Reprint (R34) Accurate Transmission Measurements Of Translucent Materials. January 2008

Transcription

1 Reprnt (R34) Accurate ransmsson Measurements Of ranslucent Materals January 2008 Gooch & Housego th Street, Orlando, FL el: Fax: Emal: sales@goochandhousego.com

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3 Accurate ransmsson Measurements of ranslucent Materals Unlke transparent materals whch transmt lght wth no apprecable attenuaton or absorpton, translucent materals often possess certan propertes and physcal structures that cause ncdent lght to become scattered as t passes through the sample. he scatterng that occurs poses some nterestng challenges when attemptng to measure the transmsson of these materals. here are three basc processes that occur when lght strkes an object; t can be ether be reflected, transmtted, or absorbed. Other process such as fluorescence can occur due to the presence of shorter wavelength radaton n the rradatng source, however we wll not be addressng ths drectly n ths artcle. In normal or regular transmsson measurements of transparent materals where the angle of the transmtted beam can be calculated f the ncdent angle s known usng Snell s law, n Sn = nsn (1) 1 θ1 2 θ2 the measurement setup s farly straghtforward (Fg. 1.). A stable source s used to provde llumnaton and dependng on the nformaton requred, a radometer, photometer, or spectroradometer s used to process the sgnals. A measurement of the source t acqured by placng t nlne wth the detector and wthout the sample n place and the ncdent flux s recorded as. he sample s then placed between the source and the detector and another measurement s taken and recorded as t. he rato of these two sgnals s the transmsson of the sample and can be reported as ether radant, lumnous (photometrc), or spectral transmttance. Lumnous ransmttance v, t τ v = (2) v, Spectral ransmttance τ( λ) t ( λ) ( λ) = (3) No specal consderatons are needed snce the ncdent beam dameter s roughly the same as the transmtted beam dameter and therefore all of the transmtted lght s receved by the detector. ransparent Sample Detector Fg. 1. Regular ransmsson Measuremet Setup

4 hs s not so wth translucent materals. When ncdent lght mpnges on a translucent materal, t tends to scatter and dffuse and therefore the angle of the transmtted lght no longer obeys Snell s law (Fg. 2.). hs results n much of the transmtted lght mssng the detector. It s ths dffuse transmsson of lght that requres specal attenton and proper selecton of detecton optcs to ensure all transmtted lght s collected and processed. ranslucent Sample Detector Fg. 2. Regular ransmsson Measurement Setup o effectvely collect all dffusely and non-dffusely transmtted lght, t becomes necessary to select a large angle collecton optc. By vrtue of ther desgn, ntegratng spheres are perfect for ths type of measurement. By placng the sample up to the nput port t becomes possble to collect even the hghest angle rays (Fg 3). It s not, however, a straghtforward measurement as when measurng transparent samples. ranslucent Sample Ext Integratng Sphere Fg. 3. Dffuse ransmsson Measurement Setup At frst glance t may seem as though we can smply take a measurement of the lght ncdent on the nput port,, place our sample up to the sphere port and take another measurement of the transmtted lght, t, calculate the spectral transmttance. Although ths sounds lke a logcal approach, we have not taken nto account the fact that by placng the sample up to the sphere port, we have effectvely changed the response of the sphere. Wthout a sample n place, t s clear that some of the ncdent lght s allowed to escape back through the nput port. Placng a sample aganst the port also allows some of the transmtted lght to escape through the nput port, however n dfferent proporton than wth no sample n place. In addton, lght s also re-reflected back nto the sphere from the sphere sde of the sample and s spectrally dependent on the composton of the sample. (Fg. 4). Fg. 4 Lght path wth and wthout sample n place

5 o account for the changes n ntegratng sphere response due to sample placement, t becomes necessary to utlze two sources, one for rradatng the sample and the other for determnng the spectral response changes. It s mportant to note that both sources must have the same spectral dstrbuton when usng the double beam method for transmsson measurements. A representatve setup for measurng total transmsson, τ, s shown (Fg. 5), otal ransmsson τ τ + τ = (4) where τ R s the regular transmsson and τ D s the dffuse transmsson. R D Correcton Incdent Ext Fg. 5 ypcal setup for measurng dffuse transmsson usng double beam method he double beam method for measurng dffuse transmsson of translucent materals conssts of 4 separate steps: 1) ake measurement of Incdent wthout sample n place and Correcton off 2) ake measurement of Correcton wthout sample n place and Incdent off 3) Keepng Incdent off, nstall sample aganst port and agan take measurement wth Correcton 4) ake measurement wth Incdent wth Correcton off. Steps one and two combne to provde and general calbraton for the system by dvdng the flux obtaned from the Correcton, CS by the flux of the Incdent,. Step three s essental for the spectral and ntensty correcton of the system due to the presence of the transmttng samples at the ntegratng sphere port. hs s generally referred to as the sample calbraton, cal, and s specfc to the 1 ndvdual sample under test. he measurement s recorded as. If the spectral characterstcs, nherent dffusvty, or transmsson of the sample changes, a new sample calbraton must be performed. A fnal measurement of the sample s obtaned wth the ncdent source and recorded as S. he total spectral transmsson, τ, of the sample can then be calculated. otal Spectral ransmsson τ ( λ) CS = cal ( λ) 1 ( λ) ( λ) cal S ( ) λ

6 Usng ths approach, test results show a marked dfference between measurements made usng the 2 beam and 4 beam methods. It s also worth notng that lookng at Fg. 6 t s clear that the dfferences obtaned usng a spectroradometer and the 4 beam method are not smply a constant shft, but rather spectrally dependent on the color and composton of the samples beng tested. Fg. 6 Comparson of 2 beam vs. 4 beam method for measurng dffuse transmsson