same analysis. 

In addn to being the general term for the field as a whole, flame photometry refers also specifically to systems where the emitted light is separated by filters and the intensities are measured by a photo tube. 

Colorimetry/photometry exploits the fact that the color intensity of the complex correlates to the metal ion concentration, at least within a certain concentration range. Therefore, sufficient indicator is added to the sample that the indicator forms a colored complex with all ions of the metal of interest in solution. The color intensity of this complex is then either compared visually against the color of reference samples (colorimetry) or the color intensity is measured at a defined wavelength with an instrument and then compared with a calibration curve (photometry). 

Sodium is still often determined by flame photometry, measuring the emission intensity of the doublet at around 589 nm, but care is necessary to make sure that excess calcium does not cause spectral interference (from molecular emission). This is unlikely to be a problem if AES is used, with a narrow spectral band-pass, and the intensity of emission at 589.0 nm from an air-acetylene flame is measured. However, at low determinant concentrations it is then advisable to add 2-5 mg ml 1 potassium or caesium as an ionization buffer. This is even more true if a nitrous oxide-acetylene flame is used for FES, although its use is rarely justified in environmental analyses because the additional sensitivity gained is rarely necessary. 

The various terms that are used for the description of the emission of electromagnetic radiation from a radiant source or for the receipt of electromagnetic radiation by a specified surface element are summarized in Tab. 3-9. The terminology of electromagnetic radiation measurement is divided into radiometry and the subset of photometry (Fig. 3-18). The former is the science that involves the energy measurement of electromagnetic radiation in general. The latter is applied for the same purpose when visible radiation is to be described or measured in relation to the human eye s response. Important photometric quantities are for example luminous flux, luminous intensity, illuminance and luminance (McCluney, 1994). Every photometric quantity has its counterpart in radiometry, and vice versa. 

The most spectacular applications of absorption spectroscopy have been, of course, to flash photolysis used in either adiabatic (Le. explosion ) or isothermal (i.e. photochemical ) studies. A brief description of the technique is to be found in the introduction to this section, and reference is made to a review article . Relative intensities of absorption may be determined by plate photometry of photographic records, or by photoelectric measurements using a monochromator. The qualitative information about the species present as intermediates may go far... 

Filter monochromators are now used almost only for flame photometry. They make use of interference filters, which may have a fairly low spectral bandpass (less than a few nm). However, it is also possible to use such filters for dynamic measurements of line and background intensities, and for transient signals, as occur in gas chromatography. The use of oscillating filters has been described, where the wavelength bandpass is slightly shifted by inclining them towards the radiation beam [65]. 

In the visible region, the terminology is somewhat different it is called photometry and is based on the human response to visible light. The unit of source intensity is the candle and the radiance of the source becomes the brightness, while the irradiance is called the illuminance (measured in lux) or the illumination (measured in footcandles). The lumen is the unit of power (in watts) while lux is the power per unit area, expressed as lumen m-2 or watts m-2. Note that the units of photometry relate to standardized human perception. Thus, a monochromatic UV source may have high irradiance but zero illuminance because none of the energy can be perceived by the human eye. However, many sources of UVR are also powerful visible light sources and are rated by manufacturers in photometric terms. 

Measurements by photographic photometry require careful calibration due to the nonlinear response of photographic plates saturation effects can lead to erroneous values. Line profiles can be recorded photoelectrically, if the stability of the source intensity and the wavelength scanning mechanism are adequate. Often individual rotational lines are composed of incompletely resolved spin or hyperfine multiplet components. The contribution to the linewidth from such unresolved components can vary with J (or TV). In order to obtain the FWHM of an individual component, it is necessary to construct a model for the observed lineshape that takes into account calculated level splitttings and transition intensities. An average of the widths for two lines corresponding to predissociated levels of the same parity and J -value (for example the P and R lines of a 1II — 1E+ transition) can minimize experimental uncertainties. A theoretical Lorentzian shape is assumed here for simplicity, but in some cases, as explained in Section 7.9, interference effects with the continuum can result in asymmetric Fano-type lineshapes. 

The first experiments of this school were directed to the understanding of ionization interference in flame photometry. Alkemade showed that the short-fall in resonance radiation intensity from alkali metals through ionization, and the suppression of such ionization, could be reconciled with the Saha relation only if an excess of electrons over alkali ions were present in acetylene flames, in amounts which decreased with height in the flame. In developing the study Borgers used the R.F. coil technique to measure electron concentration. He at first supported this conclusion for the clean acetylene flame, but then... 

---