Spectroscopy

Monochromator Introduction The term monochrome and its variants come to us from the Greek words mono “single” and chroma “colour”. No light source is truly monochromatic; no light source emits light of a single wavelength, all sources containing contributions from a finite range of wavelengths, termed its spectrum. It is often of interest to decompose a source into its component wavelengths, for the purpose of determining the spectral distribution (UV, visible, infrared e...

Theory of diffraction Diffraction describes a variety of processes which obtain when waves, such as light, approaches an obstacle of dimension of the order of their wavelength, and is characterized by the apparent bending of the waves around the object, such as is demonstrated across. What is transmitted in the one case is a sharp image of the aperture, and in the other a diffracted image of the aperture (seen , whereby most of the light is transmitted on axis, but at wider angles,...

Diffraction orders As noted above, the grating equation may be satisfied at a given angle by a number of wavelengths of different diffraction orders. ▲Figure 1-9:- Existence of diffraction orders This can lead to problems when attempting to measure light in a given diffraction order, when the detection system is capable of sensing the wavelength in the next diffraction order etc.Order sorting is therefore required, and consists of the filtering of the monochromator input wi...

Czerny-Turner Monochromator The Czerny-Turner configuration, uses a plane diffraction grating. ▲Figure 1-12:- Czerny-Turner configuration In order to control the location of diffracted light, the grating should be illuminated by collimated light. Incident light, diverging from an entrance slit is collimated by a first concave mirror. After diffraction from the grating, light is focused to an exit slit by a second concave mirror. As a function of wavelengt...

Double monochromators When using a single monochromator such as that shown in figure 1-9, it is possible that light, entering from the entrance slit, be scattered off the walls and structures constituting the monochromator, reach the exit slit. Therefore, at a given wavelength, , an artificially high signal is measured. This is termed stray light and is of concern where low light level measurements are performed where there exists a significant light component at other wavelengths. ...

Monochromator Bandwidth The monochromator bandwidth, defined in nm, is the range of wavelengths seen by the detector at one time, and is directly linked to the monochromator slits in use. This is an important quantity to take into account, particularly when measuring sources have fine spectral features such as line emission- for example the measurement of a source having two spectral lines one nanometre apart with a system bandwidth of five nanometres, will result in the measurement of...

Xe Lamp Light Generation Mechanism Xenon short-arc lamps come in two distinct varieties: pure xenon, which contain only xenon gas; and xenon-mercury, which contain xenon gas and a small amount of mercury metal. In a pure xenon lamp, the majority of the light is generated within a tiny, pinpoint-sized cloud of plasma situated where the electron stream leaves the face of the cathode. The light generation volume is cone-shaped, and the luminous intensity falls off exponentially m...

Properties of Light The light that we see everyday is only a fraction of the total energy emitted by the sun incident on the earth. Sunlight is a form of "electromagnetic radiation" and the visible light that we see is a small subset of the electromagnetic spectrum shown at the right.The electromagnetic spectrum describes light as a wave which has a particular wavelength. The description of light as a wave first gained acceptance in the early 1800's when experiments by Thomas Young, Fr...

What is Photon transfer Curve? Linear Signal Model Figure 1-1: a. Physical model of the camera and b. Mathematical model of a single pixel. Figures separated by comma represent the mean and variance of a quantity; unknown model parameters are marked in red. As illustrated in Fig. 1-1, a digital image sensor essentially converts photons hitting the pixel area during the exposure time texpby a sequence of steps finally into a digital number. During the exposure time on ave...

Air Mass (AM) Solar radiation of approximately 6000K is approximately a blackbody radiation. The energy of the radiation that reaches the top of Earth atmosphere is around 1353kW/m2. As it passes through the atmosphere, sunlight is attenuated by scattering and absorption. The more atmosphere sunlight passes, the more energy is attenuated. The spectrum outside the atmosphere is defined as AM0. Air Mass (AM) describes the relation of the length sunlight travels and spectrum and radia...

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The American Society for Testing and Materials developed and defined the solar constant 1366.1 W/m2 u (also known as the total sky irradiance) and reference spectrum ASRM E490. This AM0 solar spectral irradiance is based on data from satellites, space shuttle missions, high-altitude aircraft, rocket soundings, ground-based solar telescopes, and modeled spectral irradiance. The reference spectrum ASRM E173 AM1.5G and AM1.5D provides terrestrial uses in simulating and evaluating photovo...