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Author:Pulli, Tomi
Title:Improved Diffusers for Solar UV Spectroradiometers
Auringon UV-säteilyä mittaavan spektroradiometrin diffuuserin parantaminen
Publication type:Master's thesis
Publication year:2012
Pages:[9] + 48      Language:   eng
Department/School:Signaalinkäsittelyn ja akustiikan laitos
Main subject:Mittaustekniikka   (S-108)
Supervisor:Ikonen, Erkki
Instructor:Kärhä, Petri
Electronic version URL: http://urn.fi/URN:NBN:fi:aalto-201305163111
OEVS:
Electronic archive copy is available via Aalto Thesis Database.
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Location:P1 Ark Aalto  1312   | Archive
Keywords:radiometry
solar UV
diffusers
Monte Carlo analysis
radiometria
auringon UV
diffuuserit
Monte Carlo -analyysi
Abstract (eng): Solar UV (ultraviolet) radiation scatters heavily in the atmosphere, and the diffuse component of the radiations accounts for a significant portion of the total UV radiation that reaches the surface of the Earth.
Diffuser heads whose angular response is proportional to the cosine of the zenith angle are needed for the global irradiance measurements.
In this thesis, a combination of measurements and simulations was used to optimize such a diffuser.

At the first stage, test samples of various materials were measured for their diffuse transmittance properties in a goniometric setup to find out the most promising material candidate for use in an improved solar UV diffuser head.
Quartz materials with gas bubbles that acted as scattering centers were found to be attractive alternatives to the traditional PTFE (polytetrafluoroethylene, Teflon) materials for this purpose.

At the next stage, a Monte Carlo ray tracing software was developed to simulate light transport inside the diffusers.
The software was validated by comparing measured and simulated results of the test samples.
The effects of various simulation parameters on the overall angular response were studied extensively.
The angular response was found to be highly sensitive to variations in some of the parameter values, highlighting the importance of low manufacturing tolerances during production.

It was shown that an ideal cosine response cannot be attained without shaping the diffuser in one way or another, due to the refractive index difference between the diffuser material and its surroundings.
An integrated cosine error of 1.6 % was reached with an optimized raised flat diffuser.
An optimized diffuser with spherical front surface yielded the integrated cosine error of 0.63 %.
This value is well below the integrated cosine errors of most UV diffusers currently in use.
Abstract (fin): Auringon ultravioletti- eli UV-säteily siroaa voimakkaasti ilmakehässä, minkä seurauksena UV-säteilyn epäsuora komponentti muodostaa merkittävän osan maan pinnalle saapuvasta kokonaissäteilystä.
Diffuuseripäitä, joiden kulmavaste on verrannollinen zeniittikulman kosiniin, tarvitaan globaaleissa säteilyvoimakkuusmittauksissa.
Työssä tämänkaltainen diffuuseri optimoitiin mittausten ja simulointien avulla.

Ensimmäisessä vaiheessa lukuisten eri materiaaleista valmistettujen testinäytteiden diffuusiläpäisyominaisuudet mitattiin goniometrilaitteistossa, tavoitteena löytää lupaavin pohjamateriaali paranneltua diffuuseripäätä varten.
Kvartsinäytteiden, joissa kaasukuplat toimivat sirontaytiminä, todettiin olevan houkuttelevia vaihtoehtoja perinteisille PTFE-materiaaleille (polytetrafluorieteeni, Teflon) tähän tarkoitukseen.

Tutkimuksen seuraavassa vaiheessa kehitettiin Monte Carlo -ohjelma, jonka avulla simuloitiin valon etenemistä diffuuserin sisällä.
Ohjelman toimivuus testattiin vertailemalla testinäytteiden mittaus- ja simulointituloksia keskenään.
Eri simulointiparametrien vaikutusta kulmavasteeseen tutkittiin kattavasti.
Kulmavasteen todettiin olevan erittäin herkkä tiettyjen parametriarvojen vaihteluille, mikä korostaa pienten toleranssien tärkeyttä diffuusereiden valmistusvaiheessa.

Työssä osoitettiin, että täydellistä kosinivastetta ei voi saavuttaa muotoilematta diffuuseria tavalla tai toisella.
Tämä on seurausta diffuuserimateriaalin ja sen ympäristön taitekerroinerosta.
Optimoidulla nostetulla tasodiffuuserilla saavutettiin 1.6 %:n integroitu kosinivirhe.
Vastaavasti optimoitu diffuuseri, jonka etupinta oli muotoiltu pallomaisesti, tuotti 0.63 %:n integroidun kosinivirheen.
Arvo on selvästi matalampi kuin suurimmalla osalla tällä hetkellä käytössä olevista UV-diffuusereista.
ED:2012-12-17
INSSI record number: 45719
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