光譜學

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...

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 wavelength there...

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...

大氣質量Air Mass (AM) 太陽輻射是接近6000 K的黑體輻射，到達地球大氣層表面的輻射能量約為1353 kW/m2。輻射能量會因大氣吸收而減弱，經過不同大氣層路徑，相關的強弱也不同。定義大氣層外的太陽光譜為AM0，其中大氣質量AM (Air Mass)是用來表示大氣層吸收，導致太陽光譜與輻射能量減弱的表徵。定義θ角為入射太陽光角度與頭頂垂直入射的夾角，則大氣質量的計算Air Mass= 1/cos θ。 ▲圖一 太陽光譜AM0與AM1.5的定義 AM0：太陽光在大氣層外的平均照度，也稱為太陽常數。世界氣象組織(WMO)於1981年公布的太陽常...

AM1.5G與AM1.5D AM為Air Mass大氣質量(超連結AM)，G為Global代表全空輻射，D為Direct代表太陽光直射。太陽光穿透大氣層時，不改變其輻射方向稱為直達輻射(Direct)，漫射輻射(Diffuse)則是被大氣層反射和散射後發生方向改變的太陽輻射。全空輻射(Global irradiance)、直達輻射(Direct irradiance)、漫射輻射(Diffuse irradiance)三者的關係如下: ▲圖 全空輻射、直達輻射與漫射輻射的關係。 因此，我們可以理解AM1.5D與AM1.5G的定義如下。 AM1.5D： 太陽光以48.2度夾角，穿過1.5倍大氣層厚度”直達”測試平面的直...

AM0、AM1.5G與AM1.5D的太陽光譜與輻射強度 國際標準ASTM E490定義了標準太陽常數(solar constant) u以及AM0的太陽輻射光譜。這個標準所建立的光譜數據是來自高空飛機、航天器與地球表面的實驗測量數據搭配太陽光譜輻照度模型而來。 國際標準ASTM E173提供了AM1.5G與AM1.5D的標準太陽光譜輻照度分布，可以用以評估地表用太陽能組件與材料的模擬或評價。 IEC 60904-3的國際標準定義的AM1.5G的光譜輻照度數據，也常應用於地面光伏(PV)太陽能裝置的測量參考。 ASTM E173與IEC 60904-3兩者所定義的AM1.5G參考光譜十分接...