Tutorials

This discussion is restricted to the general use of Incoherent Light Sources, such as arc lamp or quartz tungsten halogen sources. Diffraction and coherent effects are excluded. The emphasis throughout this section will be on the collection of light.

In this section we give a brief introduction to getting light into a monochromator, and how much you can expect to get out. While the emphasis is on coupling Oriel Light Sources to Oriel Monochromators, the same general principles apply to collecting light from any source for analysis. Specifically, all of the collection principles we will cover here may be applied to spectrographs as well as monochromators.

Radiation from the sun sustains life on earth and determines climate. The energy flow within the sun results in a surface temperature of around 5800 K, so the spectrum of the radiation from the sun is similar to that of a 5800 K blackbody with fine structure due to absorption in the cool peripheral solar gas.

You can estimate the power in the collimated beams* at any wavelength or in any wavelength range from our Series Q, Apex, or Research Lamp Housings with any of our CW and pulsed arc, quartz tungsten halogen or deuterium lamps.

Gratings are available in various groove densities (i.e. lines/mm). Higher groove densities give higher reciprocal dispersion and therefore higher resolution. The grating dispersion is similar for gratings with the same groove density. The exact dispersion is dependent upon other physical characteristics of the grating in addition to the groove density.

Everything radiates and absorbs electro-magnetic radiation. Many important radiation laws are based on the performance of a perfect steady state emitter called a blackbody or full radiator. These have smoothly varying spectra that follow a set of laws relating the spectral distribution and total output to the temperature of the blackbody.

An integrating sphere collects electromagnetic radiation from a source completely external to the optical device, usually for flux measurement or optical attenuation. Radiation introduced into an integrating sphere strikes the reflective walls and undergoes multiple diffuse reflections.

The exit beam from a typical monochromator should be monochromatic but usually contains undesired light of other wavelengths. Filters can be used to block higher order diffracted wavelengths. Other unwanted light, called stray light, has several causes and should be minimized for both monochromators and spectrographs.

Grating efficiency and its variation with wavelength and spectral order are important characteristics of a diffraction grating. For a reflection grating, efficiency is defined as the energy flow (power) of monochromatic light diffracted into the order being measured, relative either to the energy flow of the incident light (absolute efficiency) or to the energy flow of specular reflection from a polished mirror substrate coated with the same material (relative efficiency).

Because of the strong ozone absorption, any photons with wavelengths shorter than 280 nm at ground level are most likely due to human activity (or lightning). This is just as well since these short wavelength photons have enough energy to break many chemical bonds. We have found many uses for this bond breaking capability in material processing.

What follows are two sample calculations of throughput. The purpose of these two examples is to help you estimate the available monochromatic power or monochromator throughput for a detection system. The two somewhat different examples can be adapted to many of our light sources or to luminescing samples.

In a monochromator, the design concentrates on the path of light from the input slit, off the grating and to the output slit. Light of other wavelengths is absorbed. At any grating setting, only a very small range of angles around the diffraction angle D, figuratively “one” wavelength, passes through the monochromator.

Here we briefly cover radiation of varying level. This requires consideration of the dependence on t, time, so that irradiance becomes Eeλ (λ,t). Most displays of radiation pulses show how the radiant power (radiant flux) Φe(λ,t) varies with time. Sometimes, particularly with laser sources, the word “intensity” is used instead of radiant flux or radiant power. In this case, intensity often does not represent the strict meaning indicated in Table 1, but beam power.

Grating installation and field calibration are typically performed at one of Newport’s manufacturing location, or by qualified field service personnel. The instructions provided in this section are for advanced users only and should be followed at the customer’s own risk.