NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

A New Method to Characterize 

Degradation of First Surface 

Aluminum Reflectors 

Preprint 

Florian Sutter, Peter Heller, Stephanie Meyen, 

and Robert Pitz-Paal 

DLR German Aerospace Center, Institute for Technical 

Thermodynamics

Cheryl Kennedy 

National Renewable Energy Laboratory

Aránzazu Fernández-García

CIEMAT, Plataforma Solar de Almeria, Ctra.

Martin Schmücker 

DLR German Aerospace Center, Institute for Material 

Sciences

Presented at SolarPACES 2010

Perpignan, France

September 21-24, 2010

Conference Paper

NREL/CP-5500-49164

October 2010
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1

A NEW METHOD TO CHARACTERIZE DEGRADATION OF FIRST 

SURFACE ALUMINUM REFLECTORS 

Florian Sutter1, Aránzazu Fernández García2, Peter Heller1, Cheryl Kennedy3, Stephanie Meyen1, 

Robert Pitz-Paal1

, Martin Schmücker4

1 Solar Researcher, DLR German Aerospace Center, Institute for Technical Thermodynamics, PSA, Ctra. Senés Km. 4, 

P.O.Box 44, 04200, Tabernas, Almería (Spain), +34 950 277684, Florian.Sutter@dlr.de

2

Solar Researcher, CIEMAT, Plataforma Solar de Almeria, Ctra. Senés Km. 4, 04200 Tabernas, Almería (Spain) 

3

Solar Researcher, NREL National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, CO 80401-3393 USA

4

Material Scientist, DLR German Aerospace Center, Institute for Material Sciences, Linder Höhe, D-51147 Cologne, 

Germany 

Abstract 

This paper reports the development of a new optical instrument capable of characterizing the aging process of 

enhanced first surface aluminum reflectors for concentrating solar power (CSP) application. Samples were 

exposed outdoors at different sites and in accelerated exposure tests. All samples exposed outdoors showed 

localized corrosion spots. Degradation originated from points of damage in the protective coating, but 

propagated underneath the protective coating. The degraded samples were analyzed with a microscope and 

with a newly designed space-resolved specular reflectometer (SR)2

 that is capable of optically detecting and 

characterizing the corrosion spots. The device measures the specular reflectance at three acceptance angles 

and the wavelengths with spatial resolution using a digital camera’s CMOS sensor. It can be used to measure 

the corrosion growth rate during outdoor and accelerated exposure tests. These results will allow a correlation 

between the degraded mirror surface and its specular reflectance. 

Keywords: accelerated and outdoor weathering, aging of reflectors, specular reflectance, reflectometer, 

filiform corrosion of aluminum, soiling 

1. Introduction 

Radiation emitted by the sun is focused onto the absorber tube of a parabolic trough collector with solar 

mirrors if the aperture angle of the reflected beam is less than 20 mradians (mrad) half-cone angle (Fig.1a). 

Including collector tracking, absorber tube position, and mirror slope errors, reflector surface scattering 

within a 12.5 mrad angle is acceptable for a parabolic trough collector [1]. 

Fig. 1. a) Specular reflectance at a parabolic trough collector and b) definition of terms 

The Alliance for Sustainable Energy, LLC (Alliance), is the manager and operator of the National Renewable Energy Laboratory (NREL). Employees of the Alliance, under 

Contract No. DE-AC36-08GO28308 with the U.S. Dept. of Energy, have authored this work. The United States Government retains and the publisher, by accepting the article for 

publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this 

work, or allow others to do so, for United States Government purposes. 

Absorber 

tube 

Collector 

~5 mrad 

~20 mrad 

12.5 mrad 

Sun 
2

Specular reflectance is the amount of light reflected into the acceptance half angle θ shown in Fig.1b. 

Reflectance is dependent on the wavelength (λ), the angle of incidence (φ) that incoming light makes with the 

mirror surface normal, and the acceptance half angle (θ) that marks the greatest offset of a reflected beam to 

the ideal specular reflected beam. Hemispherical reflectance can be understood as the reflectance within an 

acceptance half angle of π. Reflectance is written with all three parameters ρ( λ, φ, θ). The most important 

parameter to evaluate if a reflector material is suited for concentrating solar power (CSP) parabolic trough 

application is its solar-weighted specular reflectance value at 12.5 mrad. Other important parameters for solar 

mirrors are their cost and their resistance to weathering with a required durability of at least 20 years.

The specular reflectance is usually measured with a field portable 15R specular reflectometer from Devices 

and Services Instruments (D&S). Measurements can be taken at different half-cone acceptance angles (3.5, 

7.5, and 12.5 mrad), one certain wavelength λ = 660 nm, and φ=15°. The solar-weighted specular reflectance 

value can be approximated using a standard terrestrial solar spectrum [2] and the solar weighting formula 

described in [2, 3].

The existing instruments used to measure the specular reflectance have several disadvantages in terms of 

characterizing degradation processes: the measuring spot is usually very small (~1 cm in diameter), the 

measurements can only be taken at one specified wavelength, and the measurements are not spatially 

resolved. For reflectors that degrade locally, several measurements are necessary to statistically evaluate the 

influence of degradation on the reflectance properties. Problems associated with the measurement of specular 

reflectance are also described in [4].

2. Space-resolved specular reflectometer (SR)2

In order to monitor and optically characterize local degradation, the space-resolved specular reflectometer 

(SR)2 was developed (Fig.2) [5]. The major advantage of the (SR)2 compared to other reflectometers is the 

possibility of measuring the specular reflectance on an extended measuring spot of 5.5 cm with a spatial 

resolution of 12 pixel/mm. Additionally, measurements can be taken at three different acceptance half angles 

(θ = 3.5, 7.5, and 12.5 mrad) and at three different wavelengths (λ = 410 nm, 500 nm, and 656 nm).

Fig. 2. Space-resolved specular reflectometer (SR)2 instrument scheme

The parallel light emitted from a 75W xenon arc lamp is focused onto a 2-mm pinhole. The light passing

through the pinhole is collected by two achromatic lenses that focus the beam into the center of the aperture

wheel. To measure the specular reflectance, the mirror sample is placed in the optical path and held in place 

by vacuum suction to a flat plate. The sample holder design reduces the uncertainty due to mirror slope 

errors. The acceptance angle is changed by rotating the aperture wheel. The instrument is designed so that all 

of the light passing through the aperture falls onto the complementary metal-oxide-semiconductor (CMOS) 

sensor of the digital camera. Measurements are taken in a dark room to avoid errors through ambient diffuse 

light. The prototype of the system is shown in Fig.3a and b. 

Aperturewheel

Pinhole

Light source

Camera

Filterwheel

Lens 0

Lens 1

Lens 2

Iris

Acceptance angle

Mirror 

sample
3

Fig. 3. Prototype at DLR a) interior view of the dark box, b) exterior view

The photographs obtained in RAW-format are linearized and converted to ppm format with the open source 

software dcraw. In the linear photograph, the number of photons that fall on each pixel is proportional to the 

pixel intensity. The instrument is calibrated with a reference mirror for which the specular and hemispherical 

reflectance properties are identical (ρspecular,λ= ρhem,λ). The ρspecular,410nm, ρspecular,500nm, and ρspecular,656nm can then 

be determined using the Perkin-Elmer Lambda1050 UV-Vis-NIR spectrophotometer. Measurements taken in 

the (SR)2 are corrected for errors from dust on the optical components or inhomogeneous illumination of the 

mirror sample by using a reference photograph of the calibration mirror. 

The instrument can be employed to evaluate the surface reflectance characteristics of flat mirrors with the 

creation of a reflectance map. In combination with aging tests, the instrument can monitor gradual changes 

like the growth of degradation (Fig.4). The fraction of degraded surface relative to the intact surface of the 

mirror can be evaluated as well. The instrument can also be used to characterize the scattering from soiled 

mirrors before and after cleaning.

Fig. 4. Corrosion monitoring of accelerated testing to determine the corrosion growth rate and its

effect on specular reflectance. (Reflectance map at λ = 656nm, φ = 15° and θ = 12.5 mrad, blue = 0%, 

red=100% specular reflectance)

3. Aging analysis of a first surface enhanced aluminum reflector

Aggressive cost reductions and performance improvements in the solar field are required for CSP 

technologies with thermal storage to produce electricity competitively. This has lead to the study of solar

reflector candidate materials with the potential to reduce cost, weight, and installation labor while increasing 

optical performance and durability.

One such promising solar reflector candidate is the enhanced first surface aluminum mirror shown in Fig.5. 

The polished aluminum substrate is anodized, and a pure aluminum layer is deposited by physical vapor 

Pair of 

achromatic 

lenses

Vaccum sample 

holder for 

10x10cm 

samples

Nikon D300, 

35mm

Aperture 

wheel, 12.5, 

7.5 or 

3.5mrad

Stabilized light 

source

Filter wheel, 

410, 500,656nm

Focussing 

lens Pinhole, 2 mm

85.53% average specular reflectance 67.30% average specular reflectance 42.57% average specular reflectance
4

deposition (PVD). The reflectance of the aluminum is enhanced by 1⁄4 wavelength thick, low- and high-indexof-refraction oxide coatings and protected with a sol-gel nanocomposite oxide layer. The initial solarweighted hemispherical reflectance is 91.8%; initial specular reflectance at 12.5-mrad half angle and 660 nm 

is 83.7% and at 3.5-mrad is 71.7%. 

Fig. 5. Composition of the enhanced aluminum reflector 

3.1. Outdoor weathering

Samples of the enhanced aluminum reflector were exposed at three outdoor test sites in Phoenix, Arizona 

(APS), Golden, Colorado (NREL), and Miami, Florida (FLA) in the United States. Outdoor exposure sites in 

Europe were in Cologne, Germany (KOL) and Tabernas, Spain (TAB). All materials are optically 

characterized prior to exposure testing and as a function of exposure time to assess optical durability.

3.2. Accelerated weathering

Two accelerated exposure chambers were used at NREL: an Atlas Ci5000 WeatherOmeter (WOM) and a 

BlueM damp-heat accelerated exposure chamber (BlueM). The Ci5000 WOM uses a two-sun xenon-arc light 

source with filters designed to closely match the terrestrial AM1.5 global solar spectrum. The Ci5000 

operates continuously at 60°C and 60% relative humidity (RH). A single day of testing (24 hours) in the 

Ci5000 is roughly equivalent to six times outdoors. The BlueM operates continuously without light at 85°C 

and 85% RH. While the BlueM does not have the same measurable acceleration factor as the WOMs, from 

experiments, the acceleration factor approaches 25 times outdoors.

The solar-weighted hemispherical and specular reflectance over exposure time at the different exposure sites

and in the accelerated exposure chambers is shown in Fig.6. 

Sol Gel SiO2

1⁄4 λ TiO2

1⁄4 λ SiO2

PVD Al (pure)

Anodization Al2O3

High gloss rolled Al-Substrate

3μm

55nm

75nm

65nm

3μm

0.5mm
5

Fig.6: Solar-weighted hemispherical reflectance (SWH) and specular reflectance at θ = 3.5 and 12.5 

mrad at λ = 660nm of enhanced aluminum reflectors after accelerated exposure in BlueM (dark / 

85°C / 85%RH), WOM (2 sun / 60°C / 60%RH) chambers, and outdoor exposure at NREL, APS, 

FLA, and TAB.

3.3. Degradation mechanism

After three years of outdoor exposure at the different sites, only slight decreases in the solar-weighted 

hemispherical reflectance (SWH) can be observed. The specular reflectance at 3.5 mrad shows a maximum 

loss of 14.1% after 34.7 months of exposure in Phoenix. However, the specular reflectance within the 

12.5mrad acceptance angle is more relevant for parabolic trough applications. Here, losses are only in the 

range of 1-2% after three years of exposure.

Small corrosion spots were observed by visual or microscopic inspection on all samples exposed at every 

outdoor site. The corrosion starts at coating defects such as micro-cracks, coating break outs or pinholes (Fig. 

7a and 7b). 

Fig. 7a. Crack in protective coating Fig. 7b. Filiform corrosion and crack viewed in the 

viewed in the SEM light microscope

Crack in 

protective 

coating

Corroded area
6

A cross section image reveals that the degradation occurs at the pure aluminum layer (Fig.8). The image was 

produced with a transmission electron microscope (TEM) in Z-Contrast mode (the intensity is proportional to 

the atomic number Z). The sample preparation occurred by focused ion beam (FIB) to avoid introducing new 

defects. Other attempts to prepare the sample (like polishing or grinding) were not successful because the 

blade caused delamination of the sample’s coating.

A vertical energy-dispersive X-ray spectroscopy (EDX) line scan with high spatial resolution detected a loss 

of the aluminum in the degraded areas. The void is able to propagate underneath the SiO2 and TiO2

enhancing layers, while they themselves remain unharmed. This effect is also known as filiform corrosion.

The natural corrosion mechanism observed in samples exposed outdoors was not reproduced in the 

accelerated weathering chambers. More germane accelerating test methods are still being developed. 

Fig.8: Cross section of a corrosion spot

4. Optical characterization of corrosion processes with the space-resolved specular 

reflectometer (SR)2

The new instrument measures the following parameters through image analysis:

• Average specular reflectance of the measurement spot of the sample

• Fraction of area with specular reflectance below a boundary value (selected by user)

• Detection of local corrosion spots and evaluation of their area and specular reflectance

• Specular reflectance of non-corroded area.

The new instrument has been used to measure samples of the enhanced aluminum reflector exposed for 57 

months outdoors in Tabernas and for 36 months in Miami. The specular reflectance, the corroded mirror area, 

the number of corrosion spots, and the specular reflectance of the non-corroded area are shown in Fig. 9 for 

both sites. The given values are mean values averaged from two samples per site. The initial specular 

reflectance was ρs(660nm, 15°, 12.5mrad) = 83.7% measured with the D&S. Samples measured after 36 

months of exposure in Miami, ρs(660nm, 15°, 12.5mrad) = 81.8% with the D&S and with the (SR)2 

ρs(656nm, 15°, 12.5mrad) = 81.8%. After 57 months of exposure in Tabernas, ρs(660nm, 15°, 12.5mrad) = 

81.2% for samples measured with the D&S and ρs(656nm, 15°, 12.5mrad) = 81.4% measured with the (SR)2

.

The results show that Miami is a more aggressive environment than Tabernas because the corroded mirror 

surface area is almost twice as big in a shorter period of time. Local corrosion accounts for a loss in specular 

reflectance of 0.4% in Tabernas and 1.1% in Miami. 

Non-degraded area Degraded area

Corrosion of Al-layer

SiO2

TiO2

SiO2

Al

Al2O3

 100nm
7

In future work, samples exposed at Köln, Germany and in salt spray chambers will be evaluated for the 

influence of climatic conditions on corrosion kinetics. The corrosion growth rates will be determined as a 

function of climatic conditions through periodic monitoring.

Fig.9: Optical characterization of four samples of the enhanced aluminium reflector exposed for 57 

months in Tabernas, Spain and 36.1 months in Miami, USA. (Initial specular reflectance was 83.7%)

5. Optical characterization of soiling processes of a glass mirror with the space-resolved 

specular Reflectometer (SR)2

The new instrument can also be employed to determine the cleanliness factor of mirror samples and to 

characterize the scattering due to soiling. The device was used to measure a new glass mirror exposed for 22 

days in Tabernas, Spain both in soiled condition and after being cleaned. The mirror surface area whose 

reflectance is less than a defined boundary value can be determined from the curves in Fig.10. For example, 

it can be seen that 91.4% of the mirror surface has a reflectance less than 70% when soiled, whereas after 

being cleaned, only 0.001% of the surface has a reflectance less than 70%.

The average specular reflectance of the soiled surface was ρ(656nm, 15°, 12.5mrad) = 52.3%; after being 

cleaned it was ρs(656nm, 15°, 12.5mrad) = 95.2%. Thus the cleanliness factor χ (as ratio of ρsoiled to ρclean) is 

χ= 0.55. The measurements show good agreement with D&S readings. The larger measurement spot (5.5cm 

in diameter for the (SR)2 compared to 1cm for the D&S) has benefits because inhomogeneous soiling 

requires a statistical evaluation of the surface.

Fig.10: Area-reflectance dependency for a new glass mirror (soiled in Tabernas, Spain for 22 days and 

cleaned afterwards; measurements at 656nm, 15° incidence angle, and 12.5mrad)

81,4

1

9,5

81,8 81,8

1,9

46,5

82,9

0,0

10,0

20,0

30,0

40,0

50,0

60,0

70,0

80,0

90,0

100,0

specular reflectance at

25mrad and 656nm [%]

corroded mirror surface

[%]

number of corrosion

spots

specular reflectance of

non-corroded surface at

25mrad and 656nm [%]

Tabernas, 57 months Florida, 36 months

Cleaned 

mirror

Soiled 

mirror
8

6. Conclusion and Outlook

A new instrument has been designed to measure spatially resolved specular reflectance. The method is suited 

to characterize local corrosion and soiling processes of solar reflectors. Analysis of an enhanced aluminum 

reflector with the new (SR)2 instrument showed that the reflectance loss of the mirror is mainly due to local 

corrosion and scattering. Microscope analysis showed that the pure aluminum layer corrodes underneath the 

unharmed protective and enhancing coatings. Samples exposed in Miami, United States corrode faster than 

samples exposed in Tabernas, Spain. 

In future work, samples exposed in the United States and Europe will be periodically monitored in order to 

determine corrosion growth rates as a function of climatic conditions. With this data, simplified service 

lifetime prediction models can be derived to predict the reflectance losses during long-term exposure. 

Acknowledgements

The authors want to thank Gary Jorgensen (NREL), Robert Tirawat (NREL), Sebastian Hübener (DLR), and 

José Rubio (DLR) for their support. NREL’s part of this work was performed under DOE contract DEAC36-99-GO10337.

References

[1] R. B. Pettit, “Characterization of Reflected Beam Profile of Solar Mirror Materials,” Solar Energy, Vol. 

19, No. 6, 1977, pp. 773-741.

[2] ASTM Standard G 173 – 03, “Standard Tables for Reference Solar Spectral Irradiances: Direct Normal 

and Hemispherical on 37° Tilted Surface," Annual Book of ASTM Standards 2008, Vol. 14.04. West 

Conshohocken, PA: American Society for Testing and Materials.

[3] DIN Standard 5036, Part 1, “Radiometric and photometric properties of materials: Definitions and 

characteristic factors,” July 1978, Berlin, Germany: Deutsches Institut für Normung e.V.

[4] S. Meyen, A. Fernandez, C. Kennedy, E. Lüpfert, “Standardization of Solar Mirror Reflectance 

Measurements – Round Robin Test,” to be published in Proceedings SolarPaces Symposium Perpignon, 

2010

[5] Deutsche Patentanmeldung “Vorrichtung und Verfahren zur Bestimmung von Reflexionseigenschaften 

eines Reflektors,” F. Sutter, P. Heller, S. Meyen, C. Prahl, June 2010
F1147-E(10/2008)

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A New Method to Characterize Degradation of First Surface 

Aluminum Reflectors: Preprint 

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DE-AC36-08GO28308

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F. Sutter, P. Heller, S. Meyen, R. Pitz-Paal; DLR German 

Aerospace Center, Institute for Technical Thermodynamics

A. F. García; CIEMAT, Plataforma Solar de Almeria, Ctra.

M. Schmücker; DLR German Aerospace Center, Institute for 

Material Sciences

C. Kennedy; NREL

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14. ABSTRACT (Maximum 200 Words)

This paper reports the development of a new optical instrument capable of characterizing the aging process of 

enhanced first surface aluminum reflectors for concentrating solar power (CSP) application. Samples were exposed 

outdoors at different sites and in accelerated exposure tests. All samples exposed outdoors showed localized 

corrosion spots. Degradation originated from points of damage in the protective coating, but propagated underneath 

the protective coating. The degraded samples were analyzed with a microscope and with a newly designed spaceresolved specular reflectometer (SR)2 that is capable of optically detecting and characterizing the corrosion spots. 

The device measures the specular reflectance at three acceptance angles and the wavelengths with spatial 

resolution using a digital camera’s CMOS sensor. It can be used to measure the corrosion growth rate during outdoor 

and accelerated exposure tests. These results will allow a correlation between the degraded mirror surface and its 

specular reflectance.

15. SUBJECT TERMS

accelerated and outdoor weathering; aging of reflectors; specular reflectance; reflectometer; filiform corrosion of 

aluminum; soil

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