Editing Spectroradiometer (section)

==How it works==

The essential components of a spectroradiometric system are as follows:

*Input optics that gather the electromagnetic radiation from the source (Diffusers, Lenses, Fiber optic light guides)
*An entrance slit, determines how much light will enter the spectrometer. A smaller slit with have greater resolution, but less overall sensitivity
*Order sorting filters for reduction of second-order effects
*Collimator directs the light to the Grating or prism
*A grating or prism for dispersion of the light
*Focusing optics to align the light onto the Detector
*A detector, CMOS sensor or CCD array
*A control and logging system to define data and store it.<ref name=Bentham>Bentham Instruments Ltd. A Guide to Spectroradiometry: Instruments & Applications for the Ultraviolet. Guide. N.p., 1997. Web. <http://www.bentham.co.uk/pdf/UVGuide.pdf></ref>

=== Input optics ===

The front-end optics of a spectroradiometer includes the lenses, diffusers, and filters that modify the light as it first enters the system. For Radiance an optic with a narrow field of view is required. For total flux an integrating sphere is required. For Irradiance cosine correcting optics are required. The material used for these elements determines what type of light is capable of being measured. For example, to take UV measurements, quartz rather than glass lenses, optical fibers, Teflon diffusers, and barium sulphate coated integrating spheres are often used to ensure accurate UV measurement.<ref name=Bentham/>

=== Monochromator ===
{{main|monochromator}}

[[Image:Czerny-Turner_Monochromator.svg|thumb|200px|Diagram of a Czerny-Turner monochromator.]]

To perform spectral analysis of a source, monochromatic light at every wavelength would be needed to create a spectrum response of the illuminant. A monochromator is used to sample wavelengths from the source and essentially produce a monochromatic signal. It is essentially a variable filter, selectively separating and transmitting a specific wavelength or band of wavelengths from the full spectrum of measured light and excluding any light that falls outside that region.<ref name=AAS>American Astronomical Society. "Study Notes: AAS Monochromator." Study Notes: AAS Monochromator. N.p., n.d. Web. 2013. <{{cite web |url=http://toolboxes.flexiblelearning.net.au/demosites/series5/508/Laboratory/StudyNotes/snAASMonochrom.htm |title=Study Notes: AAS Monochromator |access-date=2013-12-11 |url-status=dead |archive-url=https://archive.today/20131211054338/http://toolboxes.flexiblelearning.net.au/demosites/series5/508/Laboratory/StudyNotes/snAASMonochrom.htm |archive-date=2013-12-11 }}>.</ref>

A typical monochromator achieves this through the use of entrance and exit slits, collimating and focus optics, and a wavelength-dispersing element such as a diffraction grating or prism.<ref name=Schnedier/> Modern monochromators are manufactured with diffraction gratings, and diffraction gratings are used almost exclusively in spectroradiometric applications. Diffraction gratings are preferable due to their versatility, low attenuation, extensive wavelength range, lower cost, and more constant dispersion.<ref name=AAS/> Single or double monochromators can be used depending on application, with double monochromators generally providing more precision due to the additional dispersion and baffling between gratings.<ref name=Bentham/>

===Detectors===
[[File:Pmside.jpg|thumb|upright=0.4|Photomultiplier]]
The detector used in a spectroradiometer is determined by the wavelength over which the light is being measured, as well as the required dynamic range and sensitivity of the measurements. Basic spectroradiometer detector technologies generally fall into one of three groups: photoemissive detectors (e.g. [[photomultiplier]] tubes), semiconductor devices (e.g. silicon), or thermal detectors (e.g. thermopile).<ref>Ready, Jack. "Optical Detectors and Human Vision." Fundamentals of Photonics (n.d.): n. pag. SPIE. Web. <http://spie.org/Documents/Publications/00%20STEP%20Module%2006.pdf>.</ref>

The spectral response of a given detector is determined by its core materials. For example, photocathodes found in photomultiplier tubes can be manufactured from certain elements to be [[Solar-blind technology|solar-blind]] – sensitive to UV and non-responsive to light in the visible or IR.<ref>J. W. Campbell, "Developmental Solar Blind Photomultipliers Suitable for Use in the 1450–2800-Å Region," Appl. Opt. 10, 1232-1240 (1971) 
http://www.opticsinfobase.org/ao/abstract.cfm?URI=ao-10-6-1232</ref>

'''CCD (Charge Coupled Device)''' arrays typically one dimensional (linear) or two dimensional (area) arrays of thousands or millions of individual detector elements (also known as pixels) and CMOS sensors. They include a silicon or InGaAs based multichannel array detector capable of measuring UV, visible and near-infra light.

'''CMOS (Complementary Metal Oxide Semiconductor)''' sensors differs from a CCD in that they add an amplifier to each photodiode. This is called an active pixel sensor because the amplifier is part of the pixel. Transistor switches connect each photodiode to the intrapixel amplifier at the time of readout.

=== Control and logging system ===

The logging system is often simply a personal computer. In initial signal processing, the signal often needs to be amplified and converted for use with the control system. The lines of communication between monochromator, detector output, and computer should be optimized to ensure the desired metrics and features are being used.<ref name=Bentham/> The commercially available software included with spectroradiometric systems often come stored with useful reference functions for further calculation of measurements, such as CIE color matching functions and the V<math>\lambda</math> curve.<ref>Apogee Instruments. Spectroradiometer PS-100 (350 - 1000 Nm), PS-200 (300 - 800 Nm), PS-300 (300 - 1000 Nm). N.p.: Apogee Instruments, n.d. Apogee Instruments Spectroradiometer Manual. Web. <http://www.apogeeinstruments.com/content/PS-100_200_300manual.pdf>.</ref>