`
`
`PH/DT
`
`C. Joram
`
`26 October 2009
`
`
`
`This note documents transmission measurements of material samples frequently used in the PH/DT
`scintillator lab. The measurements were performed by C. David on the Perkin-Elmer (visible light)
`spectrometer available in the DT TFG workshop.
`
`The measured samples were:
`
` Poly(methyl methacrylate) (PMMA), standard material from CERN stores, also known as
`Plexiglas, Perspex, Lucite, acrylic glass, etc. The sample was a PMMA sheet of 2 mm
`thickness. The same base material is used for the fabrication of light guides. Its refractive
`index is nPMMA = 1.489.
`
` Optical grease of type ‘Dow Corning Q2-3067 OPT COUPLANT 453G’. This is the standard
`grease used for the coupling of light guides to photodetectors (PMTs). Its refractive index is
` = 589 nm). For the transmission measurement, the optical grease was
`nOG = 1.466 (at
`applied as thin film between two glass sheets, which were pressed together to be in
`mechanical contact.
`
`The measured transmission is influenced by two components:
`
`1. Bulk absorption, inherent to the optical transparency of the material
`
`
`Tb = T0 exp (-
`
`·d) = T0 exp (-d/ a)
`
`
`
`is the wavelength dependent absorption parameter (unit [m-1]) and
`absorption length [m]. d is the thickness of the material sample [m].
`
`2. The Fresnel losses due to reflections at the optical boundaries, e.g. air/plexiglass and
`plexiglass/air. For normal incidence, the reflection losses per interface are
`
`RF = (n-1)2/(n+1)2
`
`a = 1/ is the
`
`
`with n = n2/n1, the ratio of the refractive indices of the sample and the surrounding medium. In
`the case of a plexiglass sample measured in air, n1 = 1 and therefore n = n2 = 1.489. The Fresnel
`losses per interface are therefore 3.86%.
`
`26/10/2009
`PH-EP-Tech-Note-2009-003
`
`1/3
`
`
`
`Figure 1 shows the transmission curve of the 2 mm thick plexiglass sheet in air. At wavelengths
`above 400 nm, the transmission is only constrained by the Fresnel losses which amount to two times
`3.86%, i.e. approx. 8%. Below 400 nm, the bulk absorption of plexiglass becomes dominant. The
`sample doesn’t transmit any light below 360 nm.
`
`Figure 2 shows the transmission curves of two thin glass plates (in mechanical contact) without
`optical grease (red line) and with optical grease in between (green line). At wavelengths above 400
`nm, the transmission is only constrained by the Fresnel losses which amount for ordinary glass
`(n=1.52) to about 2 or 4 times 4.2%, i.e. approx 8.5% or 17%. Below 350 nm, the bulk absorption of
`the glass becomes visible. The sample with the thin optical grease film shows a higher transmission
`than the sample without grease. This is due the almost matched refractive indices of grease and
`glass, which reduce the Fresnel losses at the inner interface to almost zero. The measurements show
`that for wavelengths where the glass shows non-zero transparency ( > 300 nm), the thin layer of
`optical grease has no noticeable impact on the transmission curve.
`
`
`
`
`
`2/3
`
`
`
`Plexiglass (PMMA) thickness 2mm
`
`100
`
`90
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`10
`
`0
`
`TR [%]
`
`
`
`
`
`
`
`
`
`200
`
`250
`
`300
`
`350
`
`400
`
`450
`
`500
`
`W.L [nm]
`
`Figure 1: Transmission curve of a 2 mm thick plexiglass sheet in air. At wavelengths above 400 nm, the transmission is
`constrained by the Fresnel losses which amount to two times 3.86%, i.e. approx. 8%. Below 400 nm, the bulk absorption
`of plexiglass becomes dominant. The sample doesn’t transmit any light below 360 nm.
`
`Glass plates with/without optical grease
`
`2 plaques de verre de 1.5mm plaque
`ensemble_ 231009
`
`2 plaques de verre de 1.5mm plaque
`ensemble+graisse optique Q2-3067_
`231009
`
`200
`
`250
`
`300
`
`350
`
`400
`
`450
`
`500
`
`W.L [nm]
`
`100
`
`90
`
`80
`
`70
`
`60
`
`50
`
`40
`
`30
`
`20
`
`10
`
`0
`
`
`
`TR [%]
`
`Figure 2: Transmission curves of two thin glass plates without optical grease (red line) and with optical grease in
`between (green line). At wavelengths above 400 nm, the transmission is constrained by the Fresnel losses which amount
`for ordinary glass (n=1.52) to about 2 or 4 times 4.2%, i.e. approx 8.5% or 17%.
`
`3/3
`
`
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